Serum samples obtained from 474 wild boars (Sus scrofa) were collected from June 2017 to September 2018 from various areas of northern and southern Poland. Serum samples were examined by enzyme-linked immunosorbent assay. However, West Nile virus (WNV) antibodies were not detected. Previous studies on WNV in Poland have focused on experimental evidence and the presence of WNV antibodies in wild birds, horses, and humans, indicating a need for more surveys of domestic and wild mammals in Poland.

West Nile virus (WNV) is the etiologic agent of West Nile fever, an emerging infectious disease that has been spreading rapidly in central Europe and has become a major public health issue. West Nile virus is an arbovirus belonging to the Flaviviridae family, genus Flavivirus, and is primarily spread by competent Culex mosquito vectors, leading to meningoencephalitis or encephalitis in humans and other mammals. Mammals have been presumed to be dead-end hosts because they typically produce a short-duration viremia (Sixl et al. 1989). Many factors favor the transmission of virus strains: demographic migrations, socioeconomic status, environmental to climatic changes, population movement (migration of wild birds), and high population density can play a role in the occurrence and mosquito-borne spread of virus. In the current context of climate change, there is a need to improve and understand the ecology and epidemiology of the disease and extend the surveillance system to every country.

Recent studies indicated the presence of WNV in fox squirrel (Sciurus niger), western gray squirrel (Sciurus griseus), and eastern gray squirrel (Sciurus carolinensis; Padgett et al. 2007). Cottontail rabbits (Sylvilagus floridanus) develop viremia at a high level (Tiawsirisup et al. 2005). Farajollahi et al. (2003) indicated WNV infection in black bears (Ursus americanus) and swine (Sus scrofa; Gibbs et al. 2006), and Komar (2003) suggested that viremia at a level >105.0 plaque-forming units/mL in some species may be a minimum threshold by which mosquito species can be infected.

The aim of this study was to evaluate retrospectively the activity of WNV in wild boars in Poland by serologic surveys of wildlife. The obtained results provide new knowledge in this field, which could help implement and improve future surveillance systems.

Research studies based on older and younger individuals is important. We surveyed wild boar in 34 hunting areas in Poland (Fig. 1). The regions with the largest wild boar populations are shown in Figure 2. The animals were between 1 and 12 yr old, and most of the sampling sites were situated in habitats within areas of high mosquito density.

Figure 1

Hunting areas of wild boars (Sus scrofa) in Poland. The numbers indicate the locations in forest district administrations of Poland where blood samples were obtained from wild boars for West Nile virus antibody testing during the 2017–18 hunting seasons. 1=Szczerba; 2=Gołdap; 3=Czerwony Dwór; 4=Maskulinskie; 5=Milomlyn; 6=Elbląg; 7=Dabrowa; 8=Lotowko; 9=Trzebielino; 10=Polczyn; 11=Podanin; 12=Bierzwnik; 13=Lopuchowko; 14=Trzciel; 15=Babimost; 16=Wymiarki; 17=Ruszów; 18=Piensk; 19=Gora Slaska; 20=Legnica; 21=Zlotoryja; 22=Milicz; 23=Brzeg; 24=Kluczbork; 25=Wielun; 26=Smardzewce; 27=Spala; 28=Sobieszyn; 29=Radzyn Podl; 30=Lubartow; 31=Tomaszow Lub; 32=Walcz; 33=Rudnik; 34=Bircza.

Figure 1

Hunting areas of wild boars (Sus scrofa) in Poland. The numbers indicate the locations in forest district administrations of Poland where blood samples were obtained from wild boars for West Nile virus antibody testing during the 2017–18 hunting seasons. 1=Szczerba; 2=Gołdap; 3=Czerwony Dwór; 4=Maskulinskie; 5=Milomlyn; 6=Elbląg; 7=Dabrowa; 8=Lotowko; 9=Trzebielino; 10=Polczyn; 11=Podanin; 12=Bierzwnik; 13=Lopuchowko; 14=Trzciel; 15=Babimost; 16=Wymiarki; 17=Ruszów; 18=Piensk; 19=Gora Slaska; 20=Legnica; 21=Zlotoryja; 22=Milicz; 23=Brzeg; 24=Kluczbork; 25=Wielun; 26=Smardzewce; 27=Spala; 28=Sobieszyn; 29=Radzyn Podl; 30=Lubartow; 31=Tomaszow Lub; 32=Walcz; 33=Rudnik; 34=Bircza.

Close modal
Figure 2

Schematic concept indicating the number of wild boars hunted across Poland from which the serum samples were obtained in 2017–18 in the places where the collection of samples was performed (black circles). During the 2017–18 study, samples were collected from 476 wild boars in 51 hunting areas. The map shows the concentration of wild boars from 2018 (gray color) prepared by the Department of Swine Diseases. The size of the circle is proportional to the number of wild boars on which sampling was performed.

Figure 2

Schematic concept indicating the number of wild boars hunted across Poland from which the serum samples were obtained in 2017–18 in the places where the collection of samples was performed (black circles). During the 2017–18 study, samples were collected from 476 wild boars in 51 hunting areas. The map shows the concentration of wild boars from 2018 (gray color) prepared by the Department of Swine Diseases. The size of the circle is proportional to the number of wild boars on which sampling was performed.

Close modal

Wild boars were shot by hunters during hunting seasons, and blood samples were collected from the heart or the thoracic cavity between June 2017 and September 2018. After collection, blood samples were centrifuged, and sera were stored at –20 C until use.

A commercially available enzyme-linked immunosorbent assay (ELISA) was used to detect antibodies against WNV. The ID Screen West Nile Competition Multi-species Kit (Innovative Diagnostics, Grabels, France) is a serologic test performed with the commercially available competition ELISA that screens for West Nile competition across multiple species for detection of WNV antibodies against the prE and prM envelope proteins. All samples were run in duplicate, and microplates were read at an optical density of 450 nm. The ELISA plates were validated according to the test protocol, and residual binding (signal-to-noise; S/N) ratios were calculated as a percentage. Serum samples with S/N ratios equal to or lower than 40% were considered positive; samples with ratios higher than 50% were considered negative. The S/N values between 40% and 50% were suspect.

A total of 474 serum samples were collected from wild boars in Poland and analyzed to evaluate WNV circulation in this species within Poland. No WNV neutralizing antibodies were detected with ELISA analysis. We did not confirm any evidence of WNV serologic presence.

West Nile virus is listed by the World Organisation for Animal Health as a neurotropic factor causing the disease, by which investigators are obliged to notify the World Organisation for Animal Health. As a consequence of climate change, in 2018 there were many new cases of WNV in European countries where it had not been previously detected (European Centre for Disease Prevention and Control 2019). Specific antibodies were identified in wild boars, leading to the hypothesis that they may potentially be sentinel animals for WNV surveillance in European countries (Root et al. 2005). Hubálek et al. (2017) indicated the presence of antibodies to WNV in samples of roe deer (Capreolus capreolus), red deer (Cervus elaphus), fallow deer (Dama dama), mouflon (Ovis musimon), and wild boar (4.8%). Kožuch et al. (1976) detected antibodies to WNV in roe deer (4.8%), red deer (4.1%), fallow deer (6.3%), mouflon (9.9%), and wild boar (4.1%). Juŭricová and Hubálek (1999) detected antibodies in 150 wild boars in South Moravia, as did Halouzka et al. (2008) in 6.5% of 93 wild boars. Antibodies to WNV were detected in 4.0% of 742 wild boars in Spain (Boadella et al. 2012). In serosurveys conducted in Serbia by Escribano-Romero et al. (2015), WNV antibodies were found in 10.4% of examined wild boars, and in eastern gray squirrels (Kramer et al. 2008), western gray squirrels, red squirrels (Tamiasciurus hudsonicus; Root et al. 2005), and eastern chipmunks (Tamias striatus; Gómez et al. 2008). Serologic evidence for the presence of WNV has been collected by Marfin et al. (2001) in the western Mediterranean mouse (Mus spretus), by Chastel et al. (1980) in predators, by Bentler et al. (2007) in striped skunk (Mephitis mephitis), and raccoons (Procyon lotor). Bard and Cain (2019) detected WNV prevalence in American black bears (Ursus americanus amblyceps). Gutiérrez-Guzmán et al. (2012) indicated that red foxes (Vulpes vulpes) and wild boar were positive for WNV antibodies. Padgett et al. (2007) confirmed infection in three squirrels (Rodentia: Sciuridae) in California, US; and in Iowa, US, Santaella et al. (2005) detected WNV in white-tailed deer (Odocoileus virginianus). To sum up, WNV has now been detected in numerous countries that are not characteristic hot zones for disease prevalence. With the rising temperatures created as an effect of climate change, having novel ways to track WNV prevalence is critical.

More experimental infection studies concerning mammalian species in Poland are warranted. The presence of WNV antibodies in wild birds, horses (Equus caballus), and humans (Niczyporuk et al. 2015; Niczyporuk 2017) indicates a necessity for enhanced surveys of domestic and wild mammals. This is especially relevant in context of WNV in neighboring countries during warm seasons, when WNV presence has been confirmed (Gale et al. 2019).

We assume that in Poland WNV has already existed somewhere for some time (Niczyporuk et al. 2015), and infectious cases may become present in humans (Hermanowska-Szpakowicz et al. 2006; Niczyporuk et al. 2015; Niczyporuk 2017). However, this hypothetical epidemic would certainly be during a period of high temperatures and high quantities of mosquitoes, such as events that occur after flooding in Poland. Climate change influences the frequency of mosquito-borne disease such as West Nile infections by affecting host and vector. Climate has an influence on growth, survival, and abundance of vectors and virus replication load in a vector. It is crucial that temperature (low and high thresholds) is the major climate point for mosquito reproduction and virus replication, as well as migrations of wild birds and population movement in the emergent regions.

We acknowledge support for sample collection provided by the Department of Swine Diseases, National Veterinary Research Institute (NVRI), Pulawy, Poland. Our study was partially funded by NVRI research project W/211: Occurrence of West Nile virus infection in Poland.

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