Biology Reference
In-Depth Information
form of the virus has yet to be isolated in the Western Hemisphere, it could
arrive via migratory birds or through the importation (legal or illegal) of
infected birds (Kilpatrick et al. 2006).
HPAI H5N1 is an influenza A virus belonging to the Orthomyxoviridae fam-
ily of viruses. Influenza A viruses can be classified by the combination of
two separate surface antigens present on the virion: hemagglutinin (H) and
neuraminidase (N). There are 16 different types of the H antigen and 9 types
of the N antigen, providing for a broad range of possible viral strains. Because
of the large number of different isolates and the interchangeability of these
antigens, influenza A viruses are known to undergo antigenic shift, a pro-
cess in which at least two different strains combine to form a new strain. In
addition to this shift, the viruses undergo antigenic drift, the accumulation
of random mutations resulting in an antigenic change. These processes give
influenza A viruses the ability to mutate rapidly in a relatively short amount
of time, a quality that is central to the concern over HPAI H5N1 becoming
the next influenza pandemic.
Wild aquatic birds are the natural host for nearly all of the influenza A
viruses, though they can occasionally be transmitted to other species. The
natural host often shows no clinical signs of infection but can secrete large
quantities of the virus in its feces for several weeks (Hulse-Post et al. 2005).
Thus, the normal means of bird-to-bird transmission in the wild is through
contact with contaminated water sources. With a highly pathogenic virus
such as HPAI H5N1, the virus can appear in a flock of domestic poultry and
rapidly kill 90% to 100% of the birds (OIE Avian Influenza Fact Sheet 2008).
Bird-to-bird transmission in these cases occurs through direct contact with
feces, respiratory secretions, and contaminated surfaces.
Transmission of HPAI H5N1 to humans appears to depend on close expo-
sure to sick or dead poultry or, in a few cases, exposure to raw poultry prod-
ucts from infected birds (Perdue and Swayne 2007). The majority of the human
cases were reported to have slaughtered or defeathered sick or dead poultry
in the week prior to developing flu-like symptoms. Though a few family
clusters of infections have shown some evidence of human-to-human trans-
mission of the virus, it was limited and nonsustained (Writing Committee
of the WHO Consultation on Human Influenza A/H5 2005, 2008). This is an
important finding as the concern about HPAI H5N1 is that it will mutate to a
form as easily spread from person-to-person as the seasonal flu, which could
spark the beginning of another influenza pandemic. This pandemic would
be of special concern if the mutated influenza isolate retained its high human
mortality rate.
Surveillance for avian influenza in zoological institutions is important
for the continual assessment of the health and safety of the animals in the
collection. Many free-ranging birds, often waterfowl and shorebirds, find
the grounds of zoological institutions to be an advantageous place to live.
Plentiful food availability and protected areas offer the birds an ideal locale,
and interaction between these free-ranging birds and zoo animals in outdoor
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