Influenza A virus

Influenza A virus, or IAV is a pathogen with strains that cause seasonal flu in humans; it can also infect birds and some mammals.^[1] Strains of IAV circulate constantly in bats, pigs, horses, and dogs, while other mammals may be infected occasionally.^[2][3] It has also been the cause of a number of pandemics, most notably the Spanish Flu pandemic from 1918–1920.^[4]

Subtypes of IAV are defined by the combination of the molecules on the surface of the virus which provoke an immune response; for example, "H1N1" denotes a subtype that has a type-1 hemagglutinin (H) protein and a type-1 neuraminidase (N) protein.^[5] Variations within subtypes affect how easily the virus spreads, the severity of illness, and its ability to infect different hosts.^[6][7] The virus changes through mutation and genetic reassortment, allowing it to evade immunity and sometimes jump between species.^[8][9]

Symptoms of human seasonal flu usually include fever, cough, sore throat, muscle aches and, in severe cases, breathing problems and pneumonia that may be fatal.^[10][1] Humans can rarely become infected with strains of avian or swine influenza, usually as a result of close contact with infected animals; symptoms range from mild to severe including death.^[11][12] Bird-adapted strains of the virus can be asymptomatic in some aquatic birds but lethal if they spread to other species, such as chickens.^[13]

IAV disease in poultry can be prevented by vaccination; however, biosecurity control measures such as quarantine, segregation, and good hygiene are preferred.^[14][15] In humans, seasonal influenza can be prevented by vaccination, or treated in its early stages with antiviral medicines.^[16] The Global Influenza Surveillance and Response System (GISRS) monitors the spread of influenza worldwide and informs development of both seasonal and pandemic vaccines.^[17] Several millions of specimens are tested by the GISRS network annually through a network of laboratories in 127 countries. As well as human viruses, GISRS monitors avian, swine, and other influenza viruses which could potentially infect humans. IAV vaccines need to be reformulated regularly in order to keep up with changes in the virus.^[18]