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Aquatic birds, particularly wild ducks, are primary reservoirs of influenza viruses.
The virus multiplies in the gastrointestinal tract and usually causes no illness.
It is excreted in the feces, and the droppings contaminate the water. Thus, aquatic
birds and domestic fowls become infected in nature by coming contact with
contaminated water. The virus in birds is highly stable antigenically.
Pigs are considered intermediate hosts for influenza viruses. Pigs possess receptors
for avian influenza viruses. Thus, they can be infected readily by coming contact
with infected birds or water contaminated by them. Avian viruses may also spread to
other mammals such as whales, seals and horses and occasionally may cause outbreaks.
Humans are reservoirs of human influenza viruses. Pigs, however, may become infected
with human influenza viruses.
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