Transmission of Avian Influenza A Viruses Between Animals and People

Influenza A viruses have infected many different animals, including 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 viruses. Currently circulating Influenza A subtypes in humans are H3N2 and H1N1 viruses. Examples of different influenza A virus subtypes that have infected animals to cause outbreaks include H1N1 and H3N2 virus infections of pigs, and H7N7 and H3N8 virus infections of horses.

Influenza A viruses that typically infect and transmit among one animal species sometimes can cross over and cause illness in another species. For example, until 1998, only H1N1 viruses circulated widely in the U.S. pig population. However, in 1998, H3N2 viruses from humans were introduced into the pig population and caused widespread disease among pigs. More 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:

• Directly from birds or from avian influenza A virus-contaminated environments to people.
• Through an intermediate host, such as a pig.

Influenza A viruses have eight separate gene segments. The segmented genome allows influenza A viruses from different species to mix and create a new virus if influenza A 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 (reassortment) and produce a new influenza A virus that had most of the genes from the human virus, but a hemagglutinin gene and/or neuraminidase gene and other genes from the avian virus. The resulting new virus might then be able to infect humans and spread easily from person to person, but it would have surface proteins (hemagglutinin and/or neuraminidase) different than those currently found in influenza viruses that infect humans.

This type of major change in the influenza A viruses is known as “antigenic shift.” Antigenic shift results when a new influenza A virus subtype to which most people have little or no immune protection infects humans. If this new influenza A virus causes illness in people and is transmitted easily from person to person in a sustained manner, an influenza pandemic can occur.

It is possible that the process of genetic reassortment could occur in a person who is co-infected with an avian influenza A virus and a human influenza A virus. The genetic information in these viruses could reassort to create a new influenza A virus with a hemagglutinin gene from the avian virus and other genes from the human virus. Influenza A viruses with a hemagglutinin against which humans have little or no immunity that have reassorted with a human influenza virus are more likely to result in sustained human-to-human transmission and pose a major public health threat of pandemic influenza. Therefore, careful evaluation of influenza A viruses recovered from humans who are infected with avian influenza A viruses is very important to identify reassortment if it occurs.

Although it is unusual for people to get influenza virus infections directly from animals, sporadic human infections and outbreaks caused by certain avian influenza A viruses and swine influenza A viruses have been reported.

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