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Polish Journal of Veterinary Sciences Vol. 26, No. 3 (2023), 401–407
DOI 10.24425/pjvs.2023.145046
Original article
Correspondence to: G. Zaleśny, e-mail: grzegorz.zalesny@upwr.edu.pl
Helminth infection in horses –
a cross-sectional study from stables
in Lower Silesia (Poland)
W. Hildebrand1, P. Zielińska2, J. Hildebrand3, G. Zaleśny4
1 Hirszfeld Institute of Immunology and Experimental Therapy,
Polish Academy of Sciences, Rudolfa Weigla 12, 53-114 Wroclaw, Poland
2 Department of Surgery, Faculty of Veterinary Medicine,
Wroclaw University of Environmental and Life Sciences, Plac Grunwaldzki 51, 50-366 Wroclaw, Poland
3 Department of Parasitology, Faculty of Biological Sciences,
University of Wrocław, Przybyszewskiego 63, Wrocław, 51-148, Poland
4 Department of Invertebrate Systematics and Ecology, Institute of Environmental Biology,
Wroclaw University of Environmental and Life Sciences, Kożuchowska 5B, 51-631 Wroclaw, Poland
Abstract
Parasitosis in horses may be uncontrolled and expose breeders and owners to serious nancial
losses or, possibly, to the loss of animals. Therefore, the prevention and monitoring of the
development of parasitic diseases should play an important role in the breeding process. The aim
of this study was to conrm the inuence of factors such as age, breed, herd size, deworming
program, and type of anthelmintics, on the prevalence and intensity of parasites (helminths)
in domestic horses in Lower Silesia. The study was carried out between August and November
of 2020. The samples of horse feces were collected from 50 dierent stables in the area of Lower
Silesia, Poland. A total of 286 individuals from various breeds were examined. Detailed analysis
revealed that the mean age of infected horses was signicantly lower than in uninfected horses.
The mean time since the last deworming procedure was almost twice as low in uninfected horses
than in infected ones. Additionally, the deworming agent aects the prevalence of infection.
The analysis was also performed for the same factors in reference to quantitative data. The mean
EPG was four-fold higher in juvenile horses than in adults and three-fold higher when the horses
were dewormed with the use of fenbendazole instead of ivermectin or ivermectin with praziquan-
tel combined.
Keywords:
deworming scheme, helminths, horses, prevention
402 W. Hildebrand et al.
Introduction
In recent years, horse breeding has been treated as
an exclusive activity that can only be carried out by pro-
fessionals. Despite the clear decline in the number
of purebred horses in Poland (the number of horses
in Poland decreased from 329.5 thousand in 2002
to 185.5 thousand by 2016, i.e., by 43.7%), the interest
in recreational horse riding seems to be growing (data
taken from the Central Statistical Oce). A natural con-
sequence of such an approach is the increase in the
number of small studs oering opportunities to spend
“time in the saddle”. However, regardless of the herd
size, animal health and welfare are important to the
breeder. Invariably, over the years, parasitic invasions
caused by helminths constitute one of the major con-
cerns for breeders. Among them, infection with nema-
todes of the Strongylidae family are considered the pre-
dominant and most pathogenic group of parasites
(Gawor 1995, Love et al. 1999, Lyons et al. 1999,
Gawor et al. 2006, Lichtenfels et al. 2008, Kaplan and
Nielsen 2010, Kuzmina et al. 2016). In addition, this
group of parasites shows a high tendency to drug resis-
tance (Kaplan 2002, Kuzmina and Kharchenko 2008,
Kaplan and Vidyashankar 2012), which is a serious
problem for many horse breeders and owners. Eective
strategies of horse helminth control must consider many
factors (Osterman et al. 1999, Kornaś et al. 2004,
Gawor et al. 2006, Francisco et al. 2009, Kornaś et al.
2010, Saeed et al. 2010, Kuzmina et al. 2016, Slivinska
et al. 2016, Studzińska et al. 2017, Sallé et al. 2018).
Among them, primarily is a properly selected deworm-
ing program or a properly selected active substance.
In addition, the herd size, breed, and age of horses will
also be potential factors inuencing the dynamics
of helminth infection.
The aim of the study was to conrm the inuence
of factors, such as age, breed, herd size, time since last
deworming, and type of anthelmintics, on the preva-
lence and fecal egg count (FEC) of parasites (helminths)
in domestic horses in Lower Silesia. We assumed two
hypotheses: 1) in the group of horses infected with hel-
minths the time between deworming and the active sub-
stance contained in the anthelmintic used are strictly
correlated, and 2) the primitive breeds of horses are less
predisposed to parasite infection.
Materials and Methods
Study design
The study was carried out between August and
November of 2020. The samples of horse feces were
collected from 50 dierent stables in the area of Lower
Silesia, Poland. Horses between the ages of 1 to 25
years were examined, including mainly mares and geld-
ings, and several stallions with a total of 286 indivi-
duals from various breeds. The study was performed
in all types of stables (recreation, breeding, sports, etc.)
having from one to more than 50 horses. Fecal samples
were collected from 50% - 100% of the individuals
living in the stables with 10 or less horses, 50% from
stables with 10-20 individuals, and 25% - 30% from
stables hosting more than 20 individuals to present
a representative sample from each location. Feces were
taken from specimens who had not changed stables for
at least 6 months prior to the study. Collected samples
were fresh (not older than 24 hours) and gathered from
the stalls or paddocks immediately after each horse’s
defecation. Each sample was collected and stored sepa-
rately, refrigerated, and examined within 48 hours.
During the sampling, an interview with the owner
or person responsible was carried out. Detailed infor-
mation including age, gender, breed, health issues, and
deworming procedures with the recent anthelmintic
treatment were noted.
Each sample was examined separately, both macro-
scopically and microscopically. All of the fecal samples
were tested by the same person to avoid any disparities
within the results. The macroscopic examination con-
sisted of a visual evaluation of the feces on a sieve and
its goal was to nd potential adult or larval forms
of parasites and to examine the structure of the feces.
All of the dierences and abnormalities were noted.
The microscopic examination consisted of two parts:
quality evaluation and quantitative assessment (egg
count). The fecal examination followed a modied
McMaster centrifugation-enhanced method using
a sugar-salt otation solution with a specic gravity
of 1.3 g/ml. In the rst step, 4 g of feces were suspended
in 56 ml of tap water, mixed, and sieved through a mesh
with an aperture of 0.8 mm. The uid was centrifuged
for 5 min at 500 g, the supernatant was discarded,
and the pellet mixed in the otation solution. Two
McMaster chambers were lled (0.5 ml/chamber) and
the total number of eggs was counted (dividing into dif-
ferent groups or species) and the EPG (eggs per gram)
was calculated (Saeed et al. 2010, Sallé et al. 2018,
Selzer et al. 2021).
Database
Age: all of the individuals were assigned to one
of the three groups: “young” which included horses
from 1 to 4 years old (n=33); “adult” containing indi-
viduals between 5 and 11 (n=119) years old; and “old”
including those over 11 years old (n= 134). Sex/gender:
the horses were divided into two groups: “female” and
403
Helminth infection in horses – a cross-sectional study ...
“male”. The group of stallions was combined with the
geldings creating the “male” group as it was not numer-
ous enough to separate it.
Breed: more than 25 breeds of horses participated.
To make it statistically signicant, the individuals were
divided into three separate groups: “pony” (according
to the Polish Equestrian Federation this includes: Hucul
horse, Hainger, Polish Konik), “warm-blooded”,
and “other”. The last group, containing mainly
cold-blooded horses, was excluded from the statistics
as it was found to be non-signicant due to a lack
of representatives.
Herd size/breeding/rearing system: the animals
examined included recreation, breeding, and sport
horses. To standardize the results, they were divided
into three groups based on the number of animals
present in the herd: the “small herd”, “medium herd”,
or “big herd” groups. The rst group contained from
one to 10 specimens, the second, from 11 up to 20,
and the last group included herds with more than
20 individuals.
Anthelmintic treatment: all of the individuals taken
for testing were dewormed before the study with
a veterinary product. The owners or caretakers were
asked about the frequency of deworming, the substance
used, and the date of the last treatment. The horses were
given the treatment one, two, three, or four times per
year. Various anti-parasite products were used by the
caretakers, although when divided by the active sub-
stance in the product it was possible to divide them
into three dierent groups, which were: “ivermectin”,
“fenbendazole”, and “ivermectin with praziquantel”.
The third parameter examined was the time between
the last anti-parasite treatment and the time when
the feces were collected. The time periods varied from
1 to 13 months.
Statistical analysis of qualitative (prevalence)
and quantitative (EPG) data
In order to check if the presence of parasites in the
sample is aected by quantitative and qualitative pre-
dictors, logistic regression was performed. The analysis
was done using Statistica 13 (Tibco) software with
application of the interactive model constructor. At the
rst step, all possible eects (age of horses [age], time
since the last deworming procedure [deworm_t], size
of the herds [herd_size], horse sex [sex], horse breed
category [breed_cat], deworming substance [deworm_
subst], and presence or absence of protists in the fecal
samples [protista]) were added to the model. Next,
based on the p value of Somers’ D coecient, signi-
cant variables were selected for the model construction.
The EPG values are characterized by overdispersion
and excessive zeros, thus the data were analyzed using
zero-inated negative binomial regression (zeroin
function in the “pscl” package in R) (Zeileis et al. 2008).
We used the same set of predictors as for the logistic
regression [age, deworm_t, herd_size, sex, breed_cat,
deworm_subst, and protista], while the dependent vari-
able was the total number of EPG values (EPG_tot).
Additionally, since the deworm_t has a very strong
eect on the presence/absence of the parasites, we per-
formed a second analysis where all horses, dewormed
up to three months before fecal analysis, were removed
from the dataset.
Ethics approval and consent to participate
All methods were carried out in accordance with the
legal regulations of Poland. Our research only includes
interactions involving the following: interview with
the horse owners, collection of fecal samples (already
defecated) and laboratory examinations of fecal sam-
ples. All these procedures were non-invasive, not harm-
ful, and not stressful to the animals and therefore there
was no necessity to apply for ethical approval (Polish
Act of 15 January 2015). The participation of the horse
owners in that study was voluntary. Consent to partici-
pate is not applicable pursuant to the Polish Act
of 15 January 2015 “On the protection of animals used
for scientic or educational purposes.”
Results
In the present study 136 horses out of 286 (47.5%)
were infected with helminths. The mean EPG was
323.96 and diered between study groups; for details
see Table 1. The strongylids were the dominant group
of parasites. Based on the McMaster method, the infec-
tion with other parasites (nematodes Oxyuris equi
or Parascaris sp.) was found in three samples, and
single egg of Anoplocephala spp. (Cestoda) was found
in two samples. Since the total EPG for non-strongylid
helminths was very low (1 600 vs 90 947 for Strongylidae)
the statistical analyses were limited to strongylid infec-
tion only. The logistic regression (D Somers: 0.5806,
KS statistics: 0.4835) revealed that infection with
helminths is linked with the following variables: age of
horses, time since the last deworming procedure, and
deworming agent (for details see Table 2). Detailed
analysis revealed that the mean age (in years) of infect-
ed horses was signicantly lower (10.5 ± 0.5 S.E.M.)
than in uninfected horses (12.4 ± 0.5 S.E.M.). The mean
time (in months) since the last deworming procedure
was almost twice as low in uninfected (2.7 ± 0.2 S.E.M)
horses than the infected ones (4.3 ± 0.2 S.E.M). Addi-
tionally, the deworming agent aects the prevalence
404 W. Hildebrand et al.
of infection. The analysis was also performed for the
same factors in reference to quantitative data (Table 3),
i.e., average value of EPG. The mean EPG is four-fold
higher in juvenile horses than in adults (Fig. 1a) and
three-fold higher when horses were dewormed with the
use of fenbendazole instead of ivermectin or the combi-
nation of ivermectin with praziquantel (Fig. 1c). It is
also worth emphasizing that the EPG values in horses
of primitive breeds were lower by about 75% than
in warm-blooded horses (Fig. 1b); however, according
to multivariate statistical analysis, this result was not
statistically signicant. Moreover, according to the
results, a longer time since the last administration of the
anthelmintic results in a more intensive FEC (Fig. 2).
We did not nd a statistically signicant relationship
between the presence of helminths and the occurrence
of intestinal microparasites; however, there was a ca.
50% higher OPG (Fig. 1d) value in those individuals
infected with Protista.
Discussion
This study focused on the inuence of dierent fac-
tors, such as horse age, breed, herd size, breeding sys-
tem, and deworming programs on helminth infections
in stables located in Lower Silesia (Southwest Poland).
In recent years, an increase in the number of stables,
of various sizes and with dierent types of maintenance,
has been observed in Poland. Simultaneously, due to the
problem of deworming eectiveness being reported by
owners and breeders, there is a growing need to develop
an appropriate parasite control program, perhaps
individual for a given stable because the interaction
of various variables in dierent horse breeding condi-
tions cannot be ruled out. Since the structure of intesti-
nal parasites infecting equids, mainly strongyle nema-
todes, has been well characterized recently (Lichtenfels
et al. 2008, Kornaś et al. 2010, Kuzmina et al. 2011,
Kuzmina et al. 2016), we focused on the overall hel-
Table 1. Basic parasitological parameters (prevalence and mean EPG) of analysed horses.
Age group Sex N Prevalence [%] Mean EPG
overall
Mean EPG
Strongylidae
Mean EPG
non-Strongylidae
Juvenile male 15 73.3% 927.40 915.40 12
female 18 72.2% 801.12 758.89 42.23
Adult male 55 40.0% 206.54 206.54 0
felmale 64 48.4% 345.73 343.54 2.19
Old male 76 38.2% 169.50 163.71 5.79
female 58 51.7% 309.57 308.19 1.38
Overall 286 47.5% 318.37 323.69 5.59
Table 2. Factors aecting prevalence of helminths in horses.
Eect Level Estimate Standard error Wald
Statistics P value
Intercept 0.447 0.491 0.826 0.3631
age -0.087 0.025 12.220 0.0004
deworm_t 0.469 0.075 39.582 0.0001
deworm_subst iwerm_praz -1.486 0.383 15.063 0.0001
deworm_subst iwerm -1.244 0.388 10.269 0.0013
Table 3. Factors aecting mean EPG of helminths in horses.
Eect Estimate Std. error z value P value
Intercept -1.472 0.784 -1.876 0.0506
age 0.097 0.025 3.898 >0.001
breed -0.351 0.403 -0.870 0.384
deworm_t -0.427 0.072 -5.940 >0.001
deworm_subst (iverm) 2.433 0.692 3.514 >0.001
deworm_subst (iverm_praz) 2.505 0.678 3.694 >0.001
405
Helminth infection in horses – a cross-sectional study ...
minth infection as well as strongyle infection, and the
intensity (EPG) of infection was also taken into consi-
deration.
Our research shows that the major factor which
signicantly aects prevalence and FEC of helminths
in horses is the active substance used in the deworming
procedure. The analysis of three deworming programs,
using ivermectin, fenbendazole, and ivermectin with
praziquantel, indicated ivermectin as the most eective
anthelmintic, although particular studies show or dis-
cuss a resistance of some nematodes to ivermectin
(Martin et al. 2021, Selzer and Epe 2021). Thus, the
current conclusion is in line with recent research con-
ducted on nine stud farms from the same region, i.e.,
Southwest Poland, which indicated that strongyle resis-
tance to ivermectin in Poland is not a serious problem
(Zak et al. 2017). Additionally, according to the results,
a longer time since the last administration of the anthel-
mintic results in a more intensive FEC. The application
of the above statistical methods to qualitative and quan-
titative data made it possible to observe that younger
horses are more susceptible to infections, and this rela-
tionship also applies to warm-blooded breeds. Although
the latter relationship is not statistically signicant,
which is likely due to the number of horses studied,
but nevertheless, the trend is visible in the analyses.
These insights are partially consistent with the results
of Kuzmina et al. (2016). This study, conducted
on 197 horses of various ages and breeds and derived
from 15 farms throughout dierent regions of Ukraine,
revealed that the frequency of anthelmintic treatments
and management on the farm have a strong inuence
on infections with strongylids, more signicant than the
age and breed of the horses. Interestingly, the analysis
provided by Kuzmina et al. (2016) of the relationship
between horse breeds and EPG values revealed that
brood horses had much higher EPG values as compared
to other breeds; for example, the Hucul horse, a primi-
tive breed, shed the lowest number of strongylid eggs.
The herd size and breeding system were not signicant
factors inuencing helminth infection according to this
current analysis. Similar conclusions were drawn in studies
conducted 20 years ago in several regions of Poland,
where the decisive factor in the level of infection with
Fig. 1. The relationship between biotic and abiotic factors and mean EPG of helminths; (a) the eect of horse age; (b) the eect of horse
breed; (c) the eect of anthelminthic active substance and (d) the relationship between mean EPG and the presence of the repre-
sentatives of Protista.
406 W. Hildebrand et al.
intestinal nematodes turned out to be the deworming
program (Kornaś et al. 2004). During the microscopic
examination, each fecal sample was also checked for
the presence of Protista, which was mostly represented
by the Eimeriida order. Samples containing these
microparasites were taken for further and more specic
examination (with the use of a molecular approach).
However, individuals with coccidia present in their
feces were marked as a “Protista positive” group, while
the others constituted the “Protista negative” group.
In this study, we did not nd a statistically signicant
relationship between the presence of helminths and the
occurrence of intestinal microparasites; however,
observed values (i.e. higher OPG in individuals infec-
ted with Protista) can be a contribution and a starting
point for research on the role of Protista as a possible
factor increasing helminthiasis.
Further long-term studies, carried out on appropri-
ate groups of horses, taking into account various factors
as well as biodiversity of helminths (e.g., Petney and
Andrews 1998), an increase of species richness and bio-
diversity of parasites is known to reduce their pathoge-
nicity and overall infection) are needed to understand
the complexity of the interactions of intrinsic and
extrinsic factors and to develop eective deworming
programs, preceded by coproscopic analysis, that will
take into account the possibility of parasite resistance,
related to, among others, too high a concentration
of anthelmintics in the environment.
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