The influenza A virus is capable of infecting many different animals – ducks, chickens, pigs, whales, horses, and seals all have influenza A viruses specific to their species. Birds, however, can become infected with all known influenza A strains that infect animals. Wild birds, especially water birds, who host the virus do not usually get sick, but carry the virus in their gut and shed it in their feces, saliva, and nasal secretions. Susceptible domestic birds can become infected through fecal to oral transmission, or through contact with contaminated secretions, surfaces, water, feed, and people.
Avian influenza outbreaks among poultry occur sporadically throughout the world. In the U.S., more than 16 outbreaks of H5 and H7 influenza have occurred among poultry since 1997. Most outbreaks in poultry are caused by strains that either do not result in illness, or cause mild illness. However, in outbreaks caused by highly pathogenic strains, mortality rates can be as high as 100%. Sometimes influenza A viruses can cross over and cause illness in another species. Avian influenza outbreaks in domestic birds are a concern because of the potential of the virus to evolve into strains that are highly pathogenic, and the possibility that avian influenza could be transmitted to humans (WHO). Sporadic avian and pig influenza infections do occur in humans who are exposed to infected animals. Human influenza virus infection has caused infection in pigs, and strains specific to horses have caused infection in dogs.
The potential exists for influenza A viruses from different species to share genetic information and create a new strain capable of infecting either of the species. If, for example, a pig became infected with human influenza A and avian influenza A viruses at the same time, viral replication in the pig could result in the creation of a new virus with characteristics of both avian and human influenza that could be spread person to person. Alternatively, viral replication in a person infected with both avian and human strains of influenza A virus could result in a new virus containing genetic information from both of the infecting strains. If the new virus had surface proteins (antigens) originating from the avian influenza A virus rather than the human strain, most people would not have any immunity to it because their antibodies recognize surface antibodies of the human influenza A virus rather than the avian strain.
The spread of infection against which there is little or no immune protection could result in an influenza pandemic. Experts believe that an influenza pandemic is closer now than at any time since 1968: however, the current pandemic influenza status is Phase 3: a new influenza virus subtype is causing disease in humans, but is not yet spreading efficiently and consistently among humans (WHO).
To date, there have been no avian influenza A infections in humans in North America. Indonesia continues to be a hotspot with the country’s 69th case of human infection with H5N1 avian influenza virus reported on October 3, 2006. The patient is a 21-year-old woman who developed symptoms on September 19, was hospitalized September 25, and remains hospitalized. Her 11-year-old brother died on September 18 from the complications of H5N1 infection. Although the source of this woman’s illness is still under investigation, poultry deaths occurred before and during her brother’s illness, and it is likely that she was exposed to the poultry as well as to her brother. Of the 69 cases in Indonesia, 52 have been fatal, a mortality rate of 75%.
Further details on the numbers of avian influenza infections in animals and humans can be found at the WHO website:
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