A review of avian influenza in different bird species

Only type A influenza viruses are known to cause natural infections in bird s, but viruses of all 15 haemagglutinin and all nine neuraminidase influenz a A subtypes in the majority of possible combinations have been isolated fr om avian species. Influenza A viruses infecting poultry can be divided into two distinct groups on the basis of their ability to cause disease. The ve ry virulent viruses cause highly pathogenic avian influenza (HPAI), in whic h mortality may be as high as 100%. These viruses have been restricted to s ubtypes H5 and H7, although not all viruses of these subtypes cause HPAI. A ll other viruses cause a much milder, primarily respiratory disease, which may be exacerbated by other infections or environmental conditions. Since 1 959, primary outbreaks of HPAI in poultry have been reported 17 times (eigh t since 1990), five in turkeys and 12 in chickens. HPAI viruses are rarely isolated from wild birds, but extremely high isolation rates of viruses of low virulence for poultry have been recorded in surveillance studies, givin g overall figures of about 15% for ducks and geese and around 2% for all ot her species. Influenza viruses have been shown to affect all types of domes tic or captive birds in all areas of the world, but the frequency with whic h primary infections occur in any type of bird depends on the degree of con tact there is with feral birds. Secondary spread is usually associated with human involvement, probably by transferring infective faeces from infected to susceptible birds. (C) 2000 Elsevier Science B.V. All rights reserved.