Influenza, avian: Also called fowl plague, avian flu, and bird flu. A highly contagious viral disease with up to 100% mortality in domestic fowl. Caused by influenza A virus subtypes H5 and H7. All types of birds are susceptible to the virus but outbreaks occur most often in chickens and turkeys. The infection may be brought by migratory wild birds which can carry the virus but show no signs of disease. Humans are only rarely affected.
Type A influenza viruses can infect several animal species aside from birds, including pigs, horses, seals and whales. Birds are an especially important species because all known subtypes of influenza A viruses circulate among wild birds, which are the natural hosts for influenza A viruses. Avian influenza viruses do not usually directly infect humans or circulate among humans.
Influenza A viruses can be divided into subtypes on the basis of their surface proteins - hemagglutinin (HA) and neuraminidase (NA). 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.
The symptoms in humans range from typical influenza-like symptoms (with fever, cough, sore throat and muscle aches) to eye infections, pneumonia, acute respiratory distress, viral pneumonia, and other severe and life-threatening complications.
All influenza viruses have the potential to change. It is possible that an avian influenza virus could change so that it could infect humans and could spread easily from person to person. Because these viruses do not commonly infect humans, there is little or no immune protection against them in the human population. If an avian virus were able to infect people and gain the ability to spread easily from person to person, an influenza pandemic, a global outbreak, could begin.
Past influenza pandemics have led to high levels of illness, death, social disruption and economic loss. There were 3 pandemics in the 20th century; the Spanish flu (1918-19), Asian flu (1957-58), and Hong Kong flu (1968-69). All of them spread worldwide within a year of being detected.