Infected birds shed avian influenza virus in their saliva, nasal secretions, and feces. Domestic poultry become infected with the virus through direct contact with infected waterfowl or other infected poultry, or through contact with contaminated surfaces and material.
According to the Food and Agriculture Organization (FAO) of the United Nations, wild waterfowl play an important role in the avian influenza infection cycle because they are the natural reservoirs for avian influenza viruses. The virus passes on through contact with resident waterfowl or domestic poultry, particularly domestic ducks.
As the virus circulates within the domestic and possibly resident wild bird populations, it can mutate in new forms of the virus. Low pathogenic forms of the virus, particularly H5 and H7 subtypes, can mutate into highly pathogenic avian influenza (HPAI). The newly formed virus is pathogenic to poultry and possibly to the wild birds from which it arose.
Avian influenza virus can remain infective for long periods of time at moderate temperatures, and can survive indefinitely in frozen material. As a result, the disease can be spread through improper disposal of infected carcasses, litter, manure, or poultry by-products. Avian influenza viruses can also be transmitted via contaminated shoes, clothing, crates, egg flats, egg cases, vehicles, and other equipment. In addition, wild animals (scavengers), insects, and rodents may mechanically carry the virus from infected to susceptible poultry.
Live poultry markets may serve as virus reservoirs, thus perpetuating a cycle of infection due to the various species and sources of birds sold in such markets. Poor biosecurity and sanitation practices in some of these markets contribute to the survival and propagation of avian influenza viruses which may re-infect supply flocks and commercial poultry.