Which influenza viruses cause highly pathogenic disease in poultry?
Influenza A viruses exist in at least 16 H subtypes and 9 N subtypes. Only viruses of the H5 and H7 subtypes are known to cause the highly pathogenic form of the disease. However, not all viruses of the H5 and H7 subtypes are highly pathogenic and not all will cause severe disease in poultry. On present understanding, H5 and H7 viruses are introduced to poultry flocks in their low pathogenic form. When allowed to circulate in poultry populations, the viruses can mutate into the highly pathogenic form. This is why the presence of an H5 or H7 virus in poultry is always cause for concern, even when the initial signs of infection are mild. Under the rules of the World Organisation for Animal Health (OIE, www.oie.int), Member countries must report all instances of H5 or H7 avian influenza in poultry to the international community. What is special about the current global spread of Avian Influenza A/H5N1? The current outbreaks of highly pathogenic avian influenza, which began in South-East A
