Avian influenza (AI) is a highly contagious viral infection that 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 are called highly pathogenic avian influenza (HPAI).
The initial infection that does not cause or causes the 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.
Avian influenza in poultry and its impact on the poultry industries
The clinical signs of infection with the 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 soft-shelled eggs are produced.
There may be profuse watery diarrhea, combs and wattles may become blue and respiration may be labored. 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 diarrhea. 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.
Spread of infection
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, the 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 nasal and eye discharges.
Persistence of AI virus
Environmental conditions have a marked effect on virus survival outside the bird. Avian influenza virus can survive for at least 35 days at 4°C in manure and can be isolated from lake water where waterfowl are present. The virus can survive for up to 23 days if refrigerated and for several days in carcasses at ambient temperature. The virus can persist in poultry meat products but is eliminated by cooking