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Avian Influenza (Bird Flu)

Influenza viruses that infect birds are called avian influenza viruses. Only influenza A viruses infect birds, and all known subtypes of influenza A viruses can infect birds. However, there are substantial genetic differences between the subtypes that typically infect both people and birds.

  • Influenza type A viruses can infect people, birds, pigs, horses, and other animals, but wild birds are the natural hosts for these viruses. Influenza type A viruses are divided into subtypes and named on the basis of two proteins on the surface of the virus: hemagglutinin (HA) and neuraminidase (NA). There are substantial genetic differences between the subtypes that typically infect both people and birds. Within subtypes of avian influenza A viruses there also are different strains:
    • Avian influenza A H5 and H7 viruses can be distinguished as “low pathogenic” and “high pathogenic” forms on the basis of genetic features of the virus and the severity of the illness they cause in poultry.
    • Influenza H9 virus has been identified only in a “low pathogenic” form. Each of these three avian influenza A viruses (H5, H7, and H9) theoretically can be partnered with any one of nine neuraminidase surface proteins; thus, there are potentially nine different forms of each subtype (e.g., H5N1, H5N2, H5N3, H5N9).
  • Influenza Type B viruses are usually found only in humans. Unlike influenza A viruses, these viruses are not classified according to subtype. Influenza B viruses can cause morbidity and mortality among humans, but in general are associated with less severe epidemics than influenza A viruses. Although influenza type B viruses can cause human epidemics, they have not caused pandemics.
  • Influenza Type C viruses cause mild illness in humans and do not cause epidemics or pandemics. These viruses are not classified according to subtype.
  • The highest numbers of bird flu cases have been reported in Vietnam and Indonesia. The majority of cases have been reported in children and adults under 40 years of age. Overall mortality is approximately 60 percent and is highest in those 10 to 19 years old.
  • The virus is mainly transmitted to humans by direct contact with live, sick or dead poultry; however, it is thought that a few cases of human-to-human spread have occurred. Due to limited person-to-person transmission, there has not been a widespread epidemic of the bird flu. Human infection with bird flu is expected to continue on a sporadic basis due to contact with birds carrying the disease.
  • Although no evidence for sustained person-to-person transmission of influenza A (H5N1) has been identified, influenza viruses have the capacity to change quickly. When human viruses combine with animal viruses a genetic shift occurs to create a new virus to which humans have little or no immunity. If a novel virus has the ability to spread easily from person to person causing serious illness, then a pandemic will almost always occur.
  • High rates of illness and death could occur worldwide due to the lack of any pre-existing natural immunity in humans or the availability of an effective vaccine. Fortunately, there has been no indication of such a change in the existing strains of the virus. At the current time most experts consider the possibility of pandemic human avian influenza to be small in the short term.
  • While avian flu has shown resistance to the antiviral medications amantadine and rimantadine, the other two drugs in this category (oseltamivir and zanamivir) should still be effective against the current strains of the virus.
  • In April 2007 the U.S. Food and Drug Administration (FDA) approved a vaccine for humans against the H5N1 influenza virus. This vaccine could provide limited protection from an H5N1 pandemic while a more specific inoculation was created. It will not be available for commercial purchase as all amounts are going to the National Stockpile to ensure adequate supplies in the event of an outbreak.
  • Currently the Centers for Disease Control has an embargo on the import of all birds and unprocessed bird products from countries with the virus in domestic poultry. Birds from these affected countries can infect humans with influenza A (H5N1). This order complements a similar action taken by the U.S. Department of Agriculture (USDA).