Avian influenza viruses are highly contagious, extremely variable viruses that are widespread in birds. Wild birds in aquatic habitats are thought to be their natural reservoir hosts, but domesticated poultry and other birds can also be infected. Most viruses cause only mild disease in poultry, and are called low pathogenic avian influenza (LPAI) viruses. Highly pathogenic avian influenza (HPAI) viruses can develop from certain LPAI viruses, usually while they are circulating in poultry flocks. HPAI viruses can kill up to 90-100% of the flock, and cause epidemics that may spread rapidly, devastate the poultry industry and result in severe trade restrictions. In poultry, the presence of LPAI viruses capable of evolving into HPAI viruses can also affect international trade.
Avian influenza viruses can occasionally affect mammals, including humans, usually after close contact with infected poultry. While infections in people are often limited to conjunctivitis or mild respiratory disease, some viruses can cause severe illness. In particular, Asian lineage H5N1 HPAI viruses have caused rare but lifethreatening infections, now totaling nearly 850 laboratory-confirmed cases since 1997, and H7N9 LPAI viruses have caused more than 600 serious human illnesses in China since 2013. Avian influenza viruses can also infect other species of mammals, sometimes causing severe or fatal disease. In rare cases, avian influenza viruses can become adapted to circulate in a mammalian species. During the last century, such viruses have caused or contributed to at least three pandemics in humans, contributed to the diversity of swine influenza viruses in pigs, and also produced one of the two canine influenza viruses now circulating among dogs.
Iowa State University
Iowa State University Center for Food Security and Public Health, "Avian Influenza" (2016). Center for Food Security and Public Health Technical Factsheets. 10.