Epsilon Open Archive

Abstract

Avian influenza viruses, especially highly pathogenic avian influenza viruses (HPAIV), affect a wide range of species, including humans and have thus become a major concern for veterinary medicine and public health. A HPAIV-H5N1 belonging to clade 2.2, originally from South East Asia, spread across Eurasia and reached Sweden in 2006. Currently the most commonly isolated HPAIV-H5N1 from wild birds belong to clade 2.3.2. There is a growing concern that the H5N1 virus has evolved in such a way that it can be maintained in the wild bird population without causing severe disease. At the same time the role of natural hosts, such as mallards (Anas platyrhynchos), in the epidemiology of avian influenza is an ongoing concern. In order to characterize the natural disease in free ranging birds in Sweden and to assess the pathogenicity of clade 2.3.2 viruses, histopathology, polymerase chain reaction, virus isolation and immunohistochemistry (IHC) were used to investigate lesions and viral tissue targeting of HPAIV-H5N1 in naturally infected tufted ducks (Aythya fuligula) and in tufted ducks experimentally infected with a clade 2.3.2 virus. Since neurotropism is a key feature of HPAIV-H5N1 infection, the encephalitis in 9 wild bird species from the Swedish outbreak was characterized in more detail. Results were compared to mallards infected with a low pathogenic avian influenza virus H1N1. The studies highlight the range and variation of the presentation of the natural disease in wild birds. Experimentally infected ducks were highly susceptible to the current HPAIV-H5N1 clade and showed similar lesions and viral antigen distribution as the naturally infected ducks. The studies suggest that there are several routes of infection and dissemination of the virus including, respiratory, hematogenous and olfactory routes. The respiratory tract is probably the main route of excretion of HPAIV-H5N1 since no viral antigen was found in the intestine. This was in contrast to the experimentally infected mallards which had primarily intestinal replication with minimal lesions. The results highlight the importance of continued investigation of the pathobiology of both low- and HPAIV infections in wild birds which is essential in the understanding of their epidemiology and, in turn, can contribute to the design and implementation of preventive and control measures to protect the health of humans and animals.