 |
 |
|
While some subtypes of influenza A -- those including the H1, H2, or H3 surface proteins -- are common in human beings, other types, including H5, H7, and H9 are thought to be primarily diseases of aquatic birds such as ducks, geese, and swans. Other subtypes affect a wide range of animals, including horses, pigs, and whales. Following antigenic shifts, animal subtypes of the flu can occasionally cross over to humans. Such illnesses are known as "zoonotic" diseases.
Wild waterfowl -- ducks in particular -- serve as the flu's natural "reservoir," the place in nature where the virus exists between outbreaks in other species. There appears to be significant species-hopping among the flu subtypes that infect birds, and animal domestication, by bringing species together in close proximity, has provided a perfect environment for such species crossovers to occur -- and to take hold among a closely-packed captive animal population.
In most cases, avian flus -- among them some of the most destructive flu viruses -- are considered unable to infect human beings directly. Generally, it has believed that a third, intermediate animal host must serve as a mixing chamber. Pigs, which can become infected with both avian and human flu viruses, have been considered the most likely candidate; the thinking being that if a pig suffers a double infection with two flu viruses, the viruses can reassort, with the emergent new virus exhibiting both avian virulency and an ability to infect humans.
Credit: Micah Fink
|
|
|
|
|
|
|
 |
|
Briefing
Read a briefing on the dangers posed by avian influeza and the challenges nations must overcome in order to fight against it.
Interactive Map
Explore how avian flu is spreading across Asia.
Filmmaker Notes
Go behind the scenes with producer Micah Fink.
Info-Graphic
Learn how chemical and biological weapons work.
Interactive Challenge
Test your knowledge of the HIV/AIDS pandemic.
|
|
|
|
|
|
