Epidemiological survey of swine influenza A virus in selected wild boar populations in Germany
Introduction
Swine influenza (SI) is an infectious disease of pigs (Suidae) caused by type A influenza viruses of swine, human and avian origin. Influenza A viruses are enveloped negative-sense, single-stranded RNA viruses with a segmented genome. They belong to the family Orthomyxoviridae, genus Influenza virus A (Palese and Shaw, 2007). Swine influenza A viruses (SIV) are classified into subtypes based on the antigenic properties of the surface glycoproteins hemagglutinin (HA) and neuraminidase (NA). Three subtypes of influenza A viruses (H1N1, H3N2, H1N2) are circulating in pig populations worldwide (reviewed by Olsen et al., 2006). These subtypes are widespread in several European regions and may cause clinical signs in infected pigs (Van Reeth, 2007).
SIV was first observed in 1918 during the human influenza epidemic in the USA and was characterised by Shope (1931). H3N2 virus infection in pigs has occurred in different European countries since the mid and late 1970s (Tumová et al., 1980, Aymard et al., 1985, Haesebrouck et al., 1985). Circulating porcine H1N1 virus in Europe was traced back to avian origin introduced from wild ducks into pig populations in 1979 (Pensaert et al., 1981, Van Reeth, 2007). This fact indicates that wild boar may also be affected by SIV. Although the disease may run as an acute infection in wild boar, a clinical detection of SI by hunters seems to be impossible and the detection of SIV, viral antigen or RNA is very difficult due to the very short course of the disease. Therefore, serological investigations generally dominate the epidemiological surveillance in wild boar. Serological diagnosis of SIV infections is based on hemagglutination inhibition (HI) test, virus neutralisation test (VNT), indirect immunofluorescence test (IFT) and enzyme-linked immunosorbent assays (ELISAs) (OIE Diagnostic Manual, http://www.oie.int/eng/normes/mmanual/A_00137.htm).
SIV infections in wild boar are reported in some European countries, e.g. in Poland (Markowska-Daniel and Pejsak, 1999), in Spain (Vincente et al., 2002), and in Slovenia (Vengust et al., 2006). In Germany, knowledge on the epidemiological situation of SIV infections in wild boar is more restricted. Therefore, we investigated sera derived from wild boar shot in different Bundeslaender for antibodies against SIV. Furthermore, wild boar bagged in Mecklenburg-Western Pomerania, district of Nordvorpommern, were examined virologically. The aim of this study was to provide a preliminary overview on epidemiological data for SI in wild boar and to permit a risk assessment for SIV transmission from wild boar to domestic pigs.
Section snippets
Investigation areas
Wild boar were investigated in five German Bundeslaender (federal states). Altogether, 1245 blood samples were collected for detection of antibodies against SIV. They were derived from animals shot in Mecklenburg-Western Pomerania during the hunting season 1997/1998 (n = 120) and in 2006 (n = 442), in Brandenburg in 2004/2005 (n = 120), in Saxony-Anhalt in 2001 (n = 120), in Baden-Württemberg in 2001/2002 (n = 81), and in Rhineland-Palatinate in 2005 (n = 117) (Fig. 1). Furthermore, virological and
Serological survey
By examination of 1125 sera derived from wild boar shot from 2001 to 2006, antibodies against SIV subtypes H1N1 and H3N2 could be detected in animals from Mecklenburg-Western Pomerania, Brandenburg, Saxony-Anhalt, and Baden-Württemberg as well as Rhineland-Palatinate (Table 1). No wild boar was diagnosed seropositive for SIV H1N2. In addition, the investigated wild boar bagged in Mecklenburg-Western Pomerania in 1997/1998 were free from antibodies against SIV H1N1 and H3N2. The highest
Discussion
Wild boar may be a reservoir for many pathogens including SIV (Markowska-Daniel and Pejsak, 1999, Dedek et al., 1990, Vincente et al., 2002) and therefore are important not only for infections within wild boar but also for domestic pigs. From this point of view it is necessary to know the epidemiological situation regarding SIV in wild boar. Based on the serological survey in the Bundeslaender presented here, it can be considered that SIV is also present in our wild boar populations, even
Acknowledgements
The authors thank Sybilla Welsch and Kathrin Steffen for their excellent technical assistance as well as R. Dürrwald (Impfstoffwerk Dessau-Tornau) and J. Süß (formerly Federal Institute for Risk Assessment, Berlin) for providing virus strains used for HI tests. We gratefully acknowledge hunters for the collection of samples and the cooperation of A. Hänel, Chemical and Veterinary Investigation Institute Baden-Württemberg/Stuttgart, A. Hlinak, State Office Brandenburg/Frankfurt-Oder, L. Mewes,
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