More details on avian influenza

Causative Agent
Clinical Symptoms
Transmission and Spread

Causative Agent

The virus causing avian influenza is an Influenzavirus A virus of the family Orthomyxoviridae. Several virus subtypes exist, which are divided on the bases of the antigenic relationships in the virus glycoproteins haemagglutinin (H) and neuraminidase (N). At present 15 H subtypes have been recognised (H1-H15) and nine neuraminidase subtypes (N1-N9).

Influenza A viruses infecting poultry can also be divided on the basis of their pathogenicity (ability to cause disease).

The very virulent viruses cause highly pathogenic avian influenza (HPAI) with high mortality in poultry up to 100%. In the whole world there have been only 21 reported primary isolates of such viruses from domestic poultry since 1959. A severe epidemic occurred in Italy in 1999/2000 causing 413 outbreaks with 16 million birds affected.

Other AI viruses cause a much milder disease (low pathogenic avian influenza, LPAI). Clinical signs are much less evident or even absent and mortality is much lower.

Sometimes secondary infections or environmental conditions may cause exacerbation of LPAI infections leading to more serious disease.

Evidence suggests that certain avian influenza virus subtypes of low pathogenicity may, after circulation for some time in a poultry population, mutate into highly pathogenic virus strains.

To date only viruses of H5 and H7 subtype have been shown to cause HPAI in susceptible species, but not all H5 and H7 viruses are highly pathogenic.

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Clinical Symptoms

The main symptoms of HPAI in poultry are depression, loss of appetite, cessation of egg laying, nervous signs, swelling and blue discoloration of combs and wattles due to disturbance of blood circulation, coughing, sneezing and diarrhoea. Sudden death can occur without any previous signs. The mortality rate may reach up to 100% depending on the species, their age, the virus type involved and environmental factors like concurrent bacterial infections.

Clinical signs of LPAI consist primarily of mild respiratory disease, depression and drop in egg production in laying birds.

The incubation period for these viruses range from as short as a few hours to 3 days in individual birds and up to 14 days to spread throughout a flock.

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Transmission and Spread

All the available evidence suggests that the most common primary introduction of both HP and LP AI viruses into an area is by wild birds, usually waterfowl, but gulls and shorebirds have also been implicated. While domestic poultry are generally highly susceptible to the clinical manifestation of the disease, wild birds, and especially waterfowls, are usually naturally resistant and often don't show any clinical sign. Therefore, wild waterfowl represent a natural reservoir for these viruses and can be responsible for the primary introduction of infection into domestic poultry.

Direct contact between wild bird and poultry is not always necessary for introduction of virus into poultry farms, as infected waterfowl may spread AI viruses by infective faeces into an area and these may then be introduced to poultry farms by a variety of mechanisms that may transfer the virus mechanically. If contaminated with influenza viruses, surface water used as drinking water may also be a source of infection. Poultry kept in free range or poultry which have access to surface water are at specific risk.

AI is transmitted within a farm by direct contact of infected animals with healthy animals, or indirect contacts with contaminated equipment or farm staff. Spread of AI viruses from farm to farm is mainly by mechanical transfer of infective faeces, in which virus may be present at high concentrations and may survive. Shared water or food may also become contaminated.

However, man is a very important cause of secondary spread of AI for domestic poultry. Caretakers, farmers, workers, trucks and drivers visiting farms, moving birds or delivering food have caused the spread of AI virus both on to and within farms.

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