Influenza A viruses, members of the Orthomyxoviridae
family, can infect a variety of animals although their natural hosts are aquatic birds [1
]. Although the host range barriers are generally strong, a hallmark of influenza A virus epidemiology is the transmission of the virus between species. These interspecies transmissions generally result in a transient infection characterized by limited spread, whereas, on occasion, a stable lineage is established in the new host. The mechanisms that decide the fate of a virus in a new host are, however, poorly understood. Morbidity and mortality are rarely seen in the aquatic bird reservoir, and it is primarily when the virus enters another host that disease results.
Influenza A viruses are found in host populations across the globe with viruses often transmitting across large geographic distances. A combination of the following transmission mechanism may be a likely explanation for the spread of influenza strains: trade of live animals, movement of animals within production systems, spread via feed, water, or equipment, wild birds’ migrations, people movement, illegal importations of animals or animal products, and animal competitive events (cocks fights, horses-races, etc). Perhaps not too surprisingly, in many cases the epidemiology of the virus is closely linked to the epidemiology of the host.
In birds, and mostly for reasons of commerce, strains are classified as either low or highly pathogenic avian influenza (LPAI and HPAI respectively) viruses. H5 and H7 subtypes may be HPAI while all the 16 known hemagglutinin subtypes (H1 to H16) may be LPAI. The difference in morbidity and mortality between LPAI and HPAI viruses may be extreme (from absence of symptoms to 100% mortality). Irrespective of the virus pathogenicity, very different clinical signs have been associated with influenza viruses depending on the strain and the host. Respiratory and/or intestinal tracts are the main targets for replication of influenza viruses but the tissue distribution may be much wider especially with HPAI strains.
Changes in agricultural practices, enhanced animal health surveillance, and/or viruses’ evolution may have contributed to the apparent increase in animal influenza outbreaks reported in recent times; influenza is an increasing concern for veterinarians worldwide.