Abstract :
Influenza A viruses are enveloped viruses belonging to the family Orthomyxoviridae that
encompasses four more genera: Influenza B, Influenza C, Isavirus and Thogotovirus. Type A
viruses belong to the only genus that is highly infectious to a variety of mammalian and avian
species. They are divided into subtypes based on two surface glycoproteins, the hemagglutinin
(HA) and neuraminidase (NA). So far, 16 HA and 9 NA subtypes have been identified
worldwide, making a possible combination of 144 subtypes between both proteins. Generally,
individual viruses are host-specific, however, interspecies transmission of influenza A viruses is
not uncommon. All of the HA and NA subtypes have been isolated from wild birds; however,
infections in humans and other mammalian species are limited to a few subtypes. The
replication of individual influenza A virus in a specific host is dependent on many factors
including, viral proteins, host system and environmental conditions. In this review, the key
findings that contribute to the transmission of influenza A viruses amongst different species are
summarized.
