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).
Overview
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
ustralia has developed industry plans (called biosecurity plans) to prevent avian influenza viruses gaining entry to commercial poultry flocks. The plans are aimed at limiting possible contact between wild birds and domestic poultry through contaminated water and food supplies and transfer of infection by the mechanical movement of infection on fomites such as on the clothing and footwear of persons and on equipment, containers, vehicles etc. Treatment of surface water by chlorination to inactivate the virus is essential if it is to be supplied to poultry and aviary birds
Generally, outbreaks of AI infections in poultry or wild birds have not been associated with cases of human influenza. However cases of human influenza due to H5 and H7 subtype HPAI viruses have been recorded in a few humans associated with recent poultry outbreaks in Europe (2003), eastern Asia (1997–2006) and western Asia (2006).
Human pandemic influenza occurs irregularly, having occurred in 1919, 1957, and 1968 when strains of influenza virus with a novel haemagglutinin type adapted to humans and spread rapidly around the world. It is feared that another human pandemic will arise from the current AI outbreak in eastern Asia. Illness and death have been associated with outbreaks of HPAI viruses of both the H5 and H7 subtypes, particularly the H5N1 virus in eastern Asia. The pandemic of human influenza will not occur until there have been mutations in the poultry H5N1 virus or its recombination with a human influenza virus that will allow the resulting virus to easily infect and transmit between humans. To this time, contact with live infected birds or their manure has been necessary for infection to establish in humans and few human cases have infected other humans in contact with them. A pandemic cannot commence without an AI virus acquiring the ability to easily infect and spread between humans. Avian influenza in birds and human pandemic influenza can be expected to be caused by genetically different viruses.
The recognised source of AI infection for humans is direct contact with infected live birds or their manure, so poultry products are relatively safe. Handling raw poultry meat products or eggs and eating cooked poultry products have not been recorded as causing infection. Cooking poultry products in the normal way destroys AI viruses
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.
