Transmission
of Influenza A Viruses
Between Animals and People
Influenza
A viruses are found in 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 subtypes
of influenza A. Subtypes that have caused widespread illness in people either
in the past or the current period are H3N2, H2N2, H1N1, and H1N2. H1N1 and H3N2
subtypes have caused outbreaks in pigs and H7N7 and H3N8 viruses have caused outbreaks
in horses.
Influenza
A viruses normally seen in one species sometimes can cross over and cause illness
in another species. For example, up 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.
Avian
influenza viruses may be transmitted 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
viruses have eight separate gene segments. The segmented genome allows 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. For example, if a pig
were infected with a human influenza virus and an avian influenza virus at the
same time, the viruses could 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 might then 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.
It
also is possible that the process of reassortment could occur in a human. For
example, a person could be infected with avian influenza and a human strain of
influenza at the same time. These viruses could reassort to create a new virus
that had 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.
Thus, careful evaluation of influenza viruses recovered from humans who are infected
with avian influenza is very important to identify reassortment if it occurs.
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 and
pig influenza viruses have been reported. (For more information see Avian
Influenza Infections in Humans.) These sporadic human infections and
outbreaks, however, rarely result in sustained transmission among humans. Centers
for Diseases Control and Prevention
http://www.cdc.gov/
May 24, 2005
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