Introduction
Parasites of the genus Eimeria (Apicomplexa: Coccidia) may cause severe disease (coccidiosis) in a wide range of domestic and wildlife species, including leporids (Pakandl, Reference Pakandl2009; Duszynski and Couch, Reference Duszynski and Couch2013; Duszynski and Morrow, Reference Duszynski and Morrow2014; Duszynski et al., Reference Duszynski, Kvičerová and Seville2018; Mesa-Pineda et al., Reference Mesa-Pineda, Navarro-Ruíz, López-Osorio, Chaparro-Gutiérrez and Gómez-Osorio2021; Bangoura et al., Reference Bangoura, Bhuiya and Kilpatrick2022). The life cycle of Eimeria, its inter-population and seasonal variability, and major risk factors have been well described in many host species, with a focus mainly on farm and domestic animals (Pakandl, Reference Pakandl2009; Geng et al., Reference Geng, Ye, Lei, Shen, Fang, Hu, Zhao and Zhou2021; Mesa-Pineda et al., Reference Mesa-Pineda, Navarro-Ruíz, López-Osorio, Chaparro-Gutiérrez and Gómez-Osorio2021; Bangoura et al., Reference Bangoura, Bhuiya and Kilpatrick2022). Due to the mode of their transmission and dispersal, Eimeria infections typically manifest as an emergent disease, affecting local populations rather than individuals (Mesa-Pineda et al., Reference Mesa-Pineda, Navarro-Ruíz, López-Osorio, Chaparro-Gutiérrez and Gómez-Osorio2021; Bangoura et al., Reference Bangoura, Bhuiya and Kilpatrick2022).
Limited movements and high population densities may promote the spreading of the infection at a fast rate (Geng et al., Reference Geng, Ye, Lei, Shen, Fang, Hu, Zhao and Zhou2021; Mesa-Pineda et al., Reference Mesa-Pineda, Navarro-Ruíz, López-Osorio, Chaparro-Gutiérrez and Gómez-Osorio2021; Bangoura et al., Reference Bangoura, Bhuiya and Kilpatrick2022). Furthermore, weather conditions are among the most important risk factors of infection in farm animals kept under extensive farming regimes and wildlife species (Bangoura et al., Reference Bangoura, Bhuiya and Kilpatrick2022). For example, the prevalence of bovine Eimeria infections in tropical areas is higher in the rainy compared to the dry season and increases with the average temperature (Rodríguez-Vivas et al., Reference Rodríguez-Vivas, Dominguez-Alpizar and Torres-Acosta1996; Pfukenyi et al., Reference Pfukenyi, Mukaratirwa, Willingham and Monrad2007; Makau et al., Reference Makau, Gitau, Muchemi, Thomas, Cook, Wardrop, Fèvre and de Glanville2017). In a French population of wild rabbits (Oryctolagus cuniculus), the prevalence was higher in the humid and relatively cold areas compared to the drier and warmer ones during 1998–1999 (Grès et al., Reference Grès, Voza, Chabaud and Landau2003). In Australian wild rabbits, the prevalence was affected by rainfall and evaporation (Stodart, Reference Stodart1968).
While the prevalence of Eimeria and oocyst excretion exhibits clear annual seasonality in ruminants (Rodríguez-Vivas et al., Reference Rodríguez-Vivas, Dominguez-Alpizar and Torres-Acosta1996; Pfukenyi et al., Reference Pfukenyi, Mukaratirwa, Willingham and Monrad2007; Rehman et al., Reference Rehman, Khan, Sajid, Iqbal, Javid, Riaz and Ahmad2012), there is a strong intra- and interannual variation in lagomorphs (Stodart, Reference Stodart1968). In wild rabbits (O. cuniculus), different Eimeria species reach the highest prevalences at various times of the year (Grès et al., Reference Grès, Voza, Chabaud and Landau2003; Foronda et al., Reference Foronda, Figueruelo, Ortega, Abreu and Casanova2005). Unlike the number of excreted oocysts, Eimeria prevalence in European hare (Lepus europaeus) did not show any seasonality from November to April in a study from the Czech Republic (Lukešová et al., Reference Lukešová, Langrová, Vadlejch, Jankovská, Hlava, Válek and Čadková2012).
Indeed, there is a considerable large-scale variation among European hare populations in the composition of Eimeria parasites and their prevalences. The number of so far described Eimeria species found parasitizing local populations ranges from 4 to 16, they differ in pathogenicity and the total prevalence ranges from 20% to 100% across Central Europe (Forstner and Ilg, Reference Forstner and Ilg1982; Chroust, Reference Chroust1984; Böckeler et al., Reference Böckeler, Mokhtari-Derakhshan and Pecher1994; Wibbelt and Frölich, Reference Wibbelt and Frölich2005; Dubinský et al., Reference Dubinský, Vasilková, Hurníková, Miterpáková, Slamečka and Jurčík2010; Chroust et al., Reference Chroust, Vodnansky and Pikula2012; Lukešová et al., Reference Lukešová, Langrová, Vadlejch, Jankovská, Hlava, Válek and Čadková2012; Kornaś et al., Reference Kornaś, Wierzbowska, Wajdzik, Kowal, Basiaga and Nosal2014; Brustenga et al., Reference Brustenga, Franciosini, Diaferia, Rigamonti, Musa, Russomanno and Veronesi2023; Faehndrich et al., Reference Faehndrich, Klink, Roller, Wohlsein, Raue, Strube, Prenger-Berninghoff, Ewers, Capucci, Lavazza, Tomaso, Schnitzler and Siebert2023).
Development of partial or even full immunity in adults has been discussed for both mammals and birds (Tellez et al., Reference Tellez, Shivaramaiah, Barta, Hernandez-Velasco and Hargis2014; Mesa-Pineda et al., Reference Mesa-Pineda, Navarro-Ruíz, López-Osorio, Chaparro-Gutiérrez and Gómez-Osorio2021; Bangoura et al., Reference Bangoura, Bhuiya and Kilpatrick2022; Sîrbu et al., Reference Sîrbu, Florea, Sîrbu, Dreghiciu, Marin, Moraru and Dărăbuş2022). Immunization has been considered as an explanation for the general reduction of Eimeria infections in older individuals compared to young ones across species, countries and ecological settings (Stodart, Reference Stodart1968; Grès et al., Reference Grès, Voza, Chabaud and Landau2003; Pfukenyi et al., Reference Pfukenyi, Mukaratirwa, Willingham and Monrad2007; Lassen et al., Reference Lassen, Viltrop, Raaperi and Järvis2009; Bangoura et al., Reference Bangoura, Mundt, Schmäschke, Westphal and Daugschies2012; Chroust et al., Reference Chroust, Vodnansky and Pikula2012; Rehman et al., Reference Rehman, Khan, Sajid, Iqbal, Javid, Riaz and Ahmad2012; Kornaś et al., Reference Kornaś, Wierzbowska, Wajdzik, Kowal, Basiaga and Nosal2014; Makau et al., Reference Makau, Gitau, Muchemi, Thomas, Cook, Wardrop, Fèvre and de Glanville2017; Faehndrich et al., Reference Faehndrich, Klink, Roller, Wohlsein, Raue, Strube, Prenger-Berninghoff, Ewers, Capucci, Lavazza, Tomaso, Schnitzler and Siebert2023). Eimeria spp.’s higher prevalence in young animals than in adults has also been reported in European hares from southern Poland (Kornaś et al., Reference Kornaś, Wierzbowska, Wajdzik, Kowal, Basiaga and Nosal2014). The only exception reported so far is Soay sheep (Ovis aries) from Hirta, St. Kilda, in which the prevalence of Eimeria granulosa increased with age (Craig et al., Reference Craig, Pilkington, Kruuk and Pemberton2007).
We studied the effects of selected risk factors on the prevalence of Eimeria in European hares, a species that has experienced a severe long-term population decline across Europe since the second half of the 20th century caused by habitat and climate change, increase in predation pressure and possibly by pathogenic factors (Smith et al., Reference Smith, Jennings and Harris2005; Panek et al., Reference Panek, Kamieniarz and Bresiński2006; Dubinský et al., Reference Dubinský, Vasilková, Hurníková, Miterpáková, Slamečka and Jurčík2010). Surprisingly, drivers of Eimeria prevalence have been rarely addressed in populations of wild hare (Kornaś et al., Reference Kornaś, Wierzbowska, Wajdzik, Kowal, Basiaga and Nosal2014), despite coccidiosis being recognized as an important source of their mortality and factor of population growth (Chroust, Reference Chroust1984; Dubinský et al., Reference Dubinský, Vasilková, Hurníková, Miterpáková, Slamečka and Jurčík2010), and communities of Eimeria parasites being described extensively in this host species (Forstner and Ilg, Reference Forstner and Ilg1982; Chroust, Reference Chroust1984; Böckeler et al., Reference Böckeler, Mokhtari-Derakhshan and Pecher1994; Dubinský et al., Reference Dubinský, Vasilková, Hurníková, Miterpáková, Slamečka and Jurčík2010; Chroust et al., Reference Chroust, Vodnansky and Pikula2012; Lukešová et al., Reference Lukešová, Langrová, Vadlejch, Jankovská, Hlava, Válek and Čadková2012; Kornaś et al., Reference Kornaś, Wierzbowska, Wajdzik, Kowal, Basiaga and Nosal2014; Faehndrich et al., Reference Faehndrich, Klink, Roller, Wohlsein, Raue, Strube, Prenger-Berninghoff, Ewers, Capucci, Lavazza, Tomaso, Schnitzler and Siebert2023). We tested the effects of age, population density, distance to water and rainfall in hares killed by road traffic. We were also interested in whether infection with Eimeria spp. may cause clustering of hare–vehicle collisions (HVCs). We addressed this issue by testing the effect of the infection status on the probability of a hare being hit by a vehicle at the hotspot of road mortality.
Materials and methods
Field procedures
A single person (Jan Hušek) searched for the hares killed by the road traffic while driving along an 83.9 km long monitoring route on paved 2-lane roads. The route was located east of the capital of the Czech Republic, Prague (50°5ʹN, 14°26ʹE; Figure 1), in an agricultural landscape composed of conventionally managed fields, mostly cultivated by common crops such as rapeseed, cereals and a low proportion of vegetables interspersed with small to midsize deciduous and coniferous forest patches and urbanized areas. The road surveys were conducted in the morning hours (8:00–10:00 local time) on average every 4.52 (SD = 5.41) days from 11 February 2022 to 24 June 2024 (a total of 192 survey days). When possible, a hare carcass was collected to determine the age and infection with oocysts of Eimeria spp. In total, 160 HVCs were detected over the study period, from which 22 whole hare carcasses were collected. All of them were examined for the presence of Eimeria spp. oocysts. Age (juvenile/adult) was determined for 17 carcasses; the remaining 5 carcasses could not be aged due to the fatal damage (Table 1).
Figure 1. The location of the monitoring route in the Czech Republic where road-killed hares were collected for the analysis of Eimeria spp.
Table 1. Overview of hare carcasses used in this study
The age of hares was determined based on dry eye lens weight following Suchentrunk et al. (Reference Suchentrunk, Willing and Hartl1991). Eye lenses were extracted from eye bulbs, which were initially stored in 10% formaldehyde. The lenses were then air-dried at 100 °C for 24 h and weighed. Hares with lenses weighing less than 275 mg were classified as juveniles.
Identification of Eimeria parasites
Fecal samples or a part of the intestine were examined for the presence of oocysts using the standard centrifugation-flotation concentration method with modified Sheather’s sucrose solution of gravity 1.30 followed by light microscopy (Zajac and Conboy, Reference Zajac and Conboy2006). Samples positive for coccidian oocysts were analysed morphologically and morphometrically using an Olympus BX53 light microscope equipped with a DP-73-1-51 high-resolution image-cooled digital camera, and Olympus cellSens Standard v.1.13 imaging software (Olympus, Tokyo, Japan).
Explanatory variables
We used spring hunter estimates of hare abundance within a radius of 1 km from each site of HVC as a proxy for hare density. The 1 km radius was considered based on the home range size of the local hare population (Ševčík et al., Reference Ševčík, Krivopalova and Cukor2023). Every year on a single day at the end of March, non-professional hunters derive estimates of the size of the spring hare populations using a direct count of all hares seen while walking through their hunting grounds in combination with the local knowledge of hare population from continuous monitoring during previous weeks. The spring hunter estimates of hare abundance from March 2022 were obtained for each hunting territory from the Forestry and Game Management Research Institute. The annual mean abundance was transformed to the density of hares per 100 ha of agricultural land (hereafter referred to as ‘hare density’). Distance between each HVC site and the nearest water course was obtained using QGIS 3.10.1 (QGIS.org, 2025).
Oocysts of Eimeria can survive in the environment for several months (Svensson, Reference Svensson1995; Burrell et al., Reference Burrell, Tomley, Vaughan and Marugan-Hernandez2020; Bangoura et al., Reference Bangoura, Bhuiya and Kilpatrick2022). Hence, we assumed rainfall in the past 3 months (84 days) preceding the date of the carcass finding. The rainfall data for the closest town to each collision site were downloaded from https://www.meteocentrum.cz/archiv-pocasi. The period of 3 months was assessed based on the length of the Eimeria life cycle and the high resistance of oocysts in the environment (Svensson, Reference Svensson1995; Pakandl, Reference Pakandl2009; Burrell et al., Reference Burrell, Tomley, Vaughan and Marugan-Hernandez2020; Mesa-Pineda et al., Reference Mesa-Pineda, Navarro-Ruíz, López-Osorio, Chaparro-Gutiérrez and Gómez-Osorio2021).
Statistical analysis
We used logistic regression to test the effect of age (juvenile/adult), population density, distance to the nearest water and rainfall on the probability of infection with Eimeria spp. (binary variable: 1 infected/0 uninfected). Due to the low sample size, we tested the effect of each candidate factor in a separate model rather than employing multiple logistic regression.
To identify the hotspots of HVCs, we used a kernel smoothing function from a point pattern within a buffer of 1 km along the monitoring route in QGIS 3.10.1. As a hotspot, we defined kernel densities of HVCs >6. We assigned a location of each hare tested for Eimeria infection to whether it was within or outside of one of the hotspots of all HVCs and used logistic regression to test the effect of the infection status (infected/uninfected) on the probability of a sample being from within or outside of an HVC hotspot.
Results
From the 22 hares analysed, 13 (59.1%) tested positive for Eimeria spp. The following 6 species were recorded: Eimeria europaea, E. hungarica, E. leporis, E. robertsoni, E. stefanskii and E. townsendi. Table 2 presents the prevalences of each species and those reported by other studies from the Czech Republic for comparison. Of the hares tested positive for Eimeria, 23.1% were infected with a single species, 53.8% with 2 species and 23.1% with 3 species. The co-infections were the following: E. robertsoni + E. europaea + E. leporis (2 hares), E. robertsoni + E. europaea + E. stefanskii (1), E. robertsoni + E. europaea (2), E. robertsoni + E. leporis (2), E. europaea + E. leporis (1), E. europaea + E. townsendii (1) and E. europaea + E. hungarica (1). Original data can be found in Table 1.
Table 2. Reported prevalence of Eimeria spp. in feces of European hare in the Czech Republic
From the 4 factors tested in separate logistic regression models, only the effect of age was significant (Table 3). The McFadden R 2 index of the model including the effect of age was 0.24, indicating that this factor explained about a quarter of the variability in the data. The model predicted that the probability of Eimeria spp. infection was 0.375 (95% CI [0.125, 0.715]) in juveniles and 0.889 (95% CI [0.500, 0.985]), i.e. 2.37× higher, in adults (Figure 2).
Figure 2. Relationship between age and probability of infection with Eimeria spp. in European hare, Czech Republic. The grey lines show 95% confidence intervals.
Table 3. Summary results of the logistic regression models on the effects of age (juvenile/adult), population density, distance to water course and rainfall in the preceding 3 months on the probability of infection by Eimeria in European hares in the Czech Republic. Significant effect in bold
The probability of a sample being collected from an HVC hotspot was not related to whether an individual tested positive or negative for Eimeria spp. (χ 21 = 1.3, P = 0.25; Figure 3).
Figure 3. Locations of road-killed hares tested positive (red points) and negative (blue points) for Eimeria infection and density hotspots of hare–vehicle collisions (HVCs, orange polygons). Density is the kernel density of a number of collisions on a given road segment during II.2022–VI.2024. The density of HVCs was defined as a hotspot when the number of HVCs is >6.
Discussion
The finding of a positive effect of age on the prevalence of Eimeria spp. in European hares was surprising as the opposite has been generally observed among birds and mammals, including leporids (Stodart, Reference Stodart1968; Grès et al., Reference Grès, Voza, Chabaud and Landau2003; Pfukenyi et al., Reference Pfukenyi, Mukaratirwa, Willingham and Monrad2007; Lassen et al., Reference Lassen, Viltrop, Raaperi and Järvis2009; Bangoura et al., Reference Bangoura, Mundt, Schmäschke, Westphal and Daugschies2012; Chroust et al., Reference Chroust, Vodnansky and Pikula2012; Rehman et al., Reference Rehman, Khan, Sajid, Iqbal, Javid, Riaz and Ahmad2012; Kornaś et al., Reference Kornaś, Wierzbowska, Wajdzik, Kowal, Basiaga and Nosal2014; Makau et al., Reference Makau, Gitau, Muchemi, Thomas, Cook, Wardrop, Fèvre and de Glanville2017; Faehndrich et al., Reference Faehndrich, Klink, Roller, Wohlsein, Raue, Strube, Prenger-Berninghoff, Ewers, Capucci, Lavazza, Tomaso, Schnitzler and Siebert2023). The pathogenicity of different Eimeria species varies from non-pathogenic to severely pathogenic, the latter often fatal (Pakandl, Reference Pakandl2009; Mesa-Pineda et al., Reference Mesa-Pineda, Navarro-Ruíz, López-Osorio, Chaparro-Gutiérrez and Gómez-Osorio2021; Bangoura et al., Reference Bangoura, Bhuiya and Kilpatrick2022). Some Eimeria species, especially E. leporis, may have caused the deaths of leverets or juveniles early during their life, leaving only more resistant juveniles in the population (Espinosa et al., Reference Espinosa, Ferreras, Benavides, Cuesta, Pérez, Iglesias, Marín and Pérez2020). A strong negative correlation between the number of oocysts of E. leporis and the proportion of juvenile hares in a population has been reported, e.g., from southwestern Slovakia (Dubinský et al., Reference Dubinský, Vasilková, Hurníková, Miterpáková, Slamečka and Jurčík2010). E. leporis had the third highest prevalence in our study, after E. robertsoni and E. europaea.
Unfortunately, no detailed overview of the pathogenicity of Eimeria of European hares has been published that would allow a more critical evaluation (Bangoura and Daugschies, Reference Bangoura, Daugschies, Florin-Christensen and Schnittger2018). However, weak and susceptible leverets and juveniles may have also been removed early from the population by mortality factors other than coccidiosis, e.g. predation and/or agricultural operations (Voigt and Siebert, Reference Voigt and Siebert2020; Cukor et al., Reference Cukor, Riegert, Krivopalova, Vacek and Šálek2024). The other tested effects, density, distance to water and rainfall, were not significant predictors of Eimeria prevalence. Again, the lack of density effect is surprising, because the positive effect has been generally reported in both domestic and wildlife species (Rodríguez-Vivas et al., Reference Rodríguez-Vivas, Dominguez-Alpizar and Torres-Acosta1996; Rehman et al., Reference Rehman, Khan, Sajid, Iqbal, Javid, Riaz and Ahmad2012; Bangoura et al., Reference Bangoura, Bhuiya and Kilpatrick2022). Inability to detect the density effect in our study may have been caused by at least 3 aspects. First, hunter estimates may not have approximated the density with a sufficient precision as their reliability may be low (Cukor et al., Reference Cukor, Havránek, Rohla and Bukovjan2017; Hušek, Reference Hušek2025). Second, assigning each road-kill site with a correct density estimate based on a hunting ground may not be straightforward because about 73% of all road-killed hares were found closer than 500 m from a hunting ground border, at a distance regularly moved by hares from our study population (Ševčík et al., Reference Ševčík, Krivopalova and Cukor2023). Third, a decrease in Eimeria transmission at low hare densities may have been compensated by a clustered rather than random or regular hare distribution at low population density promoted by a preference for patchy habitat (Kamieniarz et al., Reference Kamieniarz, Voigt, Panek, Strauss and Niewęgłowski2013; Pavliska et al., Reference Pavliska, Riegert, Grill and Šálek2018).
Relatively large home ranges of hares inhabiting conventional agricultural landscapes may have counteracted the effect of population density on Eimeria infections in our study (Ševčík et al., Reference Ševčík, Krivopalova and Cukor2023). Also, the sample size used in our study may not have been large enough to detect subtle density effects.
Our study supports a large variation in Eimeria prevalences among populations and over the years (Table 2). Yet, E. europaea, E. leporis and E. robertsoni had high prevalences in all studies from the Czech Republic. Unlike other studies, we did not detect E. babatica (Table 2).
We found no relationship between the infection status and the probability of a hare being killed by a vehicle at the hotspot of road mortality. This indicates that infection with Eimeria did not contribute to the clustering of HVCs, and it remains for further study whether Eimeria may alter levels of hare activity as has been shown in other parasites, including Apicomplexans (McElroy and de Buron, Reference McElroy and de Buron2014; Horváth et al., Reference Horváth, Martín, López, Garamszegi, Bertók and Herczeg2016; Megía-Palma et al., Reference Megía-Palma, Paranjpe, Blaimont, Cooper and Sinervo2020, Reference Megía-Palma, Paranjpe, Cooper, Blaimont and Sinervo2024).
In conclusion, the positive effect of age on the prevalence of Eimeria in European hares does not correspond with the results of a study from the Kraków Province in southern Poland (Kornaś et al., Reference Kornaś, Wierzbowska, Wajdzik, Kowal, Basiaga and Nosal2014), challenging the generality of a negative effect of age not only in hares but also in other mammals (Craig et al., Reference Craig, Pilkington, Kruuk and Pemberton2007). Future studies shall shed light on drivers of the spatiotemporal variation in the composition of Eimeria parasites in hares.
Data availability statement
The datasets analysed for this study can be found in the manuscript.
Acknowledgements
We thank Šimon Kapic and Kateřina Funková for assisting with the dissection of the eye lens.
Author contributions
J.Hu.: Conceptualization, Formal analysis, Investigation, Methodology, Visualization, Writing – original draft, Writing – review & editing. K.B.: Investigation, Writing – review & editing. J.C.: Conceptualization, Funding acquisition, Writing – review & editing. J.Hr.: Investigation, Writing – review & editing. J.K.: Investigation, Resources, Writing – review & editing.
Financial support
This study was supported by the Technological Agency of the Czech Republic (SS05010238) and the Ministry of Culture of the Czech Republic (DKRVO 2024–2028/6.I.b, National Museum of the Czech Republic, 00023272).
Competing interests
The authors declare there are no conflicts of interest.
Ethical standards
Research activities described in the above-mentioned manuscript strictly used road-killed European hares. No manipulation with live animals was done in this study. Therefore, 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.
Open access
