Avian influenza — known informally as avian flu or bird flu — refers to "influenza caused by viruses adapted to birds."The version with the greatest concern is highly pathogenic avian influenza (HPAI). "Bird flu" is a phrase similar to "swine flu," "dog flu," "horse flu," or "human flu" in that it refers to an illness caused by any of many different strains of influenza viruses that have adapted to a specific host. All known viruses that cause influenza in birds belong to the species influenza A virus. All subtypes (but not all strains of all subtypes) of influenza A virus are adapted to birds, which is why for many purposes avian flu virus is the influenza A virus. (Note, however, that the "A" does not stand for "avian"). Adaptation is not exclusive. Being adapted toward a particular species does not preclude adaptations, or partial adaptations, toward infecting different species. In this way, strains of influenza viruses are adapted to multiple species, though may be preferential toward a particular host. For example, viruses responsible for influenza pandemics are adapted to both humans and birds. Recent influenza research into the genes of the Spanish flu virus shows it to have genes adapted to both birds and humans, with more of its genes from birds than less deadly later pandemic strains. While its most highly pathogenic strain (H5N1) had been spreading throughout Asia since 2003, avian influenza reached Europe in 2005, and the Middle East, as well as Africa, the following year. On January 22, 2012, China reported its second human death due to bird flu in a month following other fatalities in Vietnam and Cambodia. Companion birds in captivity and parrots are highly unlikely to contract the virus, and there has been no report of a companion bird with avian influenza since 2003. 84% of affected bird populations are composed of chicken and farm birds, while the 15% is madeup of wild birds according to capture-and-release operations in the 2000s, during the SARs pandemic. Most human contractions of the avian flu are a result of either handling dead infected birds or from contact with infected fluids. While most wild birds mainly have only a mild form of the H5N1 strain, once domesticated birds such as chickens or turkeys are infected, it could become much more deadly because the birds are often within close contact of one another. There is currently a large threat of this in Asia with infected poultry due to low hygiene conditions and close quarters. Although it is easy for humans to become infected from birds, it's much more difficult to do so from human to human without close and lasting contact. Spreading of H5N1 from Asia to Europe is much more likely caused by both legal and illegal poultry trades than dispersing through wild bird migrations, being that in recent studies, there were no secondary rises in infection in Asia when wild birds migrate south again from their breeding grounds. Instead, the infection patterns followed transportation such as railroads, roads, and country borders, suggesting poultry trade as being much more likely. While there have been strains of avian flu to exist in the United States, such as Texas in 2004, they have been extinguished and have not been known to infect humans.

• published: 25 Dec 2013
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