Avian
influenza or 'bird flu' is a highly contagious disease of birds, caused by influenza A viruses. In birds, the viruses
can present with a range of symptoms from mild
illness and low mortality to a highly contagious disease with a near 100% fatality rate.
The bird flu virus currently affecting poultry and some people in Asia and other areas is the highly pathogenic H5N1 strain of the virus.
As the virus can remain viable in contaminated droppings for long periods, it can be spread among birds, and from birds to other animals, through ingestion or inhalation.
All bird species are thought to be susceptible to avian
influenza. Migratory birds such as wild ducks and geese can carry the viruses, often without any symptoms of illness, and show the greatest resistance to infection. Domestic poultry flocks, however, are particularly vulnerable to epidemics of a rapid, severe and fatal form of the disease.
There are many different subtypes of influenza A virus. The most virulent are called highly pathogenic avian influenza (HPAI) and can reach epidemic levels among birds. Of these, subtype H5, and more particularly subtype H5N1 currently, pose the greatest concern for human health. Two other subtypes - H9 and H7 - have caused illness in people but neither has caused outbreaks in poultry as severe as H5N1-related ones.
According to the World Health Organization (WHO), there is mounting evidence that the H5N1 strain has a unique capacity to jump the species barrier and cause severe disease, with high mortality, in people.