Wild boar carcasses in the center of boar activity: crucial risks of ASF transmission

Abstract

African swine fever (ASF) is a highly virulent disease rapidly spreading through Europe with fatal consequences for wild boar and domestic pigs. Understanding pathogen transmission among individuals and populations is crucial for disease control. However, the carcass attractiveness for boars was surprisingly almost unstudied. Here, we evaluated if the wild boar carcasses are perceived as an attractant compared to the control sites throughout the year. For this purpose, 28 wild boar carcasses were placed in seven forest stands and continuously monitored in 2019–2020 by camera traps combined with control locations situated at least 200 m away in comparable habitats. Overall, we have recorded 3,602 wild boar visits, from which 3,017 (83.8%) were recorded in locations with placed carcasses and 585 (16.2%) in control locations. Most visits were recorded after sunset and before sunrise, corresponding to common peaks of wild boar activity. On average, the first visits were detected 4.7 days after carcass placement. Contrarily, it was 61.5 days for the control site. In conclusion, we have proven an enormous wild boar carcass attractiveness for boars, which exhibits an entirely new aspect of wild boar behavior. Therefore, the carcass removal is a crucial measure for controlling the spread of ASF.

Full text

Frontiers in Veterinary Science 01 frontiersin.org
Wild boar carcasses in the center
of boar activity: crucial risks of
ASF transmission
JanCukor
1,2*, MonikaFaltusová
2, ZdeněkVacek
2,
RostislavLinda
1, VlastimilSkoták
1,3, PetrVáclavek
4, MilošJežek
2,
MartinŠálek
1,5,6 and FrantišekHavránek
1
1 Forestry and Game Management Research Institute, Jíloviště, Czechia, 2 Faculty of Forestry and Wood
Sciences, Czech University of Life Sciences Prague, Prague, Czechia, 3 Faculty of Forestry and Wood
Technology, Mendel University Brno, Brno, Czechia, 4 State Veterinary Institute Jihlava, Jihlava,
Czechia, 5 Czech Academy of Sciences, Institute of Vertebrate Biology, Brno, Czechia, 6 Faculty of
Environmental Sciences, Czech University of Life Sciences Prague, Prague, Czechia
African swine fever (ASF) is a highly virulent disease rapidly spreading through
Europe with fatal consequences for wild boar and domestic pigs. Understanding
pathogen transmission among individuals and populations is crucial for disease
control. However, the carcass attractiveness for boars was surprisingly almost
unstudied. Here, weevaluated if the wild boar carcasses are perceived as an
attractant compared to the control sites throughout the year. For this purpose, 28
wild boar carcasses were placed in seven forest stands and continuously monitored
in 2019–2020 by camera traps combined with control locations situated at least
200 m away in comparable habitats. Overall, wehave recorded 3,602 wild boar
visits, from which 3,017 (83.8%) were recorded inlocations with placed carcasses
and 585 (16.2%) in control locations. Most visits were recorded after sunset and
before sunrise, corresponding to common peaks of wild boar activity. On average,
the first visits were detected 4.7 days after carcass placement. Contrarily, it was
61.5 days for the control site. In conclusion, wehave proven an enormous wild
boar carcass attractiveness for boars, which exhibits an entirely new aspect of wild
boar behavior. Therefore, the carcass removal is a crucial measure for controlling
the spread of ASF.
KEYWORDS
African swine fever, disease control, biosecurity, wild boar behavior, camera-trapping
Introduction
African swine fever (ASF) is a global viral disease aecting wild boar (Sus scrofa L.) and
domestic pigs (Sus scrofa domesticus Erxleben) with a negative socioeconomic impact,
especially on the pork industry (1–3). From 2007, when ASF was detected in Eastern Europe,
the virus had rapidly spread to numerous Central and Western European countries, including
Germany, Slovakia, Poland, Czech Republic, and Italy (4–6). Moreover, the ASF is also
spreading throughout Asia, including China and other southeastern regions. In the worst-case
scenario, the global eects of ASF disease on food security can increase the number of humans
at risk of hunger by 13–14 million, especially in India and Southeast Asia (7). erefore,
controlling the spread of ASF in the wild boar population is one of the crucial topics worldwide,
not only in Europe.
Infections with virulent strains of ASFV oen lead to fatal disease in Suidae individuals.
Wild boars or domestic pigs infected with virulent strains of ASFV usually die up to 10 days
aer infection, and with the genotype II the mortality rate reaches 90% or even more (8, 9). In
OPEN ACCESS
EDITED BY
Salome Dürr,
University of Bern, Switzerland
REVIEWED BY
Barbara Thür,
Kanton Aargau, Switzerland
Karin Darpel,
Institute of Virology and Immunology (IVI),
Switzerland
*CORRESPONDENCE
Jan Cukor
cukor@fld.czu.cz
RECEIVED 17 September 2024
ACCEPTED 09 December 2024
PUBLISHED 19 December 2024
CITATION
Cukor J, Faltusová M, Vacek Z, Linda R,
Skoták V, Václavek P, Ježek M, Šálek M and
Havránek F (2024) Wild boar carcasses in the
center of boar activity: crucial risks of ASF
transmission.
Front. Vet. Sci. 11:1497361.
doi: 10.3389/fvets.2024.1497361
COPYRIGHT
© 2024 Cukor, Faltusová, Vacek, Linda,
Skoták, Václavek, Ježek, Šálek and Havránek.
This is an open-access article distributed
under the terms of the Creative Commons
Attribution License (CC BY). The use,
distribution or reproduction in other forums is
permitted, provided the original author(s) and
the copyright owner(s) are credited and that
the original publication in this journal is cited,
in accordance with accepted academic
practice. No use, distribution or reproduction
is permitted which does not comply with
these terms.
TYPE Original Research
PUBLISHED 19 December 2024
DOI 10.3389/fvets.2024.1497361

Cukor et al. 10.3389/fvets.2024.1497361
Frontiers in Veterinary Science 02 frontiersin.org
the acute-lethal course of ASF, most animals die within 7 to 14 days
aer infection (10, 11). However, previous evidence suggests that
some animals may survive longer or completely recover (12). However,
seropositive animals, which theoretically could spread the virus, are
rather exceptional in the wild boar population and thus do not play
an epidemiological role regarding virus perpetuation (13). In domestic
pigs, ASFV transmission by survivor pigs was observed in one study
(14), whereas another study showed no transmission (15) over the
entire in-contact phase from survivors to sentinels during infections
with moderately virulent virus strains. e spreading of the ASF virus
diers according to conditions in the area of the virus occurrence. e
sylvatic cycle, tick-pig cycle, and domestic cycle are described for the
sub-Saharan Africa region (16, 17). e situation is unlike Europe,
where most outbreaks were found in wild boar populations (5, 16).
Since 2007, ca. 50,000 cases of ASF have been reported in Europe, and
the vast majority (86%) were conrmed in wild boar (18). Based on
diering European climates and environments in comparison to
sub-Saharan Africa, the new epidemiologic cycle of wild boar habitat
was dened. e wild boar habitat cycle is characterized by direct
transmission between infected and susceptible wild boar and indirect
transmission through carcasses and contaminated environment (19).
e possible ways of ASF transmission through infected carcasses
were described by Probst etal. (20). e risky behavior of wild boar
towards infected carcasses consisted of direct contacts especially by
sning and poking on the carcass and much less by chewing bare
bone once skeletonization of the carcasses was complete which was
most frequently documented for piglets (20). Moreover, wild boar
cannibalism was initially detected in another study during the winter
when the carcass biomass is stable and preserved by low temperatures
(21). is behavior represents a very eective way of infection
transmission. On the other hand, it seems that in hot, semiarid climate
conditions, the carcass decomposes rapidly reducing opportunity for
live wild pigs to interact with carcass compared to milder climates in
Central Europe (22).
e risk of ASF transmission through carcasses is signicant due
to the relatively long-term virus stability. e long-term survival of the
virus in the environment depends on several environmental and
climatic factors, with temperature as one of the most important (23).
e ASF virus can survive for over a year in the blood at 4°C, several
months in boned meat, and several years in frozen carcasses (24, 25).
Moreover, ASF virus can persist in contaminated soils where the virus
stability depends on the soil type, pH, organic material percentage,
and to a lesser extent, the ambient temperature (26, 27). e low
temperatures are crucial in the process of overwintering when the
virus can persist in the carcass from the autumn through winter with
the following risk of cannibalism of infected body mass in spring,
which could result in the subsequent ASF outbreaks in the wild boar
population (21).
Based on the abovementioned ndings, it is evident that the
infected carcasses play a critical role in ASF transmission in the wild
boar population. is leads to various biosecurity measures including
carcass disinfection (28, 29) or removal of carcasses from infected
areas (30, 31). Surprisingly, there is still insucient evidence
describing the attractiveness of the wild boar carcass for their fellow
boar, which may bea crucial behavioral aspect for setting eective
disease control strategies. erefore, the main aims of this study were
to (i) describe the attractiveness of wild boar carcass for individuals;
(ii) evaluate the sex and age structure of individuals in the location
with a carcass and the control site; and (iii) evaluate the eect of
daytime and season on visit intensity of the carcass compared to the
control site on randomly chosen locations in comparable habitat.
Methods
Data acquisition
e research was conducted in seven forest stands in the
CzechRepublic, Central Europe. e selected sites were previously
described by Cukor etal. (21), and this research builds on the data
collected during that study by placing additional carcasses on sites in
the subsequent seasons. e forests mainly consisted of Norway
spruce (Picea abies [L.] Karst.) with young forest stands (39 years on
average) in altitudes ranging from 358 to 626 m a.s.l. (see Table1). e
study sites have humid continental and oceanic climates, characterized
by warm to hot summers and cold winters, and, respectively, by cool
summers and mild winters with a relatively narrow annual
temperature range (32). e population density of wild boar is
comparable among individual selected sites (Cukor, unpublished
data). All of the study sites are located in the CzechRepublic, that has
one of the highest wild boar population densities (1.15 to 5.31
ind./100 ha) in Central Europe (33).
e individual studied forest stands were selected before the study
using the GIS environment according to the age of young forest
stands. e young forests stands (< 40 years), mainly composed by
coniferous tree species (especially by Norway spruce), represent the
primary habitats where the ASF-infected wild boar carcasses and
deathbed patches were found in the Czech outbreak in 2017 (31).
Similar results were conrmed also by a recent study in Lithuania,
where ASF-infected wild boars sought shelter in quiet areas (34),
which corresponds to conditions of coniferous stands. erefore,
those forest stands are key deathbed patches of ASF-infected
individuals in the real outbreak. For the carcass attractiveness
evaluation, seven wild boar carcasses were placed in seven preselected
sites during every season (i.e., winter, spring, summer, and autumn)
during the monitored study period from January 2019 to February
2020, which means in total 28 carcasses were used e control
locations were randomly selected empty spaces within 200 meters
form the carcass in the same forest stands to avoid confounding eects
of environmental conditions (e.g., altitude, vegetation cover, tree
species composition, local and landscape habitat structure). Similarly,
control locations were placed in the eld at the same time as the
cameras which monitored the carcasses. To ensure comparable, slower
decomposition of the carcasses, all wild boars were hunted and killed
by a single head shot following Czech legislative regulations. e
carcass data, such as sex, age class, weight, and placement date, are
listed in Table1.
e wild boar presence and activity on study and control sites
were monitored by camera traps UOVision UV 595 HD with a
resolution of 12 megapixels, HD video (1,080 P), and trigger speed of
0.65s.
1
e game cameras were installed on a selected tree at a distance
of 4 to 8 meters from the carcass. Cameras were set in video mode
1 www.uovision.com

Cukor et al. 10.3389/fvets.2024.1497361
Frontiers in Veterinary Science 03 frontiersin.org
with automatic recording of the date and time of the wild boar visit.
e video length was set to 30 s with a window of 1 min between
recordings. e carcasses and cameras were inspected every 2 weeks
to check the carcass status and battery charge. e monitoring was
completed when all edible biomass of the carcass was consumed or
removed by scavengers or wild boar, and no evidence of the carcass
was on the monitored plots. All video sequences with wild boar
presence were analyzed from the aspect of number, sex, and
approximate age of individuals (i.e., adult male, adult female,
unspecied adult, subadult, and piglet). For each recording,
weevaluated additional parameters such as duration of carcass setting
(in days), and time duration from sunrise and sunset. Sunrise and
sunset data were obtained from the web source Sunrise Sunset
2
for
each location. As the main aim of the article was to highlight the
attractiveness of carcass, werecorded only the presence of wild boar
without conducting a deeperanalysis of behavior. In addition, the
percentage of direct contact between the wild boar and the carcass was
2 https://api.sunrise-sunset.org/
evaluated, considering only instances where physical contact between
the wild boar and the carcass was recorded.
Statistical analyses
e analyses were separated into four parts: analysis of the
number of wild boar recordings, sex-age proportions, wild boar
detection time, and the time span before the rst contact with carcass.
Regarding the analysis of the number of wild boar recordings,
basic summary statistics were computed to provide a general overview
of collected data. Subsequently, analysis of the number of detected
individuals for each study location and season was conducted, and
these data were statistically compared between locations where the
carcass was placed in comparison to control locations using paired-
sample Wilcoxon rank-sum test (35). Non-parametric statistics were
selected because the assumption of normality, tested by the Shapiro–
Wilk test was violated (36).
Chi-squared test was used to analyze whether the distribution of
detected individuals across sex-age categories depended on the type
of location (carcass site vs. control site), conducting the analysis
TABLE1 Overview of the carcasses and sites included in the study.
Site GPS Age class, gender,
body weight of
the carcasses
Date of Date of
placement of the
carcasses
End of monitoring Forest stand
type and altitude
Onomyšl (I) N 49°55.34427′ E
15°6.65680’
piglet ♂ 36 kg
yearling ♂ 48 kg
adult ♀ 73 kg
piglet ♂ 23 kg
11 JAN 2019
03 MAY 2019
07 AUG 2019
02 NOV 2019
28 APR 2019
05 AUG 2019
07 NOV 2019
06 FEB 2020
Picea abies and Pinus
sylvestris, 30 years;
414 m a.s.l.
Kostelec nad
Černými lesy (II)
N 49°56.92620′ E
14°54.36413’
adult ♀ 74 kg
adult ♀ 82 kg
yearling ♂ 63 kg
piglet ♀ 18 kg
19 JAN 2019
03 MAY 2019
30 JUL 2019
13 NOV 2019
21 MAY 2019
06 AUG 2019
08 SEP2019
04 JAN 2020
Picea abies, 40 years;
443 m a.s.l.
Slapy– Buš (III) N 49°47.43740′ E
14°24.28262’
adult ♂ 68 kg
yearling ♂46 kg
yearling ♀ 62 kg
piglet ♀ 20 kg
22 JAN 2019
11 MAY 2019
6 AUG 2019
4 NOV 2019
21 JUN 2019
13 JUL 2019
XXXXXXX
19 JAN 2020
Betula pendula, 20 years;
358 m a.s.l.
Drahany (IV) N 49°27.01468′ E
16°48.58247’
piglet ♂ 43 kg
yearling ♂ 52 kg
yearling ♀ 71 kg
piglet ♂ 20 kg
15 JAN 2019
10 MAY 2019
02 AUG 2019
01 NOV 2019
13 APR 2019
05 AUG 2019
23 SEP2019
18 NOV 2019
Picea abies, 40 years;
614 m a.s.l.
Loket (V) N 50°11.40495′ E
12°46.78647’
piglet ♀ 38 kg
adult ♂103 kg
piglet ♀ 17 kg
piglet ♀ 22 kg
06 FEB 2019
02 MAY 2019
01 AUG 2019
06 NOV 2019
23 MAR 2019
10 AUG 2019
08 SEP2019
14 DEC 2019
Picea abies, 100 years
with natural
regeneration; 626 m a.s.l.
Podveky (VI) N 49°50.17067′ E
14°59.70358’
piglet ♀ 38 kg
yearling ♀ 55 kg
yearling ♂ 53 kg
piglet ♂ 19 kg
18 JAN 2019
01 MAY 2019
29 JUL 2019
30 SEP2019
26 MAY 2019
01 AUG 2019
30 OCT 2019
30 DEC 2019
Picea abies, 15 years;
452 m a.s.l.
Zalíbená (VII) N 49°48.88495′ E
14°58.77662’
piglet ♂ 45 kg
yearling ♂ 57 kg
yearling ♀ 52 kg
yearling ♀ 56 kg
18 JAN 2019
06 MAY 2019
01 JUL 2019
03 OCT 2019
02 MAY 2019
09 JUN 2019
29 SEP2019
28 DEC 2019
Picea abies, 30 years;
418 m a.s.l.
e camera trap was stolen in the case of location III (Autumn season).

Cukor et al. 10.3389/fvets.2024.1497361
Frontiers in Veterinary Science 04 frontiersin.org
separately for each season. e times of wild boar detections were
analyzed in relation to sunrise and sunset on the current day.
We analyzed the recorded time dierence to sunrise/sunset to
eliminate the eect of the day length in dierent seasons. For each
recording, the time dierence from sunrise/sunset was computed
(depending on which was closer to the time of detection), and those
values were compared between locations with carcass and control in
every season. For such comparison between carcass and control
locality, the Wilcoxon rank-sum test was used separately for each
season (the assumption of data normality, tested by the Shapiro–Wilk
test, was violated in some cases). Weused the Levene test (37) to assess
whether the variances in time dierences from sunrise or sunset were
signicantly dierent between carcass locations and control locations.
e time of recording relating to sunrise/sunset was also analyzed via
circular statistics. Besides the visual representation of the numbers of
detected wild boar in the carcass and control locations, we have
specically tested for “uniformity” of observations (i.e., whether the
observations were evenly distributed across all hours) via the Rayleigh
Test (38) and for the dierences between the time of detection of
individuals in relation to sunrise/sunset for the carcass and control
locations via the Watson-Williams Test (39). Wehave divided time
data into an hour scale for this analysis.
Lastly, the analysis of the time duration in days from carcass and
photo-trap setting to the rst recorded activity of wild boar was
performed. Besides basic statistics and graphical representation of
data, the paired-sample Wilcoxon rank-sum test was used for testing
for dierences between carcass and control locations.
All statistical procedures were performed using R soware (40) at
a condence level alpha = 0.05. Wilcoxon-rank sum test and Shapiro–
Wilk normality test were conducted via functions in package “stats,”
which is integrated to R distribution. Package “car” (41) was used for
performing Levene test and package “circular” (42) for Rayleigh test
and Watson-Williams test.
Results
e overall comparison of numbers of wild boar visits in the
carcass and control locations showed signicant results (data for each
study area in each season were compared; paired-sample Wilcoxon
rank-sum test, V = 5.5, p< 0.001; Figure1). In particular, the number
of recordings of wild boar inlocations with a carcass (3,017 records
during 1,156 visits, i.e., mean group size of 2.61 individuals) were 5.2
higher than on control locations (585 records during 242 visits, i.e.,
mean group size of 2.42 individuals), which suggest an extreme level
of attractiveness of wild boar to the carcass. At locations with carcass,
49.9% of visits were of one individual, 17.3% of two individuals and
the rest (32.8%) in group consisting of more than two individuals. e
largest recorded group consisted of 19 individuals. For control
locations, 58.3% of visits were of one individual, 12.0% of two
individuals and the rest (29.7%) in group consisting of more than two
individuals. e largest recorded group also consisted of 19
individuals. We observed signicant seasonal dierences in visit
frequency between carcass and control locations. Specically, in
spring, there were 1,248 visits inlocations with a carcass compared to
247 at control locations, indicating 5.05 times more visits to the
carcass. In summer, locations with a carcass received 642 recordings,
while the control locations had 96, reecting a 6.7-fold increase. In
autumn, locations with a carcass had 541 recordings compared to 75
at the control, showing 7.2 times more visits. Finally, in winter, there
were 586 recordings inlocations with a carcass versus 167 at the
control, representing a 3.5-fold increase (see Figure1).
Moreover, the analysis of wild boar visits to carcass sites focused
on evaluating the percentage of direct contact between wild boars and
the carcasses. e percentage of direct contact varied by season. In
autumn, direct contacts were observed in 340 out of 541 visits (62.8%);
in spring, in 889 out of 1,248 visits (71.2%); and in summer, in 478 out
of 642 visits (74.5%). e highest percentage of direct contacts
occurred in winter, with 493 out of 586 visits (84.1%).
From the total of 3,602 detected wild boar visits, wewere able to
determine the age of individuals in the case of adults, as well as the sex
for 3,437 individuals (95%). e other 165 individuals were recognized
as adults without further sex specication (5%). e most frequent
category was piglets, with 1,817 recordings (50%), followed by
subadults (942 recordings, 26%), adult females (509 individuals, 14%),
and adult males (169 individuals, 5%).
In spring, only 8% (17 individuals) of all detected subadults were
identied at the control location, and other subadult individuals (203
individuals) were recorded at location with a carcass. For additional
sex-age categories, the dierence was not as pronounced: adult females
(31 individuals, i.e., 13% at the control location, 203 individuals at the
location with a carcass), piglets (173 individuals, i.e., 19% at the
control location, 730 individuals at the location with a carcass), and
adult males (13 individuals, i.e., 24% at the control location, 41
individuals at the location with a carcass). e chi-squared test
indicated a signicant association between the sex-age categories and
number of detections per location type (carcass vs. control) during
spring (chi-squared = 20.87, df = 3, p< 0.001).
A similar trend was observed in the summer: only 5% of subadults
were observed at the control locations (7 vs. 141 individuals at the
location with a carcass), followed by adult females (11 vs. 106
individuals, 9%), piglets (66 vs. 362 individuals, 15%), and adult males
(10 vs. 21 individuals, 32%) as in the previous example. For summer,
the chi-squared test also indicated a signicant association between
the sex-age categories and number of detections per location type
(carcass vs. control, chi-squared = 22.70, df = 3, p< 0.001).
No signicant dierences in ratios of detected individuals divided
by sex-age categories and location were found in the autumn. e
numbers of individuals detected at control locations were as follows:
10% for subadults (21 vs. 217 individuals), 8% for adult females (7 vs.
77 individuals), 12% for adult males (5 vs. 36 individuals), and 15%
for piglets (32 vs. 177 individuals). e chi-squared test did not
indicate a signicant association between the sex-age categories and
number of detections per location type (carcass vs. control) during
autumn (chi-squared = 5.55, df = 3, p= 0.14).
In winter, the ratios of detected individuals at the control
locations were higher than in other seasons. e lowest ratio was
found for piglets (15%, 41 vs. 236 individuals), followed by subadults
(26%, 86 vs. 250 individuals), adult females (32%, 24 vs. 50
individuals), and adult males (33%, 14 vs. 29 individuals). Similarly
as for spring and summer, the chi-squared test indicated a signicant
association between the sex-age categories and number of detections
per location type (carcass vs. control, chi-squared = 17.88, df = 3,
p< 0.001).
Relative frequencies of individuals divided into sex-age for each
season are shown in Figure2.

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Frontiers in Veterinary Science 05 frontiersin.org
In spring, recordings at the carcass location were obtained
signicantly sooner before sunrise compared to the control location
(mean: 1.49 h before sunrise for the carcass location and 0.88 h before
sunrise for the control location, p= 0.006). Comparisons for other
seasons were not signicant. Obtained p-values are as follows:
summer—p = 0.61, autumn—p = 0.24, winter—p = 0.12. Mean
dierence values from sunrise in hours are negative in all cases, i.e.,
the majority of wild boar were recorded before sunrise. e mean
hour dierences for seasons with insignicant dierences are as
follows: summer—3.18 h before sunrise for the location with a carcass,
2.67 h before sunrise for control locations, autumn—4.60 and 4.18 h,
and winter—2.31 and 2.04 h.
In the case of the time dierence of recordings from sunset,
signicant dierences were observed between control locations and
locations with a carcass in both spring and summer (p< 0.001in both
cases, with recordings at the carcass locations occurring later). e
mean time dierences to sunset for each season are as follows:
spring—0.85 h aer sunset for the carcass locations and 1.10 h before
sunset for the control locations; summer—1.98 h aer sunset for the
carcass locations and 0.90 h aer sunset for the control locations;
autumn—5.43 h aer sunset for the carcass locations and 5.60 h aer
sunset for the control locations; and winter—1.70 h aer sunset for the
carcass locations and 2.16 h for the control locations.
We also tested variations in variances between wild boar
recordings to sunrise and sunset at the carcass and control locations.
For sunrise, signicant dierences were observed in all seasons except
for spring—p-values: spring—p= 0.74, summer—p< 0.001, autumn—
p= 0.03, and winter—p= 0.005. Variation was consistently higher at
the control locations for signicant results. For sunset data, the results
were the opposite; the only signicant result was obtained for spring
(p < 0.001, with variation for the control location again being higher).
Other p-values are as follows: summer—p= 0.25, autumn—p= 0.24,
and winter—p= 0.78. For a graphical depiction of the results, see
Figure3.
Circular statistical analyses revealed that the timing of wild boar
recordings does not follow a uniform distribution (see Figure4).
Weconducted four separate Watson tests to analyze the dierences in
the number of recordings detected in each hour (i.e., the uniformity
of observations between hours, as described above) for each
combination of carcass and control locations with sunrise and sunset.
FIGURE1
Number of detected wild boar in particular study locations (I–VII) with a carcass and control sites in dierent seasons.

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ese tests showed signicant results in all cases (the numbers of
detections per hour signicantly diered from uniform distribution,
p< 0.01in all cases). Wefurther analyzed the data by splitting it into
AM(before and aer sunrise) and PM (before and aer sunset) hours.
For both time periods separately, we tested whether there were
dierences in the distribution of recordings across hours between
carcass and control locations. e results showed signicant
dierences in the distribution of observations between carcass and
control locations across all seasons (p< 0.001 for both AMand PM
hours). For example, in carcass locations, animals were oen recorded
in times, when no animals or only small numbers of them were
detected in control locations (animals were observed in broader time
window around sunrise/sunset in carcass locations), e.g., −5 and +6 h
around sunrise in spring or−6 and−3 h before sunset in summer.
e analysis of the time of the rst visit to the carcass showed that
wild boar found the carcass in a relatively short time (Figure5). e
average values amongst all locations were around 2 days in spring and
summer, around 6 days in autumn, and 8 days in winter. Also, during
spring and summer, the maximum recorded times to nd the dead
body were 7 and 5 days in particular locations. In autumn, the
maximum days needed to nd the dead body were 19 days, and in
winter, up to 36 days. Nevertheless, in all seasons, wild boar were able
in some locations to nd the dead body on the same day it was placed
were observed. e comparison of days between the date of carcass
setting and the rst recording of wild boar activity showed signicant
results (Wilcoxon paired-sample test, p= 0.03). On control locations,
the rst wild boar detection was found aer a much longer period
compared to locations with the carcass (spring—2.6 days on average
for the carcass location vs. 61 days on average for control locations,
summer—2 vs. 69 days, autumn—5.7 vs. 104 days, and winter—8.4 vs.
11.8 days), although very high variance was observed for all seasons.
Minimum values for control locations were 0 days for winter and
4 days for spring, while maximal values were 36 days for winter,
followed by 128 days for spring, 207 days for summer, and 275 days
for autumn. e average values through the year were 4.7 days to the
rst visit to the carcass location and 61.5 for the control.
Discussion
African swine fever transmission is driven by several factors that
are changing across the geographic conditions where the virus is
present, both in wild boar and domestic pigs’ populations (9). In
Europe, it seems that the infected carcasses play the most crucial role
in transmission (30, 31, 43, 44) besides the human factor, which
transports the virus long distances, for hundreds of kilometers, mostly
through pork products (45). erefore, it is necessary to understand
all aspects of wild boar behavior toward the carcasses of its own
species, about which westill have limited information. However, data
on carcass attractiveness can only be compared to the general
knowledge of wild boar activity within their home range, as previous
studies have investigated wild boar interactions with carcasses (20, 21)
FIGURE2
The proportion of sex-age categories inlocations with a carcass and control locations for dierent seasons. The outer circle shows the sex-age
categories in a location with a carcass, and the inner circle represents the control location.

Cukor et al. 10.3389/fvets.2024.1497361
Frontiers in Veterinary Science 07 frontiersin.org
but have not included a comparison with a control location, as was
done in this study. One of the main ways to compare and express
carcass attractiveness is by comparing the number of wild boar visits
to the control location in comparable conditions, which was over ve
times higher throughout the year. Based on those ndings, it is
apparent that the carcass is perceived by wild boar as an attractant. e
highest dierence in the number of visits inlocation with carcass and
control location was found in the spring and summer seasons. During
the warmer period, the wild boar activity around the carcass was
greater compared to the control location. is can beexplained by the
rapid carcass decomposition by scavenging insects, which is followed
by a strong odor of decaying carcasses (46) and therefore, carcasses
could bemore easily detected.
In general, wehave detected 3,602 wild boar visits for carcass
and control locations combined, from which the sex and age could
bedetermined in 95% of the visits. Not surprisingly, piglets were
detected in most of the cases, which corresponds to normal wild
boar population structure and high litter size per adult female (47).
In our case, the proportion of piglet detection exceeded 50% of
recordings in the spring and summer periods in the carcass location,
with a decreasing tendency for autumn and winter. A similar trend
was also found in the control locations. It can be explained by
hunting pressure followed by decreasing piglet proportion through
the season (48). Moreover, wild boar has enough fodder
opportunities in a fragmented landscape of high-energy crops
throughout most of the year (49, 50), and therefore, the body mass
and appearance soon resemble subadults more than piglets.
Interestingly, subadults were recorded in signicantly higher
numbers at the carcass location compared to the control site,
particularly in spring. is pattern may bedue to reduced natural
food availability during this period. Most natural food sources, such
as beechnuts, acorns, and other tree seeds, have already been
FIGURE3
Detection times of wild boar inlocations with a carcass and control related to sunset/sunrise. Sunset/sunrise is depicted by a dashed line in the plot.
The plot is divided into two parts—AM, for sunrise, and PM, for sunset. The dots indicate outliers for respective variants.

Cukor et al. 10.3389/fvets.2024.1497361
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consumed by winter, and cereal crops are not yet fully grown.
erefore, the decomposing carcass could beconsidered a food
source by wild boars, as chewing on bare ribs (especially in summer)
was conrmed in a previous study by Probst etal. (20). Also, during
this period, subadult males are excluded from family groups as adult
females focus on raising the new piglet generation. is makes it
more challenging for subadults to nd food opportunities,
prompting them to intensify their search, which leads to more
frequent encounters with carcasses.
From the ASF transmission point of view, it is important to
highlight that there is an explicit assumption that the individuals, due
to the uctuating age distribution (according to camera trap
monitoring), are from dierent groups and simultaneously visited the
same carcass. Additionally, the wild boar social structure is important
in the context of ASF transmission. At the social network level, young
animals up to 2 years of age showed greater between-group
connectivity than adult ones (51) and therefore, the observed structure
of monitored individuals indicates a higher risk especially during the
spring and summer seasons, due to the high proportion of piglets in
the population. ese facts allow us to observe how quickly ASF can
spread during out-group interactions.
e time of detected wild boar activity was another aspect of
behavior that was analyzed. In common circumstances, the diurnal
activity usually involves movement between resting areas and feeding
sites (64). e highest proportion of wild boar active behavior occurs
around midnight and morning hours (65, 66). In this study, the wild
boar activity was recorded especially close to sunset/sunrise during
most of the year. e earliest visits around sunset were found in
spring, when the decomposition process is relatively fast, which is
characterized by a strong odor (43) (mentioned earlier). However, the
distribution of wild boar visits in relation to the time before and aer
sunrise and sunset was not uniform across seasons. Despite the
signicant dierences observed across dierent seasons, no clear
temporal pattern was identied throughout the entire year. ese
ndings suggest that the temporal distribution of wild boar activity is
heavily inuenced by resource availability and sensory cues, such as
odor intensity, at carcass sites. e lack of a uniform temporal pattern
throughout the year may reect the interplay between environmental
conditions, seasonal variations in decomposition rates, and the
foraging strategies of wild boar. Interpreting these dierences
emphasizes the role of carcass sites as focal points for temporally
clustered activity, contrasting with the more dispersed activity
observed in control locations.
e greatest dierences between the location with the carcass
compared to the control site were found at the time of the rst
detection of wild boar on the camera trap. On average, the rst wild
boar was detected aer 4.7 days in the carcass location and aer
61.5 days in the control site, with the highest average dierence found
in autumn (5.7 vs. 104 days). e number of wild boar recordings
between the carcass and control locations during dierent seasons
could be caused by variations in wild boar population density
throughout locations and by the home range size changes. Wild boar
shows remarkable intraspecic variations in home ranges across
various habitats. Annual home range size varies between 400 ha to
6,000 ha, with an average size of around 800 ha (52, 53). According to
wild boar home range size, ASF is spreading gradually at a steady pace
of 1.5 km per month throughout the year (54). e larger home range
sizes were conrmed in the autumn and winter periods (55), which is
inuenced by several factors, e.g., by the rut season where the wild
boar has higher daily home range sizes compared to the rest of the
year (56). Another aspect can bethe rebalance caused by the autumn
hunting season, which also aects the home range size and the wild
boar activity and space use (57). Moreover, the habitat preference of
wild boar is driven by food source availability. In the late summer, the
standard behavior patterns and habitat utilization of wild boar can
bedisrupted and changed by the crop harvest. In forested areas, the
habitat preference is aected by oak species (Quercus spp.) and
European beech (Fagus sylvatica L.), especially in the mast years of the
aforementioned deciduous trees (47, 49, 58). is means that if the
wild boar’s basic life needs are satised, it does not make much sense
for them to move over greater distances. On the contrary, most of the
carcass and control sites in our study were in Norway spruce forests
with low availability of natural food sources for wild boar, which may
explain the later visits aer sunset during autumn. It was previously
proven that in poor nutritional conditions, wild boars move more in
search of food and water, increasing their home range (56, 59).
Moreover, wild boar behavior and the time of the carcass visits can
FIGURE4
Circular plots for the location with the carcass and control location.
Y axis is represented on log-scale due to significant dierences
between the number of recordings for the location with the carcass
and control location.

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Frontiers in Veterinary Science 09 frontiersin.org
be signicantly aected by supplementary feeding provided by
hunters. is means that the movement of wild boars is inuenced by
the number and location of feeding places in their natural habitat. e
amount of supplemental food can beapproximately 1,000 kg per year
per 100 ha in particular locations, and the feeding is targeted primarily
in the autumn and winter periods (52).
erefore, it appears that the high risk of ASF transmission
through infected carcasses is prevalent throughout the year. e
potential ASF transmission is aected by subsequent wild boar
movement aer contact with the infected carcass. e daily distances
traveled by wild boar are usually between 10 to 20 km (53, 60, 61).
However, if the area lacks suitable food sources, the wild boar is
forced to increase the distances traveled, which increases the risk of
spreading ASF. On the other hand, if there is sucient food, water,
and shelter, most young wild boar (70–80%) do not disperse further
than 5 km from their natal ranges (62, 63). All sex-age wild boar
categories occasionally move long distances of 50–250 km in a
straight line in rare situations (62, 63), and in this example, wild
boar can walk 30–40 km within 24 h and 200–300 km in
10–15 days (61).
Conclusion
us far, it has not been determined whether the wild boar
carcasses are visited purposefully or whether they are visited as part
of the habitual movement of wild boar in the location. e answer to
this question is made clear by the conclusions presented in this study,
which conrmed the immense attractiveness of the carcass for the
wild boar population across the seasons. Based on the visit dierences
between locations with the carcass and the control in a comparable
habitat, it is evident how attractive the wild boar carcass is to their own
species, which has conrmed a critical role in the ASF transmission.
Our results conrm important implications for the understanding of
ASF spreading among individuals of wild boar populations. Weclearly
demonstrated that carcasses of wild boars are highly attractive for wild
boars during the dierent seasons, which pose a high risk of ASF
transmission throughout the year. erefore, there is an urgent need
for early detection and removal of infected carcasses from the
environment. is seems to beparticularly urgent in spring and
autumn months, when wild boars detect carcass earlier than in the rest
of the year.
FIGURE5
Number of days to the first recording of wild boar activity on control locations and locations with the carcass. Bars stand for mean values, whiskers for
min and max values for each season, and location type.

Cukor et al. 10.3389/fvets.2024.1497361
Frontiers in Veterinary Science 10 frontiersin.org
Data availability statement
e raw data supporting the conclusions of this article will
bemade available by the authors, without undue reservation.
Ethics statement
For the Research activities described in the abovementioned
manuscript were strictly used only legally hunted wild boar
individuals under standard conditions of wildlife management
legislation in the Czech Republic. No manipulation with live
animals was done in this study. erefore, the Ethics approval was
done from the Forestry and Game Management Research Institute,
v.v.i. under the auspices of the Ministry of Agriculture of the
CzechRepublic.
Author contributions
JC: Conceptualization, Funding acquisition, Investigation,
Methodology, Project administration, Resources, Supervision,
Validation, Writing– original dra, Writing– review & editing. MF:
Writing– original dra, Writing– review & editing. ZV: Writing–
original dra, Writing – review & editing. RL: Formal analysis,
Methodology, Visualization, Writing– original dra, Writing– review
& editing. VS: Writing– original dra, Writing– review & editing. PV:
Writing– original dra, Writing– review & editing. MJ: Writing–
original dra, Writing– review & editing. MŠ: Writing– original
dra, Writing – review & editing. FH: Conceptualization,
Investigation, Resources, Writing– original dra, Writing– review &
editing.
Funding
e author(s) declare that nancial support was received for the
research, authorship, and/or publication of this article. is study was
supported by the Czech National Agency for Agricultural Research,
project numbers QK1920184 and QK1910462, by the Institutional
support from the Ministry of Agriculture (MZE-RO0118) and by the
Czech University of Life Sciences Prague, Faculty of Forestry and
Wood Sciences (Excellent Team and A_25_22).
Acknowledgments
We acknowledge Jitka Šišáková and Richard Lee Manore for checking
English. Moreover, we would like to acknowledge to the both reviewers and
academic editor whose comments signicantly improved the manuscript.
Conflict of interest
e authors declare that the research was conducted in the
absence of any commercial or nancial relationships that could
beconstrued as a potential conict of interest.
Publisher’s note
All claims expressed in this article are solely those of the authors
and do not necessarily represent those of their aliated organizations,
or those of the publisher, the editors and the reviewers. Any product
that may beevaluated in this article, or claim that may bemade by its
manufacturer, is not guaranteed or endorsed by the publisher.
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