Bird Flu ( Avian Influenza ) - Frequently Asked Questions
Here are the FAQ on Bird Flu ( Avian Influenza ) prepared by WHO
What is avian influenza?
Avian influenza, or “bird flu”, is a contagious disease of animals
caused by viruses that normally infect only birds and, less
commonly, pigs. Avian influenza viruses are highly species-specific,
but have, on rare occasions, crossed the species barrier to infect
In domestic poultry, infection with avian influenza viruses causes
two main forms of disease, distinguished by low and high extremes
of virulence. The so-called “low pathogenic” form commonly causes
only mild symptoms (ruffled feathers, a drop in egg production) and
may easily go undetected. The highly pathogenic form is far more
dramatic. It spreads very rapidly through poultry flocks, causes
disease affecting multiple internal organs, and has a mortality that
can approach 100%, often within 48 hours.
Which viruses cause highly pathogenic disease?
Influenza A viruses1 have 16 H subtypes and 9 N subtypes2. Only
viruses of the H5 and H7 subtypes are known to cause the highly
pathogenic form of the disease. However, not all viruses of the H5
and H7 subtypes are highly pathogenic and not all will cause severe
disease in poultry.
On present understanding, H5 and H7 viruses are introduced to
poultry flocks in their low pathogenic form. When allowed to circulate
in poultry populations, the viruses can mutate, usually within a few
months, into the highly pathogenic form. This is why the presence of
an H5 or H7 virus in poultry is always cause for concern, even when
the initial signs of infection are mild.
Do migratory birds spread highly pathogenic avian influenza
The role of migratory birds in the spread of highly pathogenic avian
influenza is not fully understood. Wild waterfowl are considered the
natural reservoir of all influenza A viruses. They have probably
carried influenza viruses, with no apparent harm, for centuries. They
are known to carry viruses of the H5 and H7 subtypes, but usually in
the low pathogenic form. Considerable circumstantial evidence
suggests that migratory birds can introduce low pathogenic H5 and
H7 viruses to poultry flocks, which then mutate to the highly
In the past, highly pathogenic viruses have been isolated from
migratory birds on very rare occasions involving a few birds, usually
found dead within the flight range of a poultry outbreak. This finding
long suggested that wild waterfowl are not agents for the onward
transmission of these viruses.
Recent events make it likely that some migratory birds are now
directly spreading the H5N1 virus in its highly pathogenic form.
Further spread to new areas is expected.
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