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Nature of the disease
Virulent avian influenza (AI) – fowl plague – is a highly contagious fatal disease of poultry. Infection with the virus is accompanied by respiratory, gastro-intestinal and /or nervous signs. Wild water birds such as waterfowl and seabirds are recognised as important reservoir of infection.
OIE List A disease
Susceptible species

AI infects almost all commercial, domestic and wild bird species.

  • chickens and turkeys are highly susceptible to infection and clinical disease
  • ducks and geese are susceptible but only very virulent strains produce disease
  • guinea fowl, quail, pheasant and partridges are susceptible to infection and clinical disease
  • wild birds: viruses can be recovered from a wide range of species but no significant disease problems are known to occur

AI viruses have been found on all continents where research has been carried out. It appears to be endemic in water birds, though disease in these species is rare. Migrating waterfowl are one way that the disease can spread between continents.

Within the region, there have been several outbreaks of virulent AI in Australia, the most recent in 1997.

Clinical signs

Clinical signs are variable and influenced by the virulence of the virus strain, the species affected, age, environmental conditions and presence of pre-existing bacterial infections.

Low pathogenicity strains

  • mild to severe respiratory symptoms (may be confused with infectious laryngotracheitis)
  • egg production may drop by up to 45% and take 2—4 weeks to recover
  • mortality from 3% in caged layers up to 15% in broilers

High pathogenicity strains

  • severe respiratory distress
  • watery eyes and sinuses
  • cyanosis of the combs, wattle and shanks
  • swelling of the head
  • diarrhoea
  • nervous signs
  • sudden death - deaths may commence 24 hours after first signs and can approach 100%

Post-mortem findings

Haemorrhages in various parts of the body are common. The oviducts and intestines often have severe haemorrhages in them. As the disease progresses, pancreas, liver, spleen, kidney and lungs may display yellowish necrotic areas. Small haemorrhages cover the abdominal fat and lining of the body cavity.



Differential diagnosis
  • Newcastle disease
  • Fowl cholera
  • Infectious laryngotracheitis
  • Duck viral enteritis
Specimens required for diagnosis

Samples should be taken from live, clinically affected birds and recently dead birds. Samples should be collected from at least six birds.

Live birds

Cloacal and tracheal swabs, fresh faeces and serum. Swabs should be mixed with transport media. Blood samples from several birds should be collected for serum.

Dead birds

Alimentary tissue samples (proventriculus, intestine, caecal tonsil) and respiratory tissues (lung, trachea) should be collected fresh (not preserved). Impression smears of internal organs can be made.

If delays of more than 48 hours in transit are expected, samples should be frozen.


Direct or indirect contact with migratory water birds is the likely source of infection for poultry. Virus in faeces and water can remain viable for up to 32 days. Transmission between poultry appears to depend on close contact.

AI can spread very rapidly and can be carried over long distances by contaminated materials such as birdcages, pallets, manure and feed.

Risk of introduction
Evidence suggests that waterfowl are the likely source of many AI outbreaks in domestic poultry. The virus can be introduced through close contact with wild birds, or indirectly through untreated water from ponds streams that has been contaminated with bird droppings.

Importation of material, manure and feed from an infected country should also be considered.

Control / vaccines

Vaccines have been used in the past but have not proven very effective. Because of the risk of producing pathogenic strains, live virus vaccines are not appropriate. In the USA inactivated commercial vaccines have been developed. When vaccine is used in an outbreak, the risk that vaccination teams may spread the disease is a concern.

For eradicating the disease, a slaughter policy is preferable to a vaccination policy. Affected flocks should be slaughtered and the carcases buried or burned and the premises cleansed and disinfected.

  • Avian Influenza, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 1308
  • GEERING WA, FORMAN AJ, NUNN MJ, Exotic Diseases of Animals, Aust Gov Publishing Service, Canberra, 1995, 53-60
  • Office International des Epizooties, 2002