Avian Influenza

Synonyms
Fowl pest, fowl plague, avian influenza A.
Wild birds, especially waterfowl and shorebirds, have long
been a focus for concern by the poultry industry as a source
for influenza infections in poultry. Human health concerns
have also been raised. For these reasons, this chapter has
been included to provide natural resource managers with
basic information about avian influenza viruses.
Cause
Avian influenza is usually an inapparent or nonclinical
viral infection of wild birds that is caused by a group of
viruses known as type A influenzas. These viruses are maintained in wild birds by fecal-oral routes of transmission. This
virus changes rapidly in nature by mixing of its genetic components to form slightly different virus subtypes. Avian influenza is caused by this collection of slightly different
viruses rather than by a single virus type. The virus subtypes
are identified and classified on the basis of two broad types
of antigens, hemagglutinan (H) and neuraminidase (N); 15 H
and 9 N antigens have been identified among all of the known
type A influenzas.
Different combinations of the two antigens appear more
frequently in some groups of birds than others. In waterfowl,
for example, all 9 of the neuraminidase subtypes and 14 of
the 15 hemagglutinin subtypes have been found, and H6 and
H3 are the predominant subtypes. In shorebirds and gulls,
10 different hemagglutinin subtypes and 8 neuraminidase
subtypes have been found. Many of the antigenic combinations of subtypes are unique to shorebirds. H9 and H13 are
the predominate subtypes. More influenza viruses from shorebirds infect waterfowl than chickens. Hemagglutinin subtypes H5 and H7 are associated with virulence or the ability
to cause severe illness and mortality in chickens and turkeys.
However, two viruses with the same subtype antigens can
vary in virulence for domestic birds.
Species Affected
Avian influenza viruses have been found in many bird
species, but are most often found in migratory waterfowl,
especially the mallard duck (Fig. 22.1). However, the only
mortality event known in wild birds killed common terns in
South Africa in 1961. This was the first influenza virus from
marine birds and it was classified as subtype H5N3. Other
wild birds yielding influenza viruses include various species
of shorebirds, gulls, quail, pheasants, and ratites (ostrich and
rhea). Experimental infections of domestic birds with viruses