Human Infection with Avian Influenza Viruses
"Human influenza virus"
usually refers to those subtypes that spread widely among humans. There are only
four known A subtypes of influenza viruses (H1N1, H1N2, H3N2, and H7N2) currently
circulating among humans. It is likely that some genetic parts of current human
influenza A viruses originally came from birds. Influenza A viruses are constantly
changing, and other strains might adapt over time to infect and spread among humans.
The risk from avian influenza
is generally low to most people, because the viruses do not usually infect humans.
H5N1 is one of the few avian influenza viruses to have crossed the species barrier
to infect humans, and it is the most deadly of those that have crossed the barrier.
Most cases of H5N1 influenza
infection in humans have resulted from contact with infected poultry (e.g., domesticated
chicken, ducks, and turkeys) or surfaces contaminated with secretion/excretions
from infected birds.
So far, the spread of H5N1
virus from person to person has been limited and has not continued beyond one person.
Nonetheless, because all influenza viruses have the ability to change, scientists
are concerned that H5N1 virus one day could be able to infect humans and spread
easily from one person to another.
In the current outbreaks
in Asia, Europe,
and Africa, more than half of those infected with the H5N1 virus have died.
Most cases have occurred in previously healthy children and young adults.
However, it is possible that the only cases currently being reported are those in
the most severely ill people, and that the full range of illness caused by the H5N1
virus has not yet been defined.
Symptoms of avian influenza
in humans have ranged from typical human influenza-like symptoms (e.g., fever, cough,
sore throat, and muscle aches) to eye infections, pneumonia, severe respiratory
diseases (such as acute respiratory distress), and other severe and life-threatening
complications. The symptoms of avian influenza may depend on which virus caused
the infection.
Because these viruses do
not commonly infect humans, there is
little or no immune protection against them
in the human population. If H5N1 virus were to gain the capacity to spread easily
from person to person, a pandemic (worldwide outbreak of disease) could begin. No
one can predict when a pandemic might occur. However, experts from around the world
are watching the H5N1 situation very closely and are preparing for the possibility
that the virus may begin to spread more easily and widely from person to person.
For the most current information
about avian influenza and cumulative case numbers, see the world map on this site's
home page.
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