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- H5N1 Avian Influenza Virus
H5N1 Avian Influenza Virus
Although avian influenza viruses usually do not infect humans,
more than 100 confirmed cases of human infection with avian
influenza viruses have been reported since 1997. For example,
Health Organization maintains situation updates and cumulative
reports of human cases of avian influenza A (H5N1). Most cases
of avian influenza infection in humans are thought to have
resulted from direct contact with infected poultry or contaminated
surfaces. However, there is still a lot to learn about how
different subtypes and strains of avian influenza virus might
affect humans. For example, it is not known how the distinction
between low pathogenic and highly pathogenic strains might
impact the health risk to humans. (For more information, see
"Low Pathogenic versus Highly Pathogenic Avian Influenza
Viruses" on the CDC Influenza
Viruses Web page.
Because of concerns about the potential for more widespread
infection in the human population, public health authorities
closely monitor outbreaks of human illness associated with
avian influenza. To date, human infections with avian influenza
A viruses detected since 1997 have not resulted in sustained
human-to-human transmission. However, because influenza A
viruses have the potential to change and gain the ability
to spread easily between people, monitoring for human infection
and person-to-person transmission is important.
Symptoms of Avian Influenza in Humans
The reported symptoms of avian influenza in humans have ranged
from typical influenza-like symptoms (e.g., fever, cough,
sore throat, and muscle aches) to eye infections (conjunctivitis),
pneumonia, acute respiratory distress, viral pneumonia, and
other severe and life-threatening complications.
Antiviral Agents for Influenza
Four different influenza antiviral drugs (tamiflu,
and zanamivir) are approved by the U.S. Food and Drug Administration
(FDA) for the treatment of influenza; three are approved for
prophylaxis. All four have activity against influenza A viruses.
However, sometimes influenza strains can become resistant
to these drugs, and therefore the drugs may not always be
effective. For example, analyses of some of the 2004 H5N1
viruses isolated from poultry and humans in Asia have shown
that the viruses are resistant to two of the medications (amantadine
and rimantadine). Monitoring of avian influenza A viruses
for resistance to influenza antiviral medications is ongoing.
Instances of Avian Influenza Infections
Confirmed instances of avian influenza viruses infecting humans
since 1997 include:
- H5N1, Hong Kong, Special Administrative Region, 1997:
Highly pathogenic avian influenza A (H5N1) infections occurred
in both poultry and humans. This was the first time an avian
influenza A virus transmission directly from birds to humans
had been found.
- H9N2, China and Hong Kong, Special Administrative Region,
1999: Low pathogenic avian influenza A (H9N2) virus infection
was confirmed in two children and resulted in uncomplicated
influenza-like illness. Both patients recovered, and no
additional cases were confirmed. The source is unknown,
but the evidence suggested that poultry was the source of
infection and the main mode of transmission was from bird
- H7N2, Virginia, 2002: Following an outbreak of H7N2 among
poultry in the Shenandoah Valley poultry production area,
one person was found to have serologic evidence of infection
- H5N1, China and Hong Kong, Special Administrative Region,
2003: Two cases of highly pathogenic avian influenza A (H5N1)
infection occurred among members of a Hong Kong family that
had traveled to China. One person recovered, the other died.
How or where these two family members were infected was
not determined. Another family member died of a respiratory
illness in China, but no testing was done.
- H7N7, Netherlands, 2003: The Netherlands reported outbreaks
of influenza A (H7N7) in poultry on several farms. Later,
infections were reported among pigs and humans. In total,
89 people were confirmed to have H7N7 influenza virus infection
associated with this poultry outbreak. These cases occurred
mostly among poultry workers.
- H9N2, Hong Kong, Special Administrative Region, 2003:
Low pathogenic avian influenza A (H9N2) infection was confirmed
in a child in Hong Kong. The child was hospitalized and
- H7N2, New York, 2003: In November 2003, a patient with
serious underlying medical conditions was admitted to a
hospital in New York with respiratory symptoms. One of the
initial laboratory tests identified an influenza A virus
that was thought to be H1N1. The patient recovered and went
home after a few weeks. Subsequent confirmatory tests conducted
in March 2004 showed that the patient had been infected
with avian influenza A (H7N2) virus.
- H7N3 in Canada, 2004: In February 2004, human infections
of highly pathogenic avian influenza A (H7N3) among poultry
workers were associated with an H7N3 outbreak among poultry.
The H7N3-associated, mild illnesses consisted of eye infections.
- H5N1, Thailand and Vietnam, 2004, and other outbreaks
in Asia during 2004 and 2005: In January 2004, outbreaks
of highly pathogenic influenza A (H5N1) in Asia were first
reported by the World Health Organization. Visit the Avian
Influenza section of the World Health Organization Web site
for more information and updates.