Avian flu and Human Transmission

Influenza A viruses have infected not only birds but also other animals like, ducks, chickens, pigs, whales, horses, and seals. However, certain subtypes of influenza A virus are specific to certain species, except for birds, which are hosts to all known subtypes of influenza A. Subtypes that have caused widespread illness in people either in the past or currently are H3N2, H2N2, H1N1, and H1N2. H1N1 and H3N2 subtypes also have caused outbreaks in pigs, and H7N7 and H3N8 viruses have caused outbreaks in horses.

Influenza A viruses can cross over from one species to another to cause illness. For example, until 1998 the U.S. pig population was infected by the H1N1 virus. However, in 1998, H3N2 viruses from humans were introduced into the pig population and caused widespread disease among pigs. Most recently, H3N8 viruses from horses have crossed over and caused outbreaks in dogs.

Avian influenza A viruses may be transmitted from animals to humans in two main ways:

The Influenza A virus has eight separate gene segments. Influenza A viruses from different species can mix and create a new influenza A virus if viruses from two different species infect the same person or animal. For example, if a pig were infected with a human influenza A virus and an avian influenza A virus at the same time, the new replicating viruses could mix existing genetic information (re-assortment) and produce a new virus that had most of the genes from the human virus. The resulting new virus might then be able to infect humans and spread from person to person, but it would have surface proteins (hem agglutinin and/or neuraminidase) not previously seen in influenza viruses that infect humans.

This type of drastic change in the influenza A viruses is known as antigenic shift. Antigenic shift results when a new influenza A subtype to which most people have little or no immune protection infects humans. An influenza pandemic can occur if this new virus causes illness in people and can be transmitted easily from person to person.

It is possible that the process of genetic re-assortment could occur in a human who is also infected with avian influenza A virus and a human strain of influenza A virus. The genetic information in these viruses could rearrange to create a new virus with a hemagglutinin from the avian virus and other genes from the human virus. Influenza A viruses with a hem agglutinin against which humans have little or no immunity that have re-assorted with a human influenza virus are more likely to result in sustained human-to-human transmission and pandemic influenza.

The symptoms of avian influenza in humans range from typical influenza-like symptoms (e.g., fever, cough, sore throat, and muscle aches) to eye infections (conjunctivitis), pneumonia, acute respiratory distress, viral pneumonia, and other severe and life-threatening complications.