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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 16 different hemagglutinin
subtypes and 9 different neuraminidase subtypes, all of which
have been found among influenza A viruses in wild birds. 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 strain
of virus. Infection with certain avian influenza A viruses
(for example, some strains of H5 and H7 viruses) can cause
widespread disease and death among some species of wild and
especially domestic birds such as chickens and turkeys.
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
be transmitted easily from person to person, an influenza pandemic
can occur.
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.
For more information see: Avian
Influenza Infections in Humans.
- Page last modified December 17, 2007
- Content Source: Coordinating Center for Infectious Diseases (CCID)
- National Center for Immunization and Respiratory Diseases (NCIRD)
