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Influenza as a zoonotic disease |
Influenza in birds:
The nature of influenza virus infections in birds is dependent
upon 2 major factors- the species of bird and the pathogenicity
of the virus involved.
- Clinical disease develops following infection in chickens
and turkeys, whereas infections in ducks and most other waterfowl
are subclinical.
- Typical respiratory disease (lethargy, inappetence, nasal
discharge, sneezing) and reductions in egg production can develop
in poultry infected with influenza viruses of relatively low
pathogenicity. In contrast, highly fatal, systemic disease ("fowl
plague") characterized by edema, hemorrhages, cyanosis
of combs and wattles and infection of the CNS can occur in poultry
infected with so-called "highly pathogenic"
influenza viruses.
- These viruses are always of the H5 or H7 subtypes, but not
all H5 and H7 viruses are highly pathogenic. The highly pathogenic
viruses are characterized by an HA protein with a cleavage site
containing multiple basic amino acids, which makes the protein
cleavable by proteases ubiquitously present throughout the body
rather than only those present at mucosal surfaces.
Zoonotic avian influenza viruses:
Until recently, the species barrier for influenza viruses between
humans and birds was thought to be quite strong. However, several
pieces of evidence point to the fact that avian influenza viruses
can, under certain circumstances, be directly infectious for people
without passing through pigs as an intermediate adaptation host.
(Unfortunately, the virologic and/or epidemiologic attributes
that allow for these infections of humans remain largely undetermined.)
Examples of zoonotic avian influenza virus infections include
the following:
- There is published serologic evidence of infection of people
in SE Asia with several avian influenza virus subtypes.
- Conjunctivitis developed in a researcher who was exposed
to an influenza virus from a seal. This virus was subsequently
shown to be an avian-lineage influenza virus.
- A very unique outbreak of influenza occurred in Hong Kong
in 1997. It was unique because it involved an H5N1 avian influenza
virus, because infection in people was epidemiologically linked
to direct contact with infected chickens in the live-bird markets,
and because of the severity of the disease induced. In fact,
6 of 18 infected people died.
- This was the first evidence of severe disease in humans due
to direct infection with an avian influenza virus. This outbreak
lead to the destruction of ~1.7 million poultry in Hong Kong
and changes in the importation regulations for poultry arriving
in Honk Kong from the mainland.
- Highly pathogenic H5N1 avian influenza viruses spread throughout
Asia in 2003-2004, infecting both birds and human beings.
- In 1999, another avian influenza virus (H9N2) infected several
people in China and Hong Kong, but this virus induced only routine
signs of influenza and was not associated with mortality.
- In 2003, a highly pathogenic H7N7 virus infected both poultry
and human beings in The Netherlands, with one human death (a
veterinarian) but dozens of cases of conjunctivis and/or influenza-like
respiratory illness.
