| AnimalsRts@hotmail.com 2005-07-09, 5:53 pm |
| 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:
=B7 Directly from birds or from avian virus-contaminated environments
to people.
=B7 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.
http://www.cdc.gov/flu/avian/gen-in...-flu-humans.htm
|