Influenza A virus, together with Influenza B virus, Influenza C virus, Isavirus
and Thogotovirus, are the five genera forming the family Orthomyxoviridae.
Orthomyxoviridae are enveloped, negative-stranded RNA viruses with a segmented
genome. Influenza A viruses can be further categorized into subtypes based on two
surface glycoproteins, the hemagglutinin (HA) and the neuraminidase (NA). To date,
16 different HAs and 9 different NAs have been found and may occur in virtually all
possible combinations. Free-ranging waterbirds are considered to be the natural
reservoir of all influenza A viruses. Even though some mammalian species—
humans, pigs, horses and dogs—harbor their own influenza A viruses, there is strong
evidence that these mammalian viruses originate from avian influenza A viruses. In
addition, avian influenza A viruses may cross the species barrier to other mammals
without developing into a stable lineage in the new host species. An example is
the outbreak of avian influenza virus of the subtype H7N7 in harbor seals in 1979.
Interestingly, the primary replication site for influenza A virus differs between birds
and mammals. In birds, the virus replicates primarily in the intestinal tract, whereas
in mammals, it replicates primarily in the respiratory tract.
An important feature of avian influenza viruses is their capacity to mutate into forms
that cause high mortality in poultry. Such mutations only have been recorded for the
subtypes H5 and H7 and occur after transmission of these viruses to poultry. Based
on their capacity to cause morbidity and mortality in chickens, avian influenza viruses
are classified as low pathogenic avian influenza virus (LPAIV) or high pathogenic
avian influenza virus (HPAIV)