Epidemiology of Influenza A viruses

Natural hosts
Migratory water birds, especially wild ducks, are the natural hosts of influenza A:

• They do not show clinical disease. 
• The virus colonises the intestinal tract and is spread in the faeces
• They act as a reservoir for the infection of other species.

Wild birds introduce the virus onto poultry farms through:

• Direct contact with domestic poultry.
• Faeces – the avian influenza virus survives for a long time in faeces.
• Contamination of drinking water.

In November 2004 the results of a preliminary study on the role of domestic ducks in the spread of avian influenza were made public.
See Role of domestic ducks in the spread of Avian Influenza.

Infection of domestic poultry

Domestic poultry such as chickens and turkeys are extremely susceptible to avian influenza (AI) infection.

Within a farm avian influenza is spread by:

• direct contact,
• contact with contaminated equipment and
  farm staff.

AI is spread between farms by:

• the transfer of infected faeces,
• staff members,
• contaminated equipment and
  delivery trucks
• air transmission over short distances.

Live bird markets also play an important role in the spread of epidemics.