The Disease in Birds: Impact and Control Measures
Avian influenza is caused by Type A strains of the influenza virus. First identified in Italy more than 100 years ago, the disease occurs worldwide. All birds are thought to be susceptible, although some species are more resistant to infection than others. Infection causes a wide spectrum of symptoms, ranging from mild illness to a highly contagious, rapidly fatal disease resulting in severe epidemics. The latter is known as "highly pathogenic avian influenza."
Fifteen subtypes of influenza virus are known to infect birds, thus providing an extensive reservoir of influenza viruses. To date, all outbreaks of the highly pathogenic form have been caused by Influenza A viruses of subtypes H5 and H7.
Migratory waterfowl--most notably wild ducks--are the natural reservoir of avian influenza viruses, and these birds are also the most resistant to infection. Domestic poultry, including chickens and turkeys, are particularly susceptible. Direct or indirect contact of domestic flocks with wild migratory waterfowl has been implicated as a frequent cause of epidemics. Live-bird markets have also played an important role in epidemics.
Recent research has shown that viruses of low pathogenicity can, after circulating in a poultry population for sometimes short periods, mutate into highly pathogenic viruses. During a 1983-1984 epidemic in the United States, the H5N2 …