Waterfowl and shorebirds (wild and domesticated) are the major natural reservoir of influenza viruses. Wild waterfowl are asymptomatic, may excrete virus in the feces for long periods, may be infected with more than one subtype, and often do not develop a detectable antibody response. Influenza virus has been recovered directly from lake and pond water utilized by infected wild ducks. Contact of these birds with range-reared commercial flocks is an important factor in some outbreaks. This source of infection often results in a seasonal incidence in some states.
Two other reservoirs worth mention are live bird markets and commercial swine facilities. Live bird markets have existed in large cities forever, but they are an emerging phenomenon in some areas. They serve as a focal point for gathering and housing many species of birds that are then sold in or around large cities. These facilities are usually neither cleaned nor depopulated. The continuous supply of susceptible poultry in such markets enhances opportunity for viral replication and mutation, and this in turn enhances the opportunity for viruses to be carried back to susceptible poultry flocks. Swine have been known to be infected with swine flu (H1N1) since the 1930s, but recently another subtype (H3N2) has been spreading in swine populations. Transmission of influenza from swine to turkeys has been documented.
AI viruses have been isolated from imported exotic birds. These infected birds are a potential threat to cage birds, wild birds, and poultry.
Although waterfowl shed virus for long periods, most viral shedding from infected poultry stops after seroconversion. Influenza virus is released in respiratory secretions and excretions and droppings of infected birds where it is protected by organic material. The virus is labile in warm conditions, but can survive for months in a cold environment. Influenza virus has been isolated from turkey eggs and semen, but there is no evidence of egg transmission. Improper disposal of infected eggs could potentially expose other susceptible birds, but such transmission has not been observed.
Once AI is introduced into the poultry industry it is transmitted from farm to farm by direct and indirect contact. AI viruses can be transmitted on contaminated shoes, clothing, crates, and other equipment and by movement of birds.
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