Disease and cause:
Virulent avian influenza is a highly contagious viral disease, which may cause up to 100% mortality in domestic fowls. The disease is caused by a virus belonging to the family Orthomyxoviridae.

Species affected : All commercial, domestic and wild bird species are susceptible but disease outbreaks occur more frequently in chickens and turkeys. Infection may be brought into a country by migratory wild birds. Many species of waterfowl, especially geese, ducks and swans carry the virus but generally show no signs of disease. Generally, humans are not affected, but an outbreak of influenza in humans was associated with an avian source in Hong Kong (January 1998).

Distribution
Avian influenza viruses are probably ubiquitous throughout the world in wild waterbirds, with outbreaks of disease occurring as sporadic events.

Key signs
The clinical signs 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 produced. There may be a profuse watery diarrhoea, combs and wattles may become blue and respiration may be laboured. In less severe forms, 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.

Spread
Direct or indirect contact (likely through drinking water) with migratory waterfowl is the most likely source of infection in poultry. Spread can also occur through contact with contaminated equipment or humans. Transmission through the egg is not known to occur, although contamination of the shell does occur. The virus is highly concentrated in the manure and in nasal and eye discharges.

Persistence of the 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. The virus can be isolated from lake water where waterfowl are present. The virus can survive several days in carcases at ambient temperature and up to 23 days if refrigerated. Virus can persist in poultry meat products but is eliminated by adequate heating.

Control strategy
The strategy is to eradicate the virulent disease by:

immediate stamping out and disposal of infected and in-contact birds to remove the major source of virus
strict quarantine and movement controls to prevent the spread of infection
decontamination to remove and reduce the virus
tracing and surveillance to locate the source of infection, locate other infected premises and determine the extent of the infection
zoning to define infected and disease-free areas.

Vaccination is not an option

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