Influenza A viruses have infected many different animals. 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.
Influenza A viruses normally seen in one species sometimes can cross over and cause illness in another species. For example, until 1998, only H1N1 viruses circulated widely in the US pig population. 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:
· Directly from birds or from avian 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 influenza A virus if viruses from two different species infect the same person or animal. It is possible that the process of genetic reassortment could occur in a human who is co-infected with avian influenza A virus and a human strain of influenza A virus. The genetic information in these viruses could reassort to create a new virus with a hemagglutinin from the avian virus and other genes from the human virus. Theoretically, 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 pandemic influenza. Therefore, careful evaluation of influenza viruses recovered from humans who are infected with avian influenza 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 pig influenza viruses have been reported. These sporadic human infections and outbreaks, however, rarely result in sustained transmission among humans.
