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Transmission of Influenza Viruses from Animals to People

Influenza A viruses are found in many different animals, including ducks, chickens, pigs, whales, horses and seals.

Influenza B viruses circulate widely only among humans.

Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: the hemagglutinin (H) and the neuraminidase (N). There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes. All known subtypes of influenza A viruses have been found among birds, except subtype H17N10 and H18N11 which have only been found in bats. Below is a table showing the different hemagglutinin and neuraminidase subtypes and the species in which they have been detected.

Wild birds are the primary natural reservoir for all subtypes of influenza A viruses and are thought to be the source of influenza A viruses in all other animals. Most influenza viruses cause asymptomatic or mild infection in birds; however, the range of symptoms in birds varies greatly depending on the properties of the virus. Infection with certain avian influenza A viruses (for example, some H5 and H7 viruses) can cause widespread, severe disease and death among some species of wild and especially domestic birds such as chickens and turkeys. (Viruses that cause severe illness and death in birds or poultry are called “highly pathogenic.”

Pigs can be infected with both human and avian influenza viruses in addition to swine influenza viruses. Infected pigs get symptoms similar to humans, such as cough, fever and runny nose. Because pigs are susceptible to avian, human and swine influenza viruses, they potentially may be infected with influenza viruses from different species (e.g., ducks and humans) at the same time. If this happens, it is possible for the genes of these viruses to mix and create a new virus.

For example, if a pig were infected with a human influenza virus and an avian influenza virus at the same time, the viruses could mix (reassort) and produce a new virus that had most of the genes from the human virus, but a hemagglutinin and/or neuraminidase from the avian virus. The resulting new virus would likely be able to infect humans and spread from person to person, but it would have surface proteins (hemagglutinin and/or neuraminidase) not previously seen 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 subtype to which most people have little or no immune protection infects humans. If this new virus causes illness in people and can spread easily from person to person, an influenza pandemic can occur.

This diagram depicts the origins of the H7N9 virus from China and shows how the virus's genes came from other influenza viruses in birds. Another example of reassortment is the H7N9 virus. The eight genes of the H7N9 virus are closely related to avian influenza viruses found in domestic ducks, wild birds and domestic poultry in Asia. Experts think multiple reassortment events led to the creation of the H7N9 virus. These events may have occurred in habitats shared by wild and domestic birds and/or in live bird/poultry markets, where different species of birds are bought and sold for food. As this diagram shows, the H7N9 virus likely obtained its HA (hemagglutinin) gene from domestic ducks, its NA (neuraminidase) gene from wild birds, and its six remaining genes from multiple related H9N2 influenza viruses in domestic poultry.

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

Influenza A Subtypes: Species Affected

Hemagglutinin

SubTypePeoplePoultryPigsBats / Other
H1Affects PeopleAffects poultryAffects pigs 
H2Affects PeopleAffects poultryAffects pigs 
H3Affects PeopleAffects poultryAffects pigs Other Animals
H4 Affects poultryAffects pigs Other Animals
H5Affects PeopleAffects poultryAffects pigs 
H6Affects PeopleAffects poultry  
H7Affects PeopleAffects poultry  Other Animals
H8 Affects poultry  
H9Affects PeopleAffects poultryAffects pigs 
H10Affects PeopleAffects poultry  
H11 Affects poultry  
H12 Affects poultry  
H13 Affects poultry  
H14 Affects poultry  
H15 Affects poultry  
H16 Affects poultry  
H17   Affects bats
H18   Affects bats

Neuraminidase

SubTypePeoplePoultryPigsBats / Other
N1Affects PeopleAffects poultryAffects pigs 
N2Affects PeopleAffects poultryAffects pigs 
N3 Affects poultry  
N4 Affects poultry  
N5 Affects poultry  
N6Affects PeopleAffects poultry  
N7Affects PeopleAffects poultry  Other Animals
N8Affects PeopleAffects poultry  Other Animals
N9Affects PeopleAffects poultry  
N10   Affects bats
N11   Affects bats

For more information about other influenza viruses, please go to Other Flu Web Sites.

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