Department of Agriculture (HDOA)
The current series of outbreaks of Highly Pathogenic Avian Influenza (HPAI) H5N1 in Southeast Asia and other parts of the world can be traced to outbreaks in 1997 in Hong Kong, where it was first documented that humans could become infected with the influenza virus by direct contact with poultry. Since 1997, HPAI H5N1 has spread to about twenty countries and more than 200 million domestic chickens and ducks have died or been destroyed in attempts to control the spread of H5N1 HPAI. As of this date, no HPAI virus has been detected in Hawaii. In addition the Asian strain of H5N1 HPAI has not been detected within the United States.
As of February 2006, about 170 human cases have been confirmed and 92 people have died. Most infected humans regularly handle poultry and, currently, there is no evidence of sustained human-to-human transmission of H5N1.
Influenza A viruses infecting poultry can be divided into two distinct groups on the basis of their ability to cause disease. Severe disease with death rates approaching 100 percent is typical of HPAI viruses. HPAI viruses are almost always either subtypes H5 and H7; however, H5 and H7 viruses may also have low pathogenicity (LPAI). All other avian influenza viruses and H5 and H7 LPAI cause a much milder disease consisting primarily of mild respiratory disease, depression and egg production problems in laying birds.
Spread of avian influenza viruses is related chiefly to the excretion of high concentrations of virus in the feces of infected birds. Usually, LPAI viruses of H5 or H7 subtype are initially introduced into commercial poultry flocks by feral birds. These LPAI viruses may subsequently mutate to HPAI subtypes. Therefore, control measures taken to prevent the introduction of LPAI viruses and prevent their spread on a farm are extremely important.
Concern about wild birds infected with HPAI H5N1 increased when migratory waterfowl began dying in western China in May 2005. Traditionally, waterfowl and shorebirds have been reservoirs for many strains of avian influenza virus, but they rarely fall ill from these viruses. However, the current HPAI H5N1 strain has been shown to be capable of causing fatal infections in over 40 species of wild birds, including geese, ducks, storks, egrets, herons, and falcons, as well as some mammalian species. It is unknown whether waterfowl and shorebird species infected with the current strains of H5N1 will become reservoirs, or if they will consistently become carriers of the virus during migration.
Currently, the implications of HPAI H5N1 for North American wildlife remain unclear. Migratory birds from infected areas visit North America regularly. An important destination for migratory birds is Alaska and enhanced surveillance for HPAI H5N1 is ongoing. If migratory birds carry and shed the virus along flyways, the potential exists for the virus to spread to other areas of North America. While few birds migrate to Hawaii from Asia or Europe, the lesser golden plover regularly migrates from Alaska to Hawaii.
Bird migration is only one of the possible routes of introducing HPAI H5N1 to North America. Birds or other animals that are not themselves susceptible to infection may become infected and spread the virus. Shared water or food may also become contaminated. However, for domestic poultry the main source of secondary spread is man. In several specific accounts of HPAI infections, strong evidence has implicated the movements of caretakers, farm owners and staff, trucks and drivers moving birds or delivering food, and artificial inseminators in the spread of virus both on to and through a farm. Travel by infected people, along with contaminated luggage or clothing, and transportation of infected poultry, including the smuggling of illegal pet birds, and other poultry equipment and products, are additional direct means to transport the virus.
For more information on avian influenza, go to the USDA website: http://www.aphis.usda.gov/newsroom/hot_issues/avian_influenza/avian_influenza.shtml