Submitted to: American Association of Veterinary Laboratory Diagnosticians
Publication Type: Proceedings
Publication Acceptance Date: September 15, 2006
Publication Date: October 12, 2006
Citation: Pantin Jackwood, M.J. 2006. Pathobiology of avian influenza viruses. In: Proceedings of the American Association of Veterinary Laboratory Diagnosticians 49th Annual Conference, October 12-18, 2006, Minneapolis, Minnesota. p.24. Technical Abstract: Avian influenza virus causes serious disease in a wide variety of birds and mammals. Its natural hosts are wild aquatic birds, in which most infections are unapparent. Avian Influenza (AI) viruses are classified into 16 hemagglutinin (H1-16) and nine neuraminidase (N1-9) subtypes. Each virus has one H and one N antigen, apparently in any combination. Influenza viruses infecting poultry can be divided into two distinct groups based on their ability to cause disease in the major poultry species, chickens. The very virulent viruses cause highly pathogenic avian influenza (HPAI), in which mortality can be as high as 100%. These viruses have been restricted to subtypes H5 and H7, although not all of these subtypes can cause HPAI. Infections caused by these subtype viruses have been responsible for devastating epidemics in poultry. All other AI viruses can cause a much milder, primarily respiratory disease, and is referred to as low pathogenic AI (LPAI). Experimentally, HPAI viruses typically produce a similar severe, systemic disease with high mortality in chickens and other galliforme birds. However, these same viruses usually produce no infection or only mild disease in domestic ducks. Over the past decade, the emergent HPAI viruses have shifted to increased virulence for chickens as evident by shorter mean death times. Furthermore, the Asian H5N1 HPAI viruses have changed from producing inconsistent respiratory infections in ducks, to some strains being highly lethal with virus found in internal organs including the brain. Natural infection with HPAI virus in wild birds had been rare, until recently, when HPAI H5N1 outbreaks in wild birds have been reported in Asia, Europe, Near East and Africa. Across all bird species, the ability to produce severe disease and death is associated with high virus replication titers in the host, especially in specific tissues such as brain and heart. Periodically, avian influenza viruses are transmitted to other hosts, including mammals, causing transitory infections and occasionally deaths. A small number of mammalian species, including pigs, seals, whales, minks, cats, tigers, leopards, and humans, are susceptible to natural infection with influenza viruses of purely avian genetic makeup. Transmission of avian influenza to humans has occurred with AI subtypes H5N1, H9N2, H7N7, H7N3, and H7N2, causing a spectrum of clinical disease, from conjunctivitis or mild respiratory infections to severe and fatal disease. The ongoing outbreak of H5N1 AI virus is of great concern because of the high human case fatality rate and the threat of a new influenza pandemic. Mammalian models, including mice, ferrets, pigs, primates and cats, are being used to evaluate the potential of AI viruses to cross the species barriers and infect domestic animal species and humans. AI viruses in these species have shown to be either highly pathogenic, replicating systemically (mice, ferrets, cats), or of low pathogenicity and replicating only in the respiratory tract (mice, pigs, primates), indicating that the outcome of infection with AI viruses is host and virus dependent.