Avian influenza (AI) is a highly contagious viral infection which may cause up to 100% mortality in domestic chickens or turkeys. The disease is caused by a virus that belongs to the family Orthomyxoviridae. Influenza viruses have two surface proteins, haemagglutinin and neuraminidase, that determine their subtype and the animal species they infect; there are 16 haemagglutinin and nine neuraminidase types. When AI viruses of two haemagglutinin types, H5 and H7, infect domestic poultry (chickens or turkeys) they often mutate and virulent disease arises in these birds which is called highly pathogenic avian influenza (HPAI). The initial infection that does not cause or causes minimal disease is called low pathogenic avian influenza (LPAI). Wild water birds act as reservoir hosts of influenza viruses, however these viruses generally do not cause disease in these birds.
Birds susceptible to AI
All commercial, domestic and wild bird species are susceptible to infection with AI viruses but disease outbreaks occur more frequently in chickens and turkeys. LPAI viruses are traditionally spread by migratory wild birds. Many species of waterfowl, especially geese, ducks and swans, carry the virus but generally show no signs of disease. The recent HPAI outbreak in eastern Asia has seen the H5N1 virus cause disease and high mortality in ducks, geese, swans and other wild birds.
Avian influenza in poultry and impact on the poultry industries
The clinical signs of infection with HPAI virus are variable and can be affected by the existence of other diseases, the age of the birds, the environment and the severity of the virus itself. In very severe forms the disease appears suddenly and birds die quickly. Some may appear depressed, egg production falls and softshelled eggs are produced. There may be profuse watery diarrhoea, combs and wattles may become blue and respiration may be laboured. With less virulent forms of HPAI, the clinical signs may include decreased egg production, depression, respiratory signs suggestive of a cold, swelling of the face, possibly some nervous signs and diarrhoea. With LPAI, there may be no clinical signs seen following infection or mild signs relating to the respiratory, alimentary or reproductive systems may be seen.
Direct or indirect contact (likely through drinking water) with migratory waterfowl is the most likely source of infection for domestic poultry. Once established in domestic poultry, infection can also spread through contact with contaminated equipment or humans. Transmission through the egg is uncommon, although contamination of the shell does occur. Avian influenza virus is highly concentrated in the manure and in nasal and eye discharges.
