Avian
Influenza Viruses
Influenza
viruses that infect birds are called " avian influenza viruses." Only influenza
A viruses infect birds. All known subtypes of influenza A virus can infect birds.
However, there are substantial genetic differences between the subtypes that typically
infect both people and birds. Within subtypes of avian influenza viruses there
also are different strains (described in "Strains").
Avian
influenza H5 and H7 viruses can be distinguished as "low pathogenic" and "high
pathogenic" forms on the basis of genetic features of the virus and the severity
of the illness they cause in poultry; influenza H9 virus has been identified only
in a "low pathogenicity" form. Each of these three avian influenza viruses (H5,
H7, and H9) can theoretically be partnered with any one of nine neuraminidase
surface proteins; thus, there are potentially nine different forms of each subtype
(e.g., H5N1, H5N2, H5N3,.H5N9).
Below
is summary information about these three prominent subtypes of avian influenza
virus:
Influenza
A H5
- Potentially
nine different subtypes
- Can
be highly pathogenic or low pathogenic
- H5
infections have been documented among humans, sometimes causing severe illness
and death
Influenza
A H7
- Potentially
nine different subtypes
- Can
be highly pathogenic or low pathogenic
- H7
infection in humans is rare, but can occur among persons who have close contact
with infected birds; symptoms may include conjunctivitis and/or upper respiratory
symptoms
Influenza
A H9
- Potentially
nine different subtypes
- Documented
only in low pathogenic form
- Three
H9 infections in humans have been confirmed.
Centers
for Diseases Control and Prevention
http://www.cdc.gov/
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