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viruses
Article
Pan-European Study on the Prevalence of the
Feline Leukaemia Virus Infection – Reported by
the European Advisory Board on Cat Diseases
(ABCD Europe)
Nadine Studer 1, Hans Lutz 1, Claude Saegerman 2, Enikö Gönczi 1, Marina L. Meli 1,
Gianluca Boo 3, Katrin Hartmann 4, Margaret J. Hosie 5, Karin Moestl 6, Séverine Tasker 7,
Sándor Belák8, Albert Lloret 9, Corine Boucraut-Baralon 10, Herman F. Egberink 11,
Maria-Grazia Pennisi 12, Uwe Truyen 13, Tadeusz Frymus 14 , Etienne Thiry 15, Fulvio Marsilio 16,
Diane Addie 17, Manfred Hochleithner 18, Filip Tkalec 19, Zsuzsanna Vizi 20, Anna Brunetti 21,
Boyko Georgiev 22, Louisa F. Ludwig-Begall 15, Flurin Tschuor 23, Carmel T. Mooney 24,
Catarina Eliasson 25, Janne Orro 26, Helle Johansen 27, Kirsi Juuti 28, Igor Krampl 29,
Kaspars Kovalenko 30, Jakov Šengaut 31, Cristina Sobral 32, Petra Borska 33 , Simona Kovaˇríková34
and Regina Hofmann-Lehmann 1,*
1Clinical Laboratory, Department of Clinical Diagnostics and Services, and Center for Clinical Studies,
Vetsuisse Faculty, University of Zurich, 8057 Zurich, Switzerland; studer.nadine@bluewin.ch (N.S.);
Hans.Lutz@uzh.ch (H.L.); enikoe.goenczi@kispi.uzh.ch (E.G.); mmeli@vetclinics.uzh.ch (M.L.M.)
2
Department of Infectious and Parasitic Diseases, Research Unit of Epidemiology and Risk Analysis Applied
to Veterinary, Fundamental and Applied Research for Animal and Health (FARAH) Center, Faculty of
Veterinary Medicine, University of Liège, B-4000 Liège, Belgium; claude.saegerman@uliege.be
3Department of Geography, University of Zurich, 8057 Zurich, Switzerland; gianluca.boo@soton.ac.uk
4Clinic of Small Animal Medicine, Centre for Clinical Veterinary Medicine, LMU Munich, 80539 Munich,
Germany; Hartmann@uni-muenchen.de
5MRC- University of Glasgow Centre for Virus Research, Glasgow G61 1QH, UK;
Margaret.Hosie@glasgow.ac.uk
6
Institute of Virology, Department for Pathobiology, University of Veterinary Medicine, 1210 Vienna, Austria;
karinmoestl@gmail.com
7Bristol Veterinary School, University of Bristol, Bristol BS40 5DU, UK & Chief Medical Officer, Linnaeus
Group, Shirley, Solihull B90 4BN, UK; s.tasker@bristol.ac.uk
8Swedish University of Agricultural Sciences (SLU), Department of Biomedical Sciences and Veterinary
Public Health (BVF), 750 07 Uppsala, Sweden; sandor.belak@slu.se
9
Fundaci
ó
Hospital Cl
í
nic Veterinari, Universitat Aut
ò
noma de Barcelona, 08193 Bellaterra, Barcelona, Spain;
albert.lloret@uab.es
10 Scanelis laboratory, 31770 Colomiers, France; corine.boucraut@scanelis.com
11
University of Utrecht, Faculty of Veterinary Medicine, Department of Infectious Diseases and Immunology,
3584 CL Utrecht, Netherlands; H.F.Egberink@uu.nl
12 Dipartimento di Scienze Veterinarie, Universitàdi Messina, 98168 Messina, Italy;
mariagrazia.pennisi@unime.it
13 Institute of Animal Hygiene and Veterinary Public Health, University of Leipzig, 04103 Leipzig, Germany;
truyen@vetmed.uni-leipzig.de
14 Department of Small Animal Diseases with Clinic, Faculty of Veterinary Medicine, Warsaw University of
Life Sciences-SGGW, 02-787 Warsaw, Poland; tadeusz_frymus@sggw.pl
15 Veterinary Virology and Animal Viral Diseases, Department of Infectious and Parasitic Diseases, FARAH
Research Centre, Faculty of Veterinary Medicine, Liège University, B-4000 Liège, Belgium;
Etienne.Thiry@ulg.ac.be (E.T.); lludwig@ulg.ac.be (L.F.L.-B.)
16 Faculty of Veterinary Medicine, Universitàdegli Studi di Teramo, 64100 Teramo, Italy; fmarsilio@unite.it
17 Veterinary Diagnostic Services, School of Veterinary Medicine, College of Medical, Veterinary and Life
Sciences, University of Glasgow, Glasgow G61 1QH, UK; draddie@catvirus.com
18 Tierklinik Strebersdorf, 1210 Vienna, Austria; hochleithner@gmail.com
19 Veterinarska klinika Kreszinger, 10360 Sesvete, Zagreb, Croatia; ftkalec89@gmail.com
Viruses 2019,11, 993; doi:10.3390/v11110993 www.mdpi.com/journal/viruses
Viruses 2019,11, 993 2 of 27
20 University of Veterinary Medicine, 1078 Budapest, Hungary; vizizsu81@gmail.com
21 School of Veterinary Medicine, University of Glasgow, Glasgow G61 1QH, UK; Anna-Brunetti@idexx.com
22 Institute of Biology and Immunology of Reproduction, 1113 Sofia, Bulgaria; boykog@netbg.com
23 Kleintierklinik BolligerTschuor AG, Fachtierärzte für Kleintiere, 4665 Oftringen – Zofingen, Switzerland;
ftschuor@bolligertschuor.ch
24
School of Veterinary Medicine, University College Dublin, Belfield, Dublin 4, Ireland; carmel.mooney@ucd.ie
25 Jamaren - Swedish Veterinary Feline Study Group, 275 71 Lövestad, Sweden; catarina.eliasson@gmail.com
26 Loomakliinik, 51014 Tartu, Estonia; janne@orrokliinik.ee
27 Bygholm Dyrehospital, 8700 Horsens, Denmark; helle.johansen@gmail.com
28 CatVet Kissaklinikka, 00400 Helsinki, Finland; kirsi.juuti@catvet.fi
29 Slovak Small Animal Veterinary Association, 821 02 Bratislava, Slovakia; info@savlmz.sk
30
Faculty of Veterinary Medicine, Latvia University of Lifesciences and Technologies, LV-3004 Jelgava, Latvia;
kkovalenko@inbox.lv
31 Jakov Veterinary Centre, Gerosios Vilties g. 1, LT-03147 Vilnius, Lithuania; dr@vetmed.lt
32 Vetalmada, small animal clinic, 2800-052 Almada, Portugal; crisgouveiasobral@gmail.com
33 Small Animal Emergency Clinic, 637 00 Brno-Jundrov, Czech Republic; petbor@post.cz
34 Department of Animal Protection, Welfare and Behavior, Faculty of Veterinary Hygiene and Ecology,
University of Veterinary and Pharmaceutical Sciences Brno, 612 42 Brno, Czech Republic; kralovas@post.cz
*Correspondence: rhofmann@vetclinics.uzh.ch; Tel.: +41-44-635-83-11
Received: 26 September 2019; Accepted: 27 October 2019; Published: 29 October 2019
Abstract:
Feline leukaemia virus (FeLV) is a retrovirus associated with fatal disease in progressively
infected cats. While testing/removal and vaccination led to a decreased prevalence of FeLV, recently,
this decrease has reportedly stagnated in some countries. This study aimed to prospectively determine
the prevalence of FeLV viraemia in cats taken to veterinary facilities in 32 European countries. FeLV
viral RNA was semiquantitatively detected in saliva, using RT-qPCR as a measure of viraemia. Risk
and protective factors were assessed using an online questionnaire to report geographic, demographic,
husbandry, FeLV vaccination, and clinical data. The overall prevalence of FeLV viraemia in cats
visiting a veterinary facility, of which 10.4% were shelter and rescue cats, was 2.3% (141/6005; 95% CI:
2.0%–2.8%) with the highest prevalences in Portugal, Hungary, and Italy/Malta (5.7%–8.8%). Using
multivariate analysis, seven risk factors (Southern Europe, male intact, 1–6 years of age, indoor and
outdoor or outdoor-only living, living in a group of
≥
5 cats, illness), and three protective factors
(Northern Europe, Western Europe, pedigree cats) were identified. Using classification and regression
tree (CART) analysis, the origin of cats in Europe, pedigree, and access to outdoors were important
predictors of FeLV status. FeLV-infected sick cats shed more viral RNA than FeLV-infected healthy cats,
and they suffered more frequently from anaemia, anorexia, and gingivitis/stomatitis than uninfected
sick cats. Most cats had never been FeLV-vaccinated; vaccination rates were indirectly associated
with the gross domestic product (GDP) per capita. In conclusion, we identified countries where FeLV
was undetectable, demonstrating that the infection can be eradicated and highlighting those regions
where awareness and prevention should be increased.
Keywords:
FeLV; retrovirus; prevalence; risk factors; protective factors; RT-qPCR; virus shedding;
vaccination; gross domestic product at purchasing power parity per capita; veterinary sciences
1. Introduction
Feline leukaemia virus (FeLV) is a gammaretrovirus that infects domestic cats and closely related
wild felids worldwide. Since the first description of the virus in 1964 in feline lymphoma tissue [
1
],
increasing knowledge has been gained concerning the pathogenesis, diagnosis, and treatment of
the infection [
2
,
3
]. FeLV infection can lead to fatal diseases in cats with progressive infection [
4
–
9
].
Viruses 2019,11, 993 3 of 27
Testing and eradication programs and the introduction of effective vaccines led to a decrease in FeLV
prevalence in many countries in the last 30 years [
7
,
10
–
12
]. However, more recently, the decrease
of the FeLV prevalence has stagnated [
13
–
15
]. In other countries, such as Denmark, FeLV is rarely
detected nowadays [
16
]. For the USA, Canada, and Australia, large prevalence studies have been
published [
10
,
13
,
17
–
19
]. There have also been some recent studies on the prevalence of FeLV in single
European countries [
15
,
20
–
22
]. However, there has been no pan-European study determining the
current FeLV prevalence in domestic cats. FeLV prevalence can vary considerably depending on the
composition of the investigated cat population; feral or stray cats versus privately owned cats, cats in
shelters or from breeders, clinically healthy cats or sick cats [
17
,
23
–
25
]. In addition, preselection of
the samples has an influence; e.g., if samples are obtained only from cats suspected of being infected
with FeLV. A recent meta-analysis [
26
] suggested an indirect relation between FeLV prevalence and
the yearly gross domestic product (GDP) per capita using purchasing power parity (PPP) [
27
], i.e.,
the total value of all the goods and services produced by a country in a particular year, divided by
the number of people living there and corrected for purchasing power using a “basket of goods”.
However, no FeLV prevalence data were available for many European countries, or the data were
obtained several years ago.
The aim of the present study was to determine the current prevalence of FeLV viraemia in cats that
visit a veterinary facility. The study protocol included cats from 32 European countries in the survey.
In order to avoid any preselection of the cats (apart from visiting a veterinary facility), 10 animals
arriving consecutively at each facility were tested independently of the reason for the veterinary
appointment. Risk and protective factors for FeLV infection were determined using the demographic,
husbandry, clinical as well as FeLV vaccination history data from each cat. Furthermore, using our
data, we tested the recently postulated hypothesis of a correlation between the FeLV prevalence and
the GDP in European countries, and extended the analysis to test also for a potential correlation with
FeLV vaccination rates. For minimally invasive sample collection and to avoid bias by requesting
blood collection, saliva swabs were collected and analysed by RT-qPCR; the detection of viral RNA by
RT-qPCR in saliva was shown to be an excellent measure of FeLV viraemia [28].
2. Materials and Methods
2.1. Design of the Prevalence Study
The studydesign included920 Europeanveterinary facilities in32 countries; for organizational/financial
reasons, some small countries were grouped into country groups, resulting in 23 countries/country groups
(Table 1). The countries/country groups were classified into “Eastern Europe”, “Northern Europe”,
“Southern Europe”, and “Western Europe” according to the United Nations geoscheme [
29
]. According to
the policy coordinator of the European Union Commission (Directorate-General for Environment, Unit
Chemicals), no animal testing permit was required for this study, mainly because of the non-invasive
method of sample collection, sample collection occurring during regular visits to veterinary practices, and
the handling of the cats by veterinary professionals. In Switzerland, the study was officially approved
by the veterinary offices of the Swiss cantons (approval number: ZH 121/16, issued 11 August 2016) and
conducted according to Swiss laws. Written informed consent was obtained from each cat owner by the
participating veterinarians in all countries. The sample collection was conducted from September 2016 to
March 2017.
Viruses 2019,11, 993 4 of 27
Table 1.
Numbers of veterinarians, samples shipped and returned, and feline leukaemia virus (FeLV)
prevalence in the different countries or country groups.
Country or Country Group 1Vets
Planned
Participating
vets
Samples
Shipped
Samples
and Data
Returned
Return
Rate %
No of
FeLV-pos. 2
FeLV
Prevalence %
(95% CI) 3
Northern Europe
Denmark 40 40 400 277 69.3 0 0.0 (0.0–1.4)
Finland 40 40 400 290 72.5 0 0.0 (0.0–1.3)
Ireland 40 39 390 136 34.9 7 5.1 (2.5–10.2)
Lithuania, Latvia, Estonia 40 40 400 266 66.5 4 1.5 (0.6–3.8)
Lithuania 19 19 190 143 75.3 4
Latvia 12 12 120 68 56.7 0
Estonia 9 9 90 55 61.1 0
Norway 40 38 380 205 53.9 0 0.0 (0.0–1.8)
Sweden 40 40 400 343 85.8 0 0.0 (0.0–1.1)
United Kingdom 40 40 400 136 34.0 1 0.7 (3.8 ×
10−4–4.0)
England 32 32 320 119 37.1 1
Scotland 4 4 40 10 25.0 0
Northern Ireland 2 2 20 0 0.0 0
Wales 2 2 20 7 35.0 0
Eastern Europe
Bulgaria, Romania 40 24 240 90 37.5 0 0.0 (0.0–4.1)
Bulgaria 11 11 110 90 81.8 0
Romania 29 13 130 0 0 n.a.
Czech Republic 40 40 400 361 90.3 8 2.2 (1.1–4.3)
Hungary 40 40 400 219 54.8 13 5.9 (3.5–9.9)
Poland 40 40 400 340 85.0 17 5.0 (3.1–7.9)
Slovakia 40 40 400 255 63.8 5 2.0 (0.8–4.5)
Southern Europe
Croatia 40 40 400 198 49.5 9 4.5 (2.4–8.4)
Greece 40 0 0 n.a. n.a. n.a.
Portugal 40 40 400 330 82.5 29 8.8 (6.2–12.3)
Spain 40 40 400 352 88.0 9 2.6 (1.4–4.8)
Italy and Malta 40 40 400 349 87.3 20 5.7 (3.7–8.7)
Italy 39 39 390 340 87.2 20
Malta 1 1 10 9 90.0 0
Western Europe
Austria 40 40 400 309 77.3 4 1.3 (0.5–3.3)
Belgium, Luxembourg 40 40 400 287 71.8 3 1.0 (0.3–3.0)
Belgium 38 38 380 278 73.2 3
Luxembourg 2 2 20 9 45.0 0
France 40 40 400 301 75.3 3 1.0 (0.3–2.9)
Germany 40 40 400 306 76.5 1 0.3 (1.7 ×
10−4–1.8)
Netherlands 40 40 400 356 89.0 0 0.0 (0.0–1.0)
Switzerland, Liechtenstein 40 40 400 299 74.8 8 2.7 (1.4–5.2)
Switzerland 39 39 390 290 74.4 7
Liechtenstein 1 1 10 9 90.0 1
Total 920 861 8610 6005 69.7 141 2.3 (2.0–2.8)
1
Countries and country groups were classified according to the United Nations geoscheme [
29
]. As no samples
were returned from Romania, it was excluded from prevalence calculations; in Greece, no veterinarians could be
recruited, and so it was excluded from all calculations.
2
Only results from cats with data in the online questionnaire
were included.
3
Percentages and 95% confidence intervals (CI) were calculated only for country groups and not for
single countries to avoid calculations based on small sample numbers. n.a., not applicable.
2.2. Sample Collection for the Prevalence Study
Forty veterinarians were involved per country/country group; they were instructed to collect
saliva swabs from 10 cats during consecutive veterinary consultations. This resulted in an intended
maximal number of 400 samples per country/country group and a total maximal intended number of
9200 samples. In the country groups, the number of samples per single country was chosen in relation
to the human population size of the respective country [
29
], since for many countries, reliable estimates
of cat populations were unavailable. Whenever possible, the veterinarians were chosen from different
areas within a country.
Participating veterinarians were provided (by priority mail) with 10 labeled screw-cap tubes
(1.5 mL, Sarstedt, Nümbrecht, Germany) filled with 300
µ
L of RNA shield (Zymo Research Europe
GmbH, Freiburg im Breisgau, Germany), 10 cotton swabs with plastic shafts (M-Budget, Migros
Viruses 2019,11, 993 5 of 27
Genossenschafts-Bund, Switzerland), an instruction sheet that described the proper swabbing
procedure, informed consent forms to be signed by the cat owners, customs declaration, import
permits, and prepaid return address labels. The RNA shield was provided to ensure biological
safety during the shipment of the saliva samples, since rabies is encountered in some participating
countries [
30
,
31
], and also to increase the stability of FeLV viral RNA. The veterinarians were asked to
sample 10 cats during consecutive appointments, regardless of the animal’s age, sex, vaccination status,
health status, or reason for the veterinary consultation. Only one cat per home, breeder, or shelter
was to be sampled. The swab was to be rubbed gently along the cheek pouches and under the tongue
of the cat, placed in the tube, and the external tip of the swab was removed prior to closing the tube.
The samples were shipped by postal mail at ambient temperature.
2.3. Data Collection for the Prevalence Study
For each sampled cat, an online questionnaire was completed by the sampling veterinarian.
The questionnaire was available in 18 languages and included 19 questions concerning geographic
data (country and postal code of cat owner), sample identification (identification number, name of the
veterinary practice, cat and cat owner, date of collection), demographic data (age, sex, reproductive
status and breed of the cat), husbandry data (type of husbandry, such as private home, cat breeder,
animal shelter, rescue cat, number of cats per household, and outdoor access) data on FeLV vaccination
history, and the results of the physical examination (healthy versus sick and, if sick, the major clinical
problem) (for details, see Appendix ATable A1). The physical examination was performed during
regular visits of the cat by the attending veterinarian, who assessed the health condition of the cat based
on his/her professional experience. A similar questionnaire has been used in previous studies [
32
–
34
].
2.4. Sample Preparation and Molecular Analysis
Samples were processed upon receipt in the laboratory as described previously [
28
,
34
–
36
].
The tubes were vortexed and put on a shaking incubator at 42
◦
C for 10 min to resuspend the sample.
Subsequent sample preparation was performed under sterile conditions in a laminar flow cabinet.
After centrifugation at 8000
×
gfor 1 min to remove any liquid from the inside of the lid, the swabs
were inverted using a pair of sterilized tweezers and centrifuged again to recover the liquid (freed
from the cotton part of the swab) in the bottom of the tube. The swabs were removed, and the liquid
sample material was stored at
−
80
◦
C until further use. Subsequently, the liquid samples were pooled
(Pipetting robot CAS-1200, LTF Labortechnik GmbH & Co. KG, Wasserburg, Germany) such that
up to 96 samples were combined in 20 pools and the material from each sample was present in two
pools (for details, see Appendix AFigure A1). Total nucleic acid (TNA) was extracted from the sample
pools using the MagNA Pure LC Total Nucleic Acid Kit - High Performance and the MagNA Pure LC
instrument (Roche Diagnostics, Mannheim, Germany), following the instructions of the manufacturer,
with an elution volume of 90
µ
L. Two negative controls of phosphate-buffered saline (PBS) were
concurrently prepared with each batch of samples to monitor for cross-contamination.
FeLV viral RNA was detected using 5
µ
L of TNA, and a previously described real-time TaqMan
FeLV RT-qPCR [
37
] on an ABI PRISM 7500 Fast Sequence Detection System (Applied Biosystems,
Foster City, USA) with some modifications. Briefly, the 25-
µ
L RT-qPCR reaction contained 12.5
µ
L 2
×
RT-qPCR Buffer, 1
µ
L 25
×
RT-qPCR Enzyme Mix (AgPath-IDTM One-Step RT-qPCR Reagents, Thermo
Fisher Scientific), a final concentration of 900 nM of forward primer (FeLV_U3_exo_f; 5’AAC AGC
AGA AGT TTC AAG GCC 3’; 21 bp), 300 nM of reverse primer (FeLV_U3_exo_r; 5’TTA TAG CAG
AAA GCG CGC G3’; 19 bp), and 200 nM of fluorogenic probe (exoFeLV-U3-probe; 5’-FAM-CCA GCA
GTC TCC AGG CTC CCC A-TAMRA 3’; 22 bp). All oligonucleotides were synthetized by Microsynth
AG (Balgach, Switzerland). The temperature profile was 10 min at 45
◦
C, followed by 10 min at 95
◦
C
and 40 cycles of 15 s at 95
◦
C, followed by 45 s at 60
◦
C. Each PCR run was performed together with
positive (RNA standard template) [37] and negative controls (PBS).
Viruses 2019,11, 993 6 of 27
The pooling scheme allowed the identification of the individual samples that could have
contributed to the positive pool results. From all these single samples, TNA was extracted from 50
µ
L
of original liquid sample material, and FeLV real-time RT-qPCR was performed as described above.
The FeLV input copy numbers in the single samples were determined by co-amplifying 10-fold serial
dilutions of an RNA standard template as described [
37
]. All further analyses were conducted with
the FeLV RT-qPCR results of the individual samples/cats.
2.5. Pre-Experiment
The stability of FeLV in the RNA shield was tested in a pre-experiment using cell culture
supernatant from FeLV-infected FL-74 cells. Cell culture supernatant was diluted in PBS to reach
a FeLV copy number concentration that corresponded to that in the saliva of viraemic cats (threshold
cycle values of approximately 22–24) [
37
,
38
]. Aliquots of diluted cell culture supernatant were stored
at room temperature for 7, 14, 21, 28, 50, and 60 days and at 37
◦
C for 2, 3, 5, 7 and 14 days, respectively.
After incubation, TNA was extracted and analysed by FeLV real-time RT-qPCR as described above.
2.6. Statistical Analysis
2.6.1. Descriptive Analysis
The parameters of all cats were compiled and analysed using Excel (Microsoft, Wallisellen,
Switzerland) and GraphPad Prism software (GraphPad, San Diego, CA). FeLV prevalence (viraemia)
in sick and healthy cats and frequencies of FeLV viral RNA loads, classified as high and low loads
(>or ≤106copies/PCR reaction)
, were analysed using the chi-square test (p
Chi
) or Fisher’s exact test
(p
F
). Visualization of the data was performed by geocoding the data of the cat owners using RStudio
(version 1.1.383, RStudio Team, 2016, Integrated Development for R. RStudio, Inc., Boston, MA).
The following packages were used: ggmap [39], cartography [40], and spatialEco [41].
2.6.2. Regression Analysis
The relationships between firstly the percentage of FeLV prevalence and GDP per capita at PPP
and secondly the percentage of FeLV vaccination rate and GDP per capita at PPP were first evaluated
using the Pearson correlation coefficient (parametric hypothesis concerning the relation) as well as
the Spearman correlation coefficient (non-parametric hypothesis concerning the relation). The GDP
per capita converted to US dollars using PPP was assigned to each country/country group using the
International Monetary Fund World Economic Outlook (October 2018) [
27
]. For significant correlation,
regression was applied to predict the relationship between the variables investigated with a 95%
confidence interval. In addition, the normality of the distribution of studentized residuals was tested
in order to validate the regression model.
To identify possible risk and protective factors associated with FeLV infection, the responses to the
questionnaire were encoded and merged with the RT-qPCR results (classified as positive or negative)
of each cat. First, a univariate analysis was conducted and odds ratios (OR) with 95% confidence
intervals (95% CI) were attributed to each variable. Next, a multivariate logistic regression analysis
was performed using variables with p-values <0.20. In some cases, the use of the Firth logit method
allowed an inference of ORs and CIs when complete separation (zero cells) occurred [
42
]. Backward
stepwise logistic regression was used to exclude progressively variables having the highest p-value [
43
].
In that final model, all pairwise interactions between variables, if biologically relevant, were examined
for significance. The goodness of fit was assessed through a multivariate logistic regression using the
Hosmer–Lemeshow goodness-of-fit test [
43
]. Statistical analyses were carried out in STATA/SE 14.2
(StataCorp, College Station, TX, USA). The limit of statistical significance of the tests performed was
defined as 0.05.
Viruses 2019,11, 993 7 of 27
2.6.3. Classification Tree Analysis
Classification and regression tree (CART) analysis [
44
,
45
] had been evaluated previously in
medical science [
46
,
47
]. Further details and explanations of this CART analysis are available in
previous veterinary articles, including a prevalence study of feline retroviruses [
48
–
52
]. Classification
trees are trained by passing data down from a root node to leaves. The data is repeatedly split according
to predictor variables so that child nodes are more “pure” (i.e., homogeneous) in terms of the outcome
variable (i.e., FeLV status). In this study, a classification tree analysis was based on the subdivision
of the data set into randomly selected and approximately equal parts, with each “part” containing
a similar distribution of data from the populations of interest (i.e., positive versus negative results for
FeLV). Then, the analysis used the first nine parts of the data, constructing the largest possible tree,
and used the remaining 1/10th of the data to obtain initial estimates of the error rate of the selected
subtree. The process was repeated, using different combinations of the nine remaining data subsets
and a different 1/10th data subset to test the resulting tree. This process was repeated until each 1/10th
data subset had been used to test a tree that had been grown using 9/10ths of the data. Then, the results
of the 10 mini-tests were combined to calculate the error rates for trees of each possible size; these error
rates were applied to prune the tree that had been grown using the entire data set. In order to test the
diagnostic power of the final decision tree generated, a receiver operating characteristic (ROC) curve
was used for both the data set that was used to build the tree (learning data set) and the data set that
was used to test the adequacy of the tree to the data (testing data set). This complex process resulted in
a set of fairly reliable estimates of the independent predictive accuracy of the tree [
53
]. Classification
tree analysis was carried out in a Salford Predictive Modeler (Salford Systems, San Diego, CA, USA).
3. Results
3.1. Pre-Experiment
When testing the stability of FeLV in RNA shield buffer at room temperature for up to 60 days
and at 37
◦
C for up to 14 days, no significant loss of FeLV viral RNA was observed (<10-fold decrease).
3.2. Pan-European Prevalence Study
3.2.1. Sample Size and Return Rate
With the support of the country representatives, 861 veterinary facilities were enrolled, 93.6% of the
initially intended total of 920 facilities (Table 1). Subsequently, 6720 samples (78.0% of the 8610 shipped
tubes) were returned to the laboratory; these samples originated from 30 of the 32 originally included
countries (Table 1; Figure 1a).
In Greece, the recruitment of veterinarians was unsuccessful, while in Romania samples were
collected, but owing to problems with shipping none were received by the laboratory in Switzerland.
Data were provided for 6005 of the 6720 returned samples using the online questionnaire (69.7%).
Subsequently, these 6005 samples accompanied with a complete data set were included in the
further analysis.
Viruses 2019,11, 993 8 of 27
Viruses 2019, 11, x FOR PEER REVIEW 7 of 27
Classification and regression tree (CART) analysis [44,45] had been evaluated previously in
medical science [46,47]. Further details and explanations of this CART analysis are available in
previous veterinary articles, including a prevalence study of feline retroviruses [48–52]. Classification
trees are trained by passing data down from a root node to leaves. The data is repeatedly split
according to predictor variables so that child nodes are more “pure” (i.e., homogeneous) in terms of
the outcome variable (i.e., FeLV status). In this study, a classification tree analysis was based on the
subdivision of the data set into randomly selected and approximately equal parts, with each “part”
containing a similar distribution of data from the populations of interest (i.e., positive versus negative
results for FeLV). Then, the analysis used the first nine parts of the data, constructing the largest
possible tree, and used the remaining 1/10
th
of the data to obtain initial estimates of the error rate of
the selected subtree. The process was repeated, using different combinations of the nine remaining
data subsets and a different 1/10
th
data subset to test the resulting tree. This process was repeated
until each 1/10
th
data subset had been used to test a tree that had been grown using 9/10ths of the
data. Then, the results of the 10 mini-tests were combined to calculate the error rates for trees of each
possible size; these error rates were applied to prune the tree that had been grown using the entire
data set. In order to test the diagnostic power of the final decision tree generated, a receiver operating
characteristic (ROC) curve was used for both the data set that was used to build the tree (learning
data set) and the data set that was used to test the adequacy of the tree to the data (testing data set).
This complex process resulted in a set of fairly reliable estimates of the independent predictive
accuracy of the tree [53]. Classification tree analysis was carried out in a Salford Predictive Modeler
(Salford Systems, San Diego, CA, USA).
3. Results
3.1. Pre-Experiment
When testing the stability of FeLV in RNA shield buffer at room temperature for up to 60 days
and at 37 °C for up to 14 days, no significant loss of FeLV viral RNA was observed (<10-fold decrease).
3.2. Pan-European Prevalence Study
3.2.1. Sample Size and Return Rate
Figure 1.
Origin and FeLV infection and vaccination status of the cats visiting veterinarians in the 30
European countries. (
a
) FeLV viraemia: black: all cats; red: FeLV-positive cats. (
b
) FeLV vaccination
status: black: all cats; blue: FeLV vaccinated cats. The size of the circle represents the number of cats.
3.2.2. Sample Characteristics
Approximately two-thirds of the 6005 cats were clinically healthy (n=4060, 67.6%); 1945 cats
were sick at the time of veterinary consultation (32.4%; Table 2). The age distribution of the 6005 cats
is shown in Figure 2. Overall, the sex and reproductive status was known for 5886 cats (Table 2);
in 119 cats
, either the sex or the reproductive status was listed as “not sure”. Approximately two-thirds
of the cats had been spayed or castrated (68.6%); approximately one-third was sexually intact (29.4%).
Table 2. Sample characteristics (all cats and FeLV-viraemic cats).
Variables Modalities All Cats (n=6005) FeLV-Positive Cats (n=141)
Health Healthy 4060 (67.6 1) 66 (46.8 2)
Sick 1945 (32.4) 75 (53.2)
Age <1 year 1826 (30.4) 31 (22.0)
1 to ≤6 years 2271 (37.8) 80 (56.7)
>6 years 1908 (31.8) 30 (21.3)
Sex Female intact 850 (14.2) 15 (10.6)
Female spayed 1941 (32.3) 38 (27.0)
Male intact 914 (15.2) 37 (26.2)
Male castrated 2181 (36.3) 49 (34.8)
Not sure 119 (2.0) 2 (1.4)
Pedigree No 5156 (85.9) 139 (98.6)
Yes 792 (13.2) 2 (1.4)
Not sure 57 (0.9) 0 (0.0)
Habitat Private 5151 (85.9) 111 (78.7)
Breeder 177 (2.9) 0 (0.0)
Shelter 179 (3.0) 7 (5.0)
Rescue cat 446 (7.4) 21 (14.9)
Other 52 (0.9) 2 (1.4)
Multicat environment Yes 3373 (56.2) 85 (60.3)
No 2380 (39.6) 44 (31.2)
Not sure 252 (4.2) 12 (8.5)
Number of cats in group 1 2380 (39.6) 44 (31.2)
2 1670 (27.8) 32 (22.7)
3 628 (10.5) 11 (7.8)
4 295 (4.9) 6 (4.3)
≥5 708 (11.8) 36 (25.5)
Not sure 324 (5.4) 12 (8.5)
Viruses 2019,11, 993 9 of 27
Table 2. Cont.
Variables Modalities All Cats (n=6005) FeLV-Positive Cats (n=141)
Access Indoor only 2193 (36.5) 32 (22.7)
Indoor and outdoor 3245 (54.0) 83 (58.9)
Outdoor only 388 (6.5) 23 (16.3)
FeLV vaccination Yes 1462 (24.3) 14 (9.9)
No 3938 (65.6) 106 (75.2)
Not sure 605 (10.1) 21 (14.9)
Last FeLV vaccination Never 3938 (65.6) 106 (75.2)
<1 year 943 (15.7) 8 (5.7)
1 to ≤3 years 337 (5.6) 3 (2.1)
>3 years 182 (3.0) 3 (2.1)
Not sure 605 (10.1) 21 (14.9)
1Number in brackets gives the percentage of the number of all cats. 2Number in brackets gives the percentage of
the number of FeLV-positive cats.
Viruses 2019, 11, x FOR PEER REVIEW 9 of 27
>3 years 182 (3.0) 3 (2.1)
Not sure 605 (10.1) 21 (14.9)
1
Number in brackets gives the percentage of the number of all cats.
2
Number in brackets gives the
percentage of the number of FeLV-positive cats.
Figure 2. Age distribution of the cats: gray: FeLV-negative cats (left axis), black: FeLV-positive cats
(right axis). Cats aged from one to six years were significantly more frequently FeLV-positive than
younger or older cats (p
F
< 0.0001; see also Table 3).
Pedigree status was known for 5948 cats; most were non-pedigree cats (n = 5156; 85.9%). Of the
792 pedigree cats, 118 were British shorthairs, 110 were Persians, 99 were Maine Coons, 51 were
Ragdolls, 42 were Siamese, 35 were Norwegian Forest cats, and 30 were Birman cats; from the
remaining breeds, fewer than 30 cats had been included. The large majority of the cats were kept in
private homes (n = 5151; 85.9%); much less frequently sampled were rescue cats and cats from
breeders or animal shelters (Table 2). The information concerning group housing or being a single cat
was known for 5753 cats, of which 3373 cats lived in multicat environments (56.2%), and 2380 were
kept as single cats (39.6%; Table 2). Approximately half of the cats in multicat environments (n = 1670;
49.5%) had only one companion cat. Most cats had outdoor access (n = 3633; 60.5%; Table 2); these
cats could be further divided into cats that lived indoors and had outdoor access (n = 3245; 54.0%)
and cats that always lived outdoors (n = 388 cats; 6.5%). Finally, approximately one-third of all cats
lived strictly indoors (n = 2193; 36.5%). Information concerning FeLV vaccination status and the
timepoint of the most recent FeLV vaccination was available for 5400 cats (Table 2 and Figure 1b).
3.2.3. Prevalence of FeLV Viremia
Of the 6005 samples accompanied with data from the questionnaire, 141 samples tested FeLV-
positive by real-time RT-qPCR (2.3%; 95% CI: 2.0–2.8; Table 1) from saliva. The individual samples
that tested RT-qPCR positive had been identified from the sample pools that had tested positive. At
least one individual sample tested FeLV RT-qPCR positive in each set of samples that had contributed
to a positive pool. The countries/country groups with the highest prevalence of FeLV viremia were
Portugal (8.8%; 95% CI: 6.2–12.3%), Hungary (5.9%; 95% CI: 3.5–9.9%), and Italy and Malta (5.7%;
95% CI: 3.7–8.7%; Table 1 and Figure 3). None of the tested samples was FeLV-positive in the
following countries: Denmark, Finland, Norway, Sweden, Latvia, Estonia, Scotland, Wales, Bulgaria,
Malta, Luxembourg, and the Netherlands. Moreover, only one FeLV-positive sample was obtained
from Germany and England (Table 1 and Figure 3).
The prevalence of FeLV viremia in sick cats was 3.9% (75 of 1945 cats; 95% CI: 3.0–4.8%) and in
healthy cats 1.6% (66 of 4060 cats; 95% CI: 1.3–2.1%; Tables 2 and 3). Sick cats were significantly more
frequently FeLV-positive than healthy cats (p
F
< 0.0001; OR 2.4, 95% CI: 1.7–3.4%; Table 3). When the
Figure 2.
Age distribution of the cats: grey: FeLV-negative cats (left axis), black: FeLV-positive cats
(right axis). Cats aged from one to six years were significantly more frequently FeLV-positive than
younger or older cats (pF<0.0001; see also Table 3).
Table 3.
Risk and protective factors associated with positive FeLV viraemic status of cats after univariate
logistic regression.
Variables Modalities FeLV
Prevalence
95%
Confidence
Interval aOdds Ratio
95%
Confidence
Interval bp-value
Europe Eastern 3.4 2.5–4.6 Reference - -
Northern 0.7 0.4–1.7 0.21 (0.11–0.40) <0.001 *
Southern 5.5 4.2–6.9 1.64 (1.11–2.42) 0.013 *
Western 1.0 0.6–1.6 0.29 (0.17–0.51) <0.001 *
Pedigree No 2.7 2.2–3.1 Reference - -
Yes 0.3 0.0–1.0 0.09 (0.02–0.37) 0.001 *
Habitat Breeder 0.0 0.0–2.1 Reference - -
Rescue cat 4.7 2.8–6.8 17.94 (1.08–297.85) 0.044 *
Private 2.2 1.7–2.5 17.57 (0.82–372.10) 0.07
Shelter 3.9 1.5–7.6 7.85 (0.49–126.89) 0.15
Other 3.8 5.0–13.2 15.43 (0.87–272.42) 0.06
Viruses 2019,11, 993 10 of 27
Table 3. Cont.
Variables Modalities FeLV
Prevalence
95%
Confidence
Interval aOdds Ratio
95%
Confidence
Interval bp-value
Number of
cats in group
1 2.1 1.6–2.7 Reference - -
2 1.9 1.3–2.7 0.92 (0.60–1.43) 0.72
3 1.8 0.9–3.1 0.84 (0.44–1.62) 0.61
4 2.0 0.7–4.4 0.98 (0.42–2.30) 0.97
≥5 5.1 3.6–7.0 2.53 (1.65–3.88) <0.001 *
Sex Female
intact 1.8 1.0–2.9 Reference - -
Female
spayed 2.0 1.4–2.7 1.11 (0.61–2.03) 0.73
Male intact 4.0 2.9–5.5 2.35 (1.28–4.31) 0.006 *
Male
castrated 2.2 1.7–3.0 1.28 (0.71–2.29) 0.41
Not sure 1.7 0.2–5.9 0.95 (0.21–4.21) 0.95
Age <1 year 1.7 1.2–2.4 Reference - -
1 to ≤6 years 3.5 2.8–4.4 2.11 (1.39–3.22) <0.001 *
>6 years 1.6 1.1–2.2 0.92 (0.56–1.53) 0.77
Access Indoor only 1.5 1.0–2.1 Reference - -
In- and
outdoor 2.6 2.0–3.1 1.77 (1.17–2.67) 0.006 *
Outdoor
only 5.9 3.8–8.8 4.26 (2.46–7.35) 0.001 *
Not sure 1.7 0.3–4.8 1.15 (0.35–3.80) 0.82
Last FeLV
vaccination Never 2.7 2.2–3.2 Reference - -
<1 year 0.8 0.4–1.7 0.31 (0.15–0.64) 0.001 *
1 to ≤3 years 0.9 0.2–2.6 0.32 (0.10–1.03) 0.056
>3 years 1.6 0.3–4.7 0.61 (0.19–1.93) 0.40
Not sure 3.5 2.2–5.3 1.30 (0.81–2.09) 0.28
Health Healthy 1.6 1.3–2.1 Reference - -
Sick 3.9 3.0–4.8 2.43 (1.74–3.39) <0.001 *
aConfidence interval for the mean; bconfidence interval for the odds ratio; * p-value <0.05.
Pedigree status was known for 5948 cats; most were non-pedigree cats (n=5156; 85.9%). Of the
792 pedigree cats
, 118 were British shorthairs, 110 were Persians, 99 were Maine Coons, 51 were
Ragdolls, 42 were Siamese, 35 were Norwegian Forest cats, and 30 were Birman cats; from the
remaining breeds, fewer than 30 cats had been included. The large majority of the cats were kept in
private homes (n=5151; 85.9%); much less frequently sampled were rescue cats and cats from breeders
or animal shelters (Table 2). The information concerning group housing or being a single cat was
known for 5753 cats, of which 3373 cats lived in multicat environments (56.2%), and 2380 were kept as
single cats (39.6%; Table 2). Approximately half of the cats in multicat environments (n=1670; 49.5%)
had only one companion cat. Most cats had outdoor access (n=3633; 60.5%; Table 2); these cats could
be further divided into cats that lived indoors and had outdoor access (n=3245; 54.0%) and cats that
always lived outdoors (n=388 cats; 6.5%). Finally, approximately one-third of all cats lived strictly
indoors (n=2193; 36.5%). Information concerning FeLV vaccination status and the timepoint of the
most recent FeLV vaccination was available for 5400 cats (Table 2and Figure 1b).
3.2.3. Prevalence of FeLV Viraemia
Of the 6005 samples accompanied with data from the questionnaire, 141 samples tested
FeLV-positive by real-time RT-qPCR (2.3%; 95% CI: 2.0–2.8; Table 1) from saliva. The individual
samples that tested RT-qPCR positive had been identified from the sample pools that had tested
Viruses 2019,11, 993 11 of 27
positive. At least one individual sample tested FeLV RT-qPCR positive in each set of samples that
had contributed to a positive pool. The countries/country groups with the highest prevalence of FeLV
viraemia were Portugal (8.8%; 95% CI: 6.2–12.3%), Hungary (5.9%; 95% CI: 3.5–9.9%), and Italy and
Malta (5.7%; 95% CI: 3.7–8.7%; Table 1and Figure 3). None of the tested samples was FeLV-positive
in the following countries: Denmark, Finland, Norway, Sweden, Latvia, Estonia, Scotland, Wales,
Bulgaria, Malta, Luxembourg, and the Netherlands. Moreover, only one FeLV-positive sample was
obtained from Germany and England (Table 1and Figure 3).
Viruses 2019, 11, x FOR PEER REVIEW 11 of 27
Figure 3. FeLV prevalence (on the left) and FeLV vaccination rates (on the right) in the different
countries/country groups. In the FeLV vaccination rates, cats with unknown vaccination status were
not included (~10% of the cats).
3.2.4. FeLV Vaccination Status
For 5400 cats, the veterinarians provided information concerning FeLV vaccination status.
Amongst these cats, 3938 had never been vaccinated against FeLV (65.6% of all cats); 1462 cats were
known to have been vaccinated at least once (24.3% of all cats; Table 2 and Figure 1b). Young cats (<1
year of age) were less frequently vaccinated (18.7%) compared to older cats (1 to ≤6 years: 25.1%; >6
years: 28.8; p
Chi
< 0.0001). Among the 141 FeLV-positive cats, the vaccination status was known for
120 cats; amongst those cats, 14 (11.7%) had been vaccinated at some timepoint. FeLV-positive cats
had been vaccinated significantly less frequently against FeLV compared to FeLV-negative cats (p
F
<
0.0001). Vaccination rates varied considerably, ranging from 3.2% and 3.4% in Finland and Sweden,
respectively, to 67.6% and 81.5% in Switzerland and Liechtenstein and the UK, respectively (Figure
3). The vaccination rates in the three countries/country groups with the highest FeLV prevalence,
namely Portugal, Hungary, and Italy and Malta, were rather low, at 14.2%, 26.9%, and 17.8%,
respectively (Figure 3).
3.2.5. Regression Analysis
First, a univariate regression analysis was performed (Table 3). Then, all variables with p-values
< 0.20 in the univariate analysis were entered in a multivariate analysis. A backward, stepwise
strategy was used to obtain a final multivariate logistic regression analysis model (Table 4). Following
the multivariate analysis, seven risk factors (origin of cats from Southern Europe, male intact, one to
six years of age, indoor and outdoor or outdoor-only living, living in a group of ≥5 cats, sick cats),
and three protective factors (origin from Northern Europe, origin from Western Europe, and pedigree
cats) were identified (p < 0.05). There was a tendency that cats vaccinated against FeLV during the
year prior to sampling were less frequently infected (8/943) compared to cats that had never been
Figure 3.
FeLV prevalence (on the left) and FeLV vaccination rates (on the right) in the different
countries/country groups. In the FeLV vaccination rates, cats with unknown vaccination status were
not included (~10% of the cats).
The prevalence of FeLV viraemia in sick cats was 3.9% (75 of 1945 cats; 95% CI: 3.0–4.8%) and
in healthy cats 1.6% (66 of 4060 cats; 95% CI: 1.3–2.1%; Tables 2and 3). Sick cats were significantly
more frequently FeLV-positive than healthy cats (p
F
<0.0001; OR 2.4, 95% CI: 1.7–3.4%; Table 3). When
the age distribution of the FeLV-positive and FeLV-negative cats was inspected graphically (Figure 2),
it was evident that cats aged one to six years were more frequently FeLV-positive than cats of other
ages. Thus, cats were categorized accordingly for further statistical analysis: cats less than one year of
age; cats between one and six years of age; and cats older than six years.
3.2.4. FeLV Vaccination Status
For 5400 cats, the veterinarians provided information concerning FeLV vaccination status. Amongst
these cats, 3938 had never been vaccinated against FeLV (65.6% of all cats); 1462 cats were known to
have been vaccinated at least once (24.3% of all cats; Table 2and Figure 1b). Young cats (<1 year of age)
were less frequently vaccinated (18.7%) compared to older cats (1 to
≤
6 years: 25.1%;
>6 years
: 28.8;
pChi <0.0001
). Among the 141 FeLV-positive cats, the vaccination status was known for 120 cats; amongst
those cats, 14 (11.7%) had been vaccinated at some timepoint. FeLV-positive cats had been vaccinated
significantly less frequently against FeLV compared to FeLV-negative cats (p
F
<0.0001). Vaccination rates
Viruses 2019,11, 993 12 of 27
varied considerably, ranging from 3.2% and 3.4% in Finland and Sweden, respectively, to 67.6% and
81.5% in Switzerland and Liechtenstein and the UK, respectively (Figure 3). The vaccination rates in the
three countries/country groups with the highest FeLV prevalence, namely Portugal, Hungary, and Italy
and Malta, were rather low, at 14.2%, 26.9%, and 17.8%, respectively (Figure 3).
3.2.5. Regression Analysis
First, a univariate regression analysis was performed (Table 3). Then, all variables with
p-values <0.20
in the univariate analysis were entered in a multivariate analysis. A backward, stepwise strategy was
used to obtain a final multivariate logistic regression analysis model (Table 4). Following the multivariate
analysis, seven risk factors (origin of cats from Southern Europe, male intact, one to six years of age,
indoor and outdoor or outdoor-only living, living in a group of
≥
5 cats, sick cats), and three protective
factors (origin from Northern Europe, origin from Western Europe, and pedigree cats) were identified
(
p<0.05
). There was a tendency that cats vaccinated against FeLV during the year prior to sampling
were less frequently infected (8/943) compared to cats that had never been vaccinated (106/3938; p=0.06;
Tables 2and 4). The Hosmer–Lemeshow goodness-of-fit test (chi-square (8 degrees of freedom) =10.85;
p-value =0.21) indicated that the final model fit the data well.
Table 4.
Risk and protective factors associated with the feline leukaemia virus (FeLV) viraemia of cats
after multivariate analysis.
Variables Modalities FeLV
Prevalence
95%
Confidence
Interval aOdds Ratio
95%
Confidence
Interval bp-value
Europe Eastern 3.4 2.5–4.6 Reference - -
Northern 0.7 0.4–1.7 0.29 (0.15–0.56) <0.001 *
Southern 5.5 4.2–6.9 1.81 (1.20–2.72) 0.005 *
Western 1.0 0.6–1.6 0.42 (0.23–0.74) 0.003 *
Pedigree No 2.7 2.2–3.1 Reference - -
Yes 0.3 0.0–1.0 0.15 (0.04–0.60) 0.008 *
Number of cats in
group 1 2.1 1.6–2.7 Reference -
2 1.9 1.3–2.7 0.96 (0.62–1.51) 0.87
3 1.8 0.9–3.1 0.79 (0.41–1.54) 0.49
4 2.0 0.7–4.4 0.90 (0.39–2.15) 0.82
≥5 5.1 3.6–7.0 1.63 (1.03–2.58) 0.040 *
Sex Female intact 1.8 1.0–2.9 Reference - -
Female spayed 2.0 1.4–2.7 1.38 (0.72–2.64) 0.33
Male intact 4.0 2.9–5.5 2.24 (1.20–4.18) 0.01 *
Male castrated 2.2 1.7–3.0 1.48 (0.79–2.78) 0.23
Not sure 1.7 0.2–5.9 0.97 (0.21–4.53) 0.84
Age category <1 year 1.7 1.2–2.4 Reference - -
1 to ≤6 years 3.5 2.8–4.4 2.04 (1.27–3.28) 0.003 *
>6 years 1.6 1.1–2.2 1.01 (0.56–1.83) 0.97
Access Indoor only 1.5 1.0–2.1 Reference - -
In- and outdoor 2.6 2.0–3.1 1.72 (1.12–2.65) 0.01 *
Outdoor only 5.9 3.8–8.8 1.88 (1.03–3.44) 0.04 *
Not sure 1.7 0.3–4.8 1.07 (0.31–3.69) 0.92
Last FeLV vaccination Never 2.7 2.2–3.2 Reference - -
<1 year 0.8 0.4–1.7 0.49 (0.23–1.03) 0.06
1 to ≤3 years 0.9 0.2–2.6 0.39 (0.12–1.26) 0.11
>3 years 1.6 0.3–4.7 0.79 (0.24–2.62) 0.70
Not sure 3.5 2.2–5.3 1.40 (0.85–2.33) 0.19
Health Healthy 1.6 1.3–2.1 Reference - -
Sick 3.9 3.0–4.8 2.04 (1.41–2.90) <0.001 *
aConfidence interval for the mean; bconfidence interval for the odds ratio; * p-value <0.05.
When testing the relationship between the GDP per capita at PPP and the percentage of FeLV
prevalence, neither parametric nor non-parametric significant correlations were found between GDP and
FeLV prevalence (Pearson correlation coefficient =–0.29 with p=0.18 and Spearman rank correlation
Viruses 2019,11, 993 13 of 27
coefficient =–0.37 with p=0.09) (Figure 4a). However, there was a significant linear correlation between
GDP at capita PPP and the FeLV vaccination rate (p=0.045; Figure 4b). The normality of the distribution
of studentized residuals was acceptable; see Figure 4c.
Viruses 2019, 11, x FOR PEER REVIEW 13 of 27
Figure 4. Relation between FeLV prevalence or FeLV vaccination rate and the gross domestic product
(GDP) per capita purchasing power parity (PPP). (a) Relation between FeLV prevalence and GDP per
capita using PPP in US dollars (USD); (b) Linear relation between FeLV vaccination rate and GDP per
capita using PPP in USD (with the black points, the observed values; the line, the linear relation
between the FeLV vaccination rate and the GDP per capita PPP and its 95% confidence interval, with
the following equation: FeLV_vacc = –0.9698946 + (0.6052006 * GDP); (c) Density of the studentized
residuals with the kernel density estimate and the normal density as reference. Kernel =
epanechnikov, bandwidth = 0.4433.
Figure 4.
Relation between FeLV prevalence or FeLV vaccination rate and the gross domestic product
(GDP) per capita purchasing power parity (PPP). (a) Relation between FeLV prevalence and GDP per
capita using PPP in US dollars (USD); (
b
) Linear relation between FeLV vaccination rate and GDP
per capita using PPP in USD (with the black points, the observed values; the line, the linear relation
between the FeLV vaccination rate and the GDP per capita PPP and its 95% confidence interval, with
the following equation: FeLV_vacc =
−
0.9698946 +(0.6052006 * GDP); (
c
) Density of the studentized
residuals with the kernel density estimate and the normal density as reference. Kernel =epanechnikov,
bandwidth =0.4433.
3.2.6. Classification Tree Analysis
According to the classification tree analysis (Figure 5), the origin of cats within Europe
(discriminatory power (DP) of 100, with a scale between 0 and 100), having a pedigree (
DP =55.60
),
and living outdoors only (DP =29.08) were the three important predictors (or splitters) regarding a cat’s
FeLV status, with a relatively good tree sensitivity and specificity: 82.98% (95% CI: 75.74–88.78) and
61.89% (95% CI: 60.63–63.13), respectively. The areas under the ROC curve for the learning data and
the test data set were 0.73 and 0.69, respectively. These values indicate the potential of the proposed
tree to discriminate between the diagnoses (FeLV-negative versus FeLV-positive).
Viruses 2019,11, 993 14 of 27
Viruses 2019, 11, x FOR PEER REVIEW 13 of 27
Figure 4. Relation between FeLV prevalence or FeLV vaccination rate and the gross domestic product
(GDP) per capita purchasing power parity (PPP). (a) Relation between FeLV prevalence and GDP per
capita using PPP in US dollars (USD); (b) Linear relation between FeLV vaccination rate and GDP per
capita using PPP in USD (with the black points, the observed values; the line, the linear relation
between the FeLV vaccination rate and the GDP per capita PPP and its 95% confidence interval, with
the following equation: FeLV_vacc = –0.9698946 + (0.6052006 * GDP); (c) Density of the studentized
residuals with the kernel density estimate and the normal density as reference. Kernel =
epanechnikov, bandwidth = 0.4433.
Figure 5.
Classification tree analysis for FeLV viraemia. Legend: Class: 0 =negative for FeLV;
1=positive
for FeLV. EU =Europe: N, North; W, West; E, East; S, South. Access: A, Indoor only; B,
Indoor and outdoor; C, Outdoor only; D, Not sure. Pedigree: Y, Yes; N, No.
3.2.7. FeLV Viral RNA Loads in Saliva
Viral load determination in saliva samples was semiquantitative, since the volume of collected
saliva and the biological dilution of the saliva were unknown. Moreover, each sample had been stored
in 300
µ
L of RNA shield that stabilized, but also further diluted, the saliva sample. Nevertheless,
by employing quantitative FeLV real-time RT-qPCR to determine FeLV viral RNA loads in the available
samples, between three copies and 6.9
×
10
7
copies per PCR reaction were detected using 5
µ
L of
input TNA. Approximately two-thirds (63%) of the samples contained high FeLV viral RNA loads
with >10
6
copies per PCR reaction (Figure 6). Samples from sick FeLV-infected cats contained more
frequently high FeLV viral RNA loads (>10
6
copies per PCR reaction; 59 of 75 cats; 79%) than samples
from healthy FeLV-infected cats (30 of 66 cats; 45%; p
F
<0.0001; OR 4.4, 95% CI: 2.1–9.2). However,
healthy FeLV-positive cats also shed high viral RNA copy numbers in their saliva; see Figure 6.
Viruses 2019, 11, x FOR PEER REVIEW 14 of 27
Figure 5. Classification tree analysis for FeLV viremia. Legend: Class: 0 = negative for FeLV; 1 =
positive for FeLV. EU = Europe: N, North; W, West; E, East; S, South. Access: A, Indoor only; B, Indoor
and outdoor; C, Outdoor only; D, Not sure. Pedigree: Y, Yes; N, No.
3.2.6. Classification Tree Analysis
According to the classification tree analysis (Figure 5), the origin of cats within Europe
(discriminatory power (DP) of 100, with a scale between 0 and 100), having a pedigree (DP = 55.60),
and living outdoors only (DP = 29.08) were the three important predictors (or splitters) regarding a
cat’s FeLV status, with a relatively good tree sensitivity and specificity: 82.98% (95% CI: 75.74–88.78)
and 61.89% (95% CI: 60.63–63.13), respectively. The areas under the ROC curve for the learning data
and the test data set were 0.73 and 0.69, respectively. These values indicate the potential of the
proposed tree to discriminate between the diagnoses (FeLV-negative versus FeLV-positive).
3.2.7. FeLV Viral RNA Loads in Saliva
Viral load determination in saliva samples was semiquantitative, since the volume of collected
saliva and the biological dilution of the saliva were unknown. Moreover, each sample had been
stored in 300 µL of RNA shield that stabilized, but also further diluted, the saliva sample.
Nevertheless, by employing quantitative FeLV real-time RT-qPCR to determine FeLV viral RNA
loads in the available samples, between three copies and 6.9 × 10
7
copies per PCR reaction were
detected using 5 µL of input TNA. Approximately two-thirds (63%) of the samples contained high
FeLV viral RNA loads with >10
6
copies per PCR reaction (Figure 6). Samples from sick FeLV-infected
cats contained more frequently high FeLV viral RNA loads (>10
6
copies per PCR reaction; 59 of 75
cats; 79%) than samples from healthy FeLV-infected cats (30 of 66 cats; 45%; p
F
< 0.0001; OR 4.4, 95%
CI: 2.1–9.2). However, healthy FeLV-positive cats also shed high viral RNA copy numbers in their
saliva; see Figure 6.
Figure 6. Distribution of salivary FeLV viral RNA loads in FeLV-positive healthy and sick cats. Loads
are given as copies per PCR reaction conducted with 5 µL of total nucleic acid extracted from 100 µL
of liquid from saliva swab samples.
3.2.8. Clinical Signs Associated with FeLV Infection
Figure 6.
Distribution of salivary FeLV viral RNA loads in FeLV-positive healthy and sick cats. Loads
are given as copies per PCR reaction conducted with 5
µ
L of total nucleic acid extracted from 100
µ
L of
liquid from saliva swab samples.
Viruses 2019,11, 993 15 of 27
3.2.8. Clinical Signs Associated with FeLV Infection
The major clinical problems (inclusion optional in the questionnaire) that were reported in the
75 FeLV-positive sick cats included upper respiratory tract diseases (URTD: “cat flu”, nasal or ocular
discharge, conjunctivitis, sneezing; n=11), gingivitis and/or stomatitis (n=11), anaemia (n=8),
anorexia (n=6), tumour (lymphoma, rhabdomyosarcoma, liver tumour; n=4), diarrhoea (n=3), and
abscess (n=3). Less frequently reported were fever, apathy, renal failure, jaundice, and various other
clinical signs. FeLV-positive sick cats more frequently displayed anaemia (OR 9.6, 95% CI: 4.4–21.4;
pF<0.0001
), anorexia (OR 3.5, 95% CI: 1.6–8.3; p
F
=0.0122) and gingivitis and/or stomatitis (OR 2.5,
95% CI: 1.3–4.8; pF=0.0152) than FeLV-negative sick cats (Table 5).
Table 5. Major clinical problems reported in FeLV viraemic sick cats in comparison with all sick cats.
Clinical Problem FeLV-Negative
Sick Cats (%)
FeLV-Positive Sick
Cats (%)
Odds Ratio (95%
Confidence Interval) pF*
Anaemia
Yes
No
23 (1.2)
1847 (98.8)
8 (10.7)
67 (89.3) 9.6 (4.4–21.4) <0.0001
Anorexia
Yes
No
45 (2.4)
1825 (97.6)
6 (8.0)
69 (92.0) 3.5 (1.6–8.3) 0.0122
Gingivitis and/or
stomatitis
Yes
No
121 (6.5)
1749 (93.5)
11 (14.7)
64 (85.3) 2.5 (1.3–4.8) 0.0152
*pF:p-value Fisher’s exact test.
4. Discussion
The aim of the present study was to evaluate the prevalence and significance of FeLV infection
in cats taken to veterinarians in Europe. The investigation was conducted prospectively and at
a pan-European level; it is the first study of its kind. The study aimed to include 32 countries, of which
30 successfully contributed samples. To the best of the authors’ knowledge, this study provides the
first FeLV prevalence data for some European countries, i.e., Latvia, Lithuania, Bulgaria, Hungary,
and Croatia, and current data for some countries without recent information on FeLV prevalence, i.e.,
France, Finland, Czech Republic, and the Netherlands. By using saliva swabs rather than an invasive
method such as blood collection, samples could be collected from all cats visiting the veterinarians,
thereby reducing the bias of selecting only cats that were easy to handle or that were presented because
of illness and required blood collection for routine diagnostics. Saliva samples collected using buccal
swabs were analysed for FeLV viral RNA using RT-qPCR. There is an almost perfect agreement between
the shedding of FeLV viral RNA in saliva and the presence of viraemia, as reported previously [
28
,
35
];
therefore, the results obtained were comparable with those of other studies measuring free FeLV
p27 antigen in blood samples. FeLV antigenemia is in most—but not all—cats a measure for FeLV
viraemia [
54
]. Discrepancies can be found, particularly in the early stages of FeLV infection and in cats
with a focal FeLV infection [
55
–
57
]. The saliva swabs had to be transported in an RNA shield buffer to
ensure biological safety during shipment, since rabies is encountered in some of the countries included
in the study [30,31]. At the same time, the buffer increased the stability of the FeLV viral RNA.
The overall prevalence of FeLV viraemia of 2.3% (95% CI: 2.0–2.8%) found in cats presented
to veterinary facilities in Europe in the present study is within the range of other similar reports,
where both healthy and sick cats from different environmental conditions have been tested [
10
,
15
,
23
].
In a large
prevalence study conducted in the United States and Canada in 2010, the overall prevalence
was somewhat higher, at 3.1% (95% CI: 3.0–3.3%), when testing for free FeLV p27 antigen [
13
]. However,
the two surveys are difficult to compare. In the latter investigation, the samples were preselected; they
had been collected by veterinarians and by staffat animal shelters with the intention of testing the cats
Viruses 2019,11, 993 16 of 27
for FeLV. In contrast, in the current study, all cats visiting European veterinary facilities were tested.
Notably, the prevalence reported in the present study still might not reflect the overall FeLV prevalence
in all cats within the countries included, although a number of stray and rescue cats had been tested
also in the present study. Many cats, in particular those at risk of FeLV infection, might never visit
a veterinary facility and thus would evade testing.
The prevalence of FeLV viraemia, as estimated by the present study, differed significantly in cats
living in different regions of Europe. The investigations revealed that cats in Northern and Western
Europe were at a lower risk of being FeLV-infected, while cats living in Southern European countries
were at a much greater risk. In some Northern European countries, FeLV was even undetectable, i.e.,
Denmark, Finland, Sweden, and Norway. Remarkably, FeLV is a reportable disease in Sweden, and
infections are registered by the Board of Agriculture (Jorbruksverket). FeLV is an unusual infection:
in Sweden, six and seven cases were reported in 2015 and 2016, respectively [
58
]. In Denmark, FeLV
prevalence was already low in 2010 in two out of three groups of stray cats and client-owned cats (0.8%;
0–0.9%), and a decrease of FeLV prevalence was reported in the third group; however, no detailed
information was provided on FeLV tests or testing criteria [
16
]. In Finland, FeLV prevalence was also
shown to be very low (1.0%) in 1990 in the Helsinki area in 196 stray cats [
59
]. Estonia, Latvia, and
Lithuania are also Northern European countries [
29
]. Of these Baltic countries FeLV-positive cats
were found only in Lithuania, the southernmost of these three countries (four positives of 143 cats
tested; 2.8%). In Estonia, there has been one report from 173 shelter cats tested in 2014/2015, in which
three cats tested positive for FeLV p27 antigen (1.7%) [
60
]; currently, there is one Estonian clinical
case under investigation that tested positive for FeLV antigen; FeLV infection was confirmed by both
virus isolation and provirus PCR (personal communication MJH and Olga Sjatkovskaja). The FeLV
prevalence in the UK was low in the present study, with just one viraemic cat amongst 119 returned
samples in England and no positives in Scotland and Wales (overall 0.7%); however, the return rate of
samples was the lowest in the present study for the UK and Ireland compared to the other countries.
An earlier study in the UK reported a high prevalence of FeLV in sick cats suspected of having FeLV or
feline immunodeficiency virus (FIV) infections (18%) as well as in young healthy cats (5%) [
24
], but two
more recent studies reported also a decreased FeLV prevalence in the UK: 3.5% of 517 stray cats in the
Birmingham area (1.4% of the healthy cats; 6.9% of the sick cats) in 1997 were FeLV-infected, and 2.3%
of the cats tested positive in two rehoming centres in the midlands and Eastern part of England in
2011/12 [61,62].
In some of the Northern European countries, where FeLV is an unusual infection or has not been
reported for many years, it was observed that veterinarians vaccinated only infrequently against FeLV:
the vaccination rate in Finland and Sweden was 3.2% and 3.4%, respectively (Figure 3). These low
vaccination rates seem reasonable, since the infection risk is low, as long as no new FeLV cases are
introduced. In contrast, 39.0% of the cats in Denmark were vaccinated against FeLV, which exceeds the
European average (27.1%; Figure 3). Considering the very low FeLV infection risk for cats in Denmark,
any vaccination strategy against FeLV might have to be reconsidered, taking into account the relative
risk-to-benefit ratio for any subcutaneous injection [
63
]. The vaccination rate was even higher in the
United Kingdom, where 81.5% of the cats in the present study had been FeLV vaccinated, and only
0.7% of the cats were FeLV-positive. Thus, veterinarians in the United Kingdom should be advised to
vaccinate more selectively against FeLV by identifying those cats that might currently, or in the future,
be at risk of FeLV infection.
A higher FeLV prevalence among the Northern European countries was found in Ireland (5.1%).
This was higher than the prevalence reported previously in a study investigating 112 client-owned and
stray cats in the Dublin area in 2007/2008 (1.8%) [
64
]. However, among the seven FeLV-positive cats in
Ireland, five cats had undergone appointments at only two veterinary facilities in Galway and Kerry
county, respectively. This suggests that there might be some geographic differences or regional FeLV
hotspots within Ireland; this might also be the case in other countries.
Viruses 2019,11, 993 17 of 27
Amongst the Western European countries, no FeLV-infected cats were identified in the Netherlands
in the current study: none of 356 cats tested FeLV RT-qPCR positive. In 1974, the prevalence of FeLV
in the Netherlands was 9%, but it decreased to 3% by 1986 [
11
]. This success was attributed to
the “test and removal program” that identified cats that carried FeLV and prevented their contact
with uninfected cats [
11
]. In the present study, only 8.1% of the tested cats were vaccinated against
FeLV in the Netherlands; this is in accordance with previous reports. Nonetheless, none of the cats
in the Netherlands tested FeLV-positive. This highlights the importance of testing for maintaining
an infection-free status, even in the absence of vaccination. A significant decrease in FeLV prevalence
had also been reported in Switzerland: in 1990, 13.0% of sick cats were FeLV-positive [
32
] and
a decrease to 3.1% in 2003 was demonstrated in a similar population of cats, using similar FeLV
detection methods [
15
]. However, thereafter in Switzerland, the decrease in the FeLV prevalence
stagnated; the most recent data from 2013–2016 reported a prevalence of 2.0% [
15
], which is similar to
the prevalence found for Switzerland and Liechtenstein in the current study (2.7%). It is not clear why
the FeLV prevalence did not decrease further in recent years; the FeLV vaccination rate reported in the
present study was good, with 67.6% in Switzerland and Liechtenstein. However, not all cats at risk
of FeLV infection appear to have been identified and FeLV vaccinated in time. In the other Western
European countries—Germany, France, Austria, Belgium, and Luxemburg—the prevalence of FeLV
(0.3–1.3%) was below the overall European prevalence (2.3%) and lower than that reported some years
ago, e.g., in urban stray cats in the Belgic city of Ghent (1998–2000: 3.8%) [
65
], or in blood samples
submitted for FeLV testing from owned cats in Austria (1996–2011: 5.6%) [
22
], but similar to a more
recent study in Belgian stray cats sampled by veterinarians through a trap–neuter program (2010–2012:
0.7%) [
52
]. In Germany, the current data extends the decreasing trend observed between 1993 and
2002 [14].
Living in the Southern European countries was associated with a higher risk of FeLV infection
(prevalence 2.6–8.8%). This is in accordance with a report from Portugal, where FeLV prevalence in
stray cats in Lisbon was 5.7% in 2013/2014 [
66
]. For Italy, the most recent information about FeLV
prevalence comes from a nationwide survey about Leishmania infantum infection, where 2659 leftover
feline blood samples were tested also for retroviral DNA, and FeLV proviral DNA was recorded in
4.8% of cats, but unfortunately no regional or clinical data is available for these cats [
67
]. The present
study demonstrated FeLV infections particularly in some Northern areas and in the Apulia region
(Figure 1A). In previous reports, 3.8% and 6.1%, respectively, of stray cats in urban and rural colonies
in northern Italy, Lombardy, tested FeLV antigen-positive (2008–2010 and 2014, respectively) [
68
,
69
].
Prior to that, a national study carried out in 1863 cats of Northern and Central Italy examined in
clinical settings found an average prevalence of 15.3% with 12% of prevalence in Lombardy, and the
highest positivity percentages in the northern–eastern area of the country (Friuli-Venezia Giulia 28.7%;
Trentino-Alto Adige 21%) and in Tuscany (23.2%). The lowest percentages of positivity were reported
in Lazio (7.5%) and Liguria (7.3%) [
70
]. However, in this study, the cats were tested for diagnostic
purposes and most were symptomatic and/or free roaming; therefore, the sampled population was not
representative of the feline population in those areas. Nevertheless, a wide distribution and the clinical
relevance of FeLV infection were shown at that time in Northern and Central Italy. Older studies from
Southern Italy found 7% positivity in Campania [
71
]. Around the same time and in the following
years, no FeLV-positive samples were found in Sicily in studies including stray or outdoor cats [
72
–
77
],
or about 2% positivity was reported in the Sicily [
78
,
79
] and Calabria regions [
72
]. Regional differences
in the prevalence of FeLV infection have likely existed for some time within Italy.
One factor contributing to the high FeLV risk of cats in Southern Europe might be the large
numbers of stray cats found in these Mediterranean countries that lack harsh winters and have
an abundance of food sources. In some Southern European countries, cats are not expected to be owned;
they are regarded as natural co-habitants in settlements, and are considered useful for their hunting
activity. They are considered not a domestic but a synanthropic species. In some areas, these cats are
left unneutered to breed naturally and with limited veterinary support, with serious consequences
Viruses 2019,11, 993 18 of 27
for the animals and often high mortality of kittens from infectious diseases [
80
]. In Italy, free-living
outdoor cats (gatti liberi) have been protected by law (no-kill no-moving policy) since 1991 [
81
]. There
are registered cat caretakers and compulsory neutering of the cats by the Veterinary Services of the
Local Health Unit. This has led to stable cat numbers in Rome [
81
], but in other Italian regions and
other countries, there remain concerns regarding free-roaming cats in general as well as concerning
infectious diseases [
82
]. The no-kill and no-moving restriction has many benefits for the cats, but might
be a disadvantage in terms of FeLV infection. Healthy FeLV-positive shedders might stay unrecognized,
and so pose an infection risk to uninfected cats. Moreover, if cats are identified as FeLV-infected, they
cannot be removed from the cat population.
Amongst the Southern European countries, the FeLV prevalence was lowest in Spain (2.6%).
This is lower than reported previously in 2012, when the FeLV infection rate in Barcelona was 6.0%
in stray cats [
83
], and was considerably lower than the 15.6% and 30.4% described in cats taken to
veterinarians in the Madrid metropolitan area in 1999 [
25
]. The low FeLV prevalence in Spain might be
associated with the relatively high FeLV vaccination rate in the cats tested from Spain (49.5%) compared
to the FeLV vaccination rate in cats from other Southern European countries (Portugal: 14.2%; Croatia:
16.5%; Italy and Malta: 17.8%).
The FeLV prevalence in the Eastern European region was intermediate, between those of the
Northern/Western and the Southern European regions, with considerable variation amongst the
Eastern European countries. The prevalence was higher in Hungary and Poland (5.9% and 5.0%,
respectively). The FeLV-positive cats from Poland were found in the middle and southeastern parts of
the country (Figure 1a); a previous study from this region in 2006–2010 had also demonstrated a high
FeLV prevalence of 6.4% in clinically healthy cats and cats suspected to have an infectious disease [
84
],
and an even higher prevalence of 14.2% was reported in a study investigating 741 mainly sick cats in
the area of Warsaw [
85
]. Remarkably, none of the 90 tested cats from Bulgaria were FeLV-positive. This
could be related to the small number of samples tested or, alternatively, the cats taken to veterinarians
in Bulgaria might be well cared for. The FeLV prevalence in the Czech Republic and Slovakia was
similar to the overall prevalence in Europe (2.0% and 2.2%) as well as to that reported in a previous
study of stray and owned cats in Slovakia [86].
Apart from the site of origin within Europe, other risk factors of FeLV infection were identified
in the current study, one being intact males. Tomcats are still considered to be mainly at risk of
FIV infection, and male sex has also been described as a risk factor for FeLV infection in other
studies [
10
,
14
,
87
,
88
]. FeLV can no longer be considered only as infection of “social cats”, although
FeLV is easily transmitted through social interactions via infectious saliva. However, it is, of course,
also spread via the saliva and blood of viraemic cats through aggression, which is a common male
behaviour. It is possible that as more cat owners become aware of the fact that FeLV can be transmitted
socially and aim to prevent this route of infection (e.g., within a household), the more the transmission
by cat fights in cats with outdoor access becomes evident and important. This is supported by the
findings that cats exhibiting aggressive behaviour have a higher risk of FeLV infection [
14
], and cats
taken to veterinarians for fighting injuries were frequently FeLV-positive [
89
]. It is also consistent
with the observation here that the outdoor access of the cat was a risk factor for FeLV infection. Cats
living outdoors, or having outdoor access at least sometimes, had a significantly higher risk of testing
FeLV-positive than cats living indoors. Therefore, it is recommended that all cats with outdoor access
should be vaccinated against FeLV in areas/countries where FeLV occurs [90].
Another risk factor identified in this study was the age of the cat. While it was shown previously
that young cats are more at risk of developing progressive FeLV infection [
24
,
91
], the risk of being FeLV
infected in the present study was approximately twice as high in cats aged one to six years compared
to younger and older cats. Accordingly, the median age of FeLV viraemic cats was 3 years in a recent
study from Germany and 4.75 years in a recent study conducted in the UK. Moreover, in a large study
of North American cats tested for FeLV at veterinary facilities and animal shelters, it was observed that
adult cats, defined as cats older than 6 months, were more likely to be FeLV viraemic (odds ratio 2.5)
Viruses 2019,11, 993 19 of 27
than juveniles up to 6 months of age [
10
]. Thus, FeLV infection should be expected in adult cats and
not only in kittens, although the latter have a higher risk of developing progressive infection [91].
As expected, a lower FeLV risk was found in pedigree cats compared to non-pedigree cats:
pedigree cats were approximately six to seven times less frequently FeLV-positive than non-pedigree
cats. There has been high awareness amongst cat breeders of the risks of FeLV infection for many
years, and most cat-breeding facilities are kept FeLV-free; cats are tested as kittens and after every
possible exposure risk, and pedigree cats usually have no, or very limited, outdoor access. This further
demonstrates that appropriate measures, such as testing, separation, risk assessment/reduction, and if
necessary, FeLV vaccination, can significantly reduce FeLV infection within a cat population.
As for other feline viral infections [
34
,
92
,
93
], and in accordance with results from feline retrovirus
studies investigating cats in overcrowded conditions [
94
], keeping cats in groups of
≥
5 cats/group was
also a risk factor for FeLV infection in the present study. From an epidemiological standpoint, it is
generally advisable to keep cats in stable groups that are as small as possible. Moreover, sick cats were
more frequently FeLV infected than healthy cats in Europe, which has been described also in other
studies [
17
,
32
], and has been demonstrated under experimental conditions [
5
,
95
]. In contrast to the
information on whether a cat was sick or healthy, which was required in the online questionnaire,
information on clinical signs was optional and provided voluntarily and not systematically by the
attending veterinarians; this represents a limitation of the study. Clinical signs that were associated
with FeLV infection in the cats reported in the present study were primarily anaemia, but also anorexia
and gingivitis/stomatitis. Anaemia in FeLV-infected cats has been reported previously [
96
–
98
]. FeLV
infection can be associated with different types of anaemia. Pure-red cell aplasia is associated with
the rare development of FeLV C within a cat [
99
], while haemolytic regenerative anaemia is related
to opportunistic infections or immune-mediated destruction of red blood cells [
98
]. However, recent
consensus statements suggest that there is only low evidence that FeLV per se induces immune-mediated
haemolytic anaemia [
100
]. FeLV infection is known to lead to anorexia, loss of body weight, and poor
body condition [
61
]. Moreover, in cats with gingivitis/stomatitis, FeLV infection should be considered
as an important underlying cause [3].
High FeLV RNA loads (>10
6
RNA copies/PCR reaction) were measured in the saliva samples of
approximately two-thirds of the infected cats and were observed particularly in sick cats. These results
cannot be compared directly with the results from virus isolation, although they confirm earlier results
of high virus loads shed in infected cats [
36
,
101
]. Using RT-qPCR, viral RNA equivalents are measured
rather than whole infectious viral particles.
The overall FeLV vaccination rate was rather low (27.1%) in Europe. Although some animals
were vaccinated against FeLV in all countries, high variation was found (range of vaccination rate:
3.2%–81.5%). The present study provides unique data on the prevalence of FeLV viraemia and FeLV
vaccination rates for many geographic areas of Europe. Remarkably, there are still countries with high
FeLV prevalence and low vaccination rates, e.g., Portugal, Italy, Croatia, and Poland. Awareness of
FeLV infection and its consequences for the cat and its owner and the protective effect of FeLV vaccines
should be increased particularly in these countries, as well as in countries where FeLV prevalence
remains at a low level but has not decreased further in recent years.
The results of the present study did not confirm the recently proposed associations between FeLV
prevalence and income; it was reported that the highest percentages of FeLV-infected cats lived in areas
with lower incomes, whereas a decreasing FeLV infection rate was observed with increasing income [
26
].
The FeLV prevalence did not correlate with the GDP at PPP in the 30 European countries included
in our study. In the present analysis, the data were obtained using the same FeLV detection and cat
recruitment method in all countries; in contrast, only limited and/or older data were available from
European countries for the recently published meta-analysis, and FeLV prevalence was determined
using different methods and heterogeneous study populations [
26
]. However, the number of countries
included in the present study and the range in GDP were somewhat smaller than in the previous
study [
26
], which might have limited the statistical analysis to a degree. Interestingly, we observed
Viruses 2019,11, 993 20 of 27
a correlation between the FeLV vaccination rate and the GDP at PPP in the investigated European
countries. Vaccination rates were higher in areas with higher income and lower in countries/country
groups with lower income. This observation can easily be explained since vaccines are more affordable
in countries with a higher GDP. A variable vaccination rate might have an indirect effect on the FeLV
prevalence. It was noted in the current study that higher FeLV vaccination rates tended to be associated
with lower FeLV prevalence rates. However, since this was only a tendency, additional factors other
than the FeLV vaccination rate (and the GDP) appear to have an impact on the FeLV prevalence, e.g.,
the availability of information and awareness concerning FeLV for veterinarians and cat owners.
According to the CART analysis, FeLV-positive cases were grouped into two terminal nodes
(Figure 5). The first terminal node 2 corresponds to cats living in Northern and Western Europe
(i.e., the area considered at lower risk of FeLV) but living always outdoors (considered a high-risk
behaviour). In this terminal node 2, only eight FeLV-positive cats were present within a total of
3511 cats living in Northern or Western Europe. The second terminal node 1 corresponds to cats living
in Southern (i.e., area with high prevalence of FeLV) and Eastern Europe (i.e., area with intermediate
prevalence of FeLV) but with no pedigree (pedigree cats usually have no or limited outdoor access).
Proportionally more FeLV-positive cats were present in this terminal node (i.e., 109 of a total of 2494 cats
originating from the southern or eastern parts of Europe), which was expected according to the high to
intermediate prevalence of FeLV in these countries in combination with the absence of pedigree cats.
5. Conclusions
The low prevalence of FeLV viraemia in Northern Europe and in most of the Western European
countries indicates that strict testing and separation programs and vaccination can decrease or even
eliminate FeLV infections. A very low FeLV prevalence was generally demonstrated in pedigree cats;
cat breeders are aware of FeLV and prevent infection using strict preventive measures. In contrast,
in some regions in this study, particularly in Southern Europe, high FeLV prevalence rates were
identified, and extra measures will be necessary to control or eliminate FeLV infection from these
geographic regions. The present study included countries with significant populations of stray cats
and various policies for controlling (or not) these cat populations. While some rescue and shelter
cats were included in the study, it can be assumed that the prevalence of FeLV in stray and feral
cats that do not attend any veterinary facilities might even be higher in many countries since the
living environment is suboptimal, and these cats do not receive any preventive or therapeutic medical
care. Moreover, risk factors for FeLV infection, including outdoor access and intact male sex that are
commonly associated with aggressive behaviour and fighting, were identified. Thus, in countries
where FeLV remains prevalent, cats with outdoor access should be vaccinated and—also for the welfare
of the cats—neutered. Awareness of FeLV infection and vaccination should be intensified, particularly
in countries with high FeLV prevalence or in countries with lower prevalence but suspected geographic
pockets of FeLV-positive cats, e.g., Switzerland and Ireland. In these countries, all cats at risk of FeLV
infection should be tested for FeLV. Subsequently, the separation of infected cats and vaccination of
uninfected animals is recommended by the Advisory Board on Cat Diseases Europe (ABCD Europe).
Author Contributions:
Conceptualization, H.L., R.H.-L., K.H., M.J.H., K.M., S.T., S.B., A.L., C.B.-B., H.F.E., M.-G.P.,
U.T., T.F., E.T., F.M., and D.A.; supervision, methodology, R.H.-L., M.L.M., and H.L.; project administration
and resources, H.L. and R.H.-L.; funding acquisition, H.L.; project administration (communication with
country representatives and local veterinarians), investigation, N.S. and E.G.; project administration (country
representatives: local administration including translation, interaction with the local veterinarians, sample
collection and returning): K.H., M.J.H., K.M., A.L., C.B.-B., H.F.E., T.F., E.T., F.M., M.H., F.T., Z.V., A.B., B.G.,
L.F.L.-B., F.T., C.T.M., C.E., J.O., H.J., K.J., I.K., K.K., J.Š., C.S. (Cristina Sobral), P.B., and S.K.; data curation and
validation, N.S. and R.H.L.; formal (statistical) analyses, C.S. (Claude Saegerman), N.S., and R.H.-L.; visualization,
C.S. (Claude Saegerman) and R.H.-L.; geographic figures, G.B.; writing—original draft, R.H.-L., N.S., and C.S.
(Claude Saegerman); writing—reviewing and editing, all authors.
Funding:
This study was financially supported by research grants from Merial (Lyon, France; now Boehringer
Ingelheim, Germany), Virbac (Carros, France), and Zoetis (Kalamazoo, MI, USA).
Viruses 2019,11, 993 21 of 27
Acknowledgments:
The study was initiated and supported by the European Advisory Board on Cat Diseases
(ABCD; www.abcdcatsvets.org). We are grateful to all the veterinarians who contributed samples and data to this
study. The molecular biology work was performed using the logistics of the Center for Clinical Studies, Vetsuisse
Faculty, University of Zurich.
Conflicts of Interest:
The authors declare that they have no conflicts of interest. The funders had no influence on
the study design, execution and the data analysis. The final manuscript has been accepted by the funders.
Appendix A
Table A1. Online Questionnaire.
Parameter Type Value Range Value Required
Country identification select list of countries yes
Veterinary practice text yes
Six-digit swab number text according to tube label yes
Date of collection date yes
Cat name text yes
Owner name text yes
Postal code of cat owner text yes
Type of husbandry select private/breeder/shelter/rescue cat/other yes
Multicat environment select yes/no/not sure yes
If yes: how many cats in the group select 2/3/4/≥5 optional
Sex of cat select male/female/not sure yes
Reproductive status of cat select intact/neutered/not sure yes
Age of cat select 0–8 weeks/9–12 weeks/13–52 weeks/1–2
years/2–3 years/. . . /19–20 years/≥20 years yes
Pedigree cat select yes/no/not sure yes
If yes: which breed text optional
Indoor and outdoor access select Indoor and outdoor/indoor only/outdoor
only/not sure yes
Last vaccination against FeLV select <1 year/1–3 years/>3 years/never/not sure yes
Health status select healthy/sick yes
If sick: major clinical problem text optional
Comments
long text
optional
Viruses 2019, 11, x FOR PEER REVIEW 21 of 27
Conflicts of Interest: The authors declare that they have no conflicts of interest. The funders had no influence
on the study design, execution and the data analysis. The final manuscript has been accepted by the funders.
Appendix
Table A1. Online Questionnaire.
Parameter Type Value Range Value
required
Country identification select list of countries yes
Veterinary practice text yes
Six-digit swab number text according to tube label yes
Date of collection date yes
Cat name text yes
Owner name text yes
Postal code of cat owner text yes
Type of husbandry select private/breeder/shelter/rescue cat/other yes
Multicat environment select yes/no/not sure yes
If yes: how many cats in
the group select 2/3/4/≥5optional
Sex of cat select male/female/not sure yes
Reproductive status of cat select intact/neutered/not sure yes
Age of cat select 0–8 weeks/9–12 weeks/13–52 weeks/1–2 years/2–3 years/…/19–
20 years/≥20 years yes
Pedigree cat select yes/no/not sure yes
If yes: which breed text optional
Indoor and outdoor access select Indoor and outdoor/indoor only/outdoor only/not sure yes
Last vaccination against
FeLV select <1 year/1–3 years/>3 years/never/not sure yes
Health status select healthy/sick yes
If sick: major clinical
problem text optional
Comments long
text optional
References
Figure A1.
Schema of the pooling of the saliva samples: 96 samples (A1 to H12) were pooled to 20
pools. Each pool consisted of 200
µ
L; either 8
×
25
µ
L (horizontal pooling) or 12
×
16.7
µ
L (vertical
pools). After pooling, each sample was present in two pools. Example: If a sample is FeLV-positive
(black circle, B3), two pools resulted in positive RT-PCR results.
Viruses 2019,11, 993 22 of 27
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