Avian Influenza (Bird Flu)

These viruses can infect several animal species, including birds, pigs, horses, seals and whales. Influenza viruses that infect birds are called “avian influenza viruses.” Birds are an especially important species because all known subtypes of influenza A viruses circulate among wild birds, which are considered the natural hosts for influenza A viruses. Avian influenza viruses do not usually directly infect humans or circulate among humans.

There are 15 known H subtypes. While all subtypes can be found in birds, only 3 subtypes of HA (H1, H2 and H3) and two subtypes of NA (N1 and N2) are known to have circulated widely in humans.

Avian influenza usually does not make wild birds sick, but can make domesticated birds very sick and kill them.

Avian Influenza Infections in Humans

1997 in Hong Kong, avian influenza A (H5N1) infected both chickens and humans. This was the first time an avian influenza virus had ever been found to transmit directly from birds to humans. During this outbreak, 18 people were hospitalized and 6 of them died. To control the outbreak, authorities killed about 1.5 million chickens to remove the source of the virus. Scientists determined that the virus spread primarily from birds to humans, though rare person-to-person infection was noted.