At the genetic level, an AIV can be usually be identified as an HPAI virus if it has a multibasic cleavage site in the HA protein, which contains additional residues in the HA gene. Most AIVs are LPAI. Notable HPAI viruses include HPAI H5N1 and HPAI H7N9. HPAI viruses have been a major disease burden in the 21st century, resulting in the death of large numbers of birds. In H7N9s case, some circulating strains were originally LPAI but became HPAI by acquiring the HA multibasic cleavage site. Avian H9N2 is also of concern because although it is LPAI, it is a common donor of genes to H5N1 and H7N9 during reassortment.Migratory birds can spread influenza across long distances. An example of this was when an H5N1 strain in 2005 infected birds at Qinghai Lake, China, which is a stopover and breeding site for many migratory birds, subsequently spreading the virus to more than 20 countries across Asia, Europe, and the Middle East. AIVs can be transmitted from wild birds to domestic free-range ducks and in turn to poultry through contaminated water, aerosols, and fomites. Ducks therefore act as key intermediates between wild and domestic birds. Transmission to poultry typically occurs in backyard farming and live animal markets where multiple species interact with each other. From there, AIVs can spread to poultry farms in the absence of adequate biosecurity. Among poultry, HPAI transmission occurs through aerosols and contaminated feces, cages, feed, and dead animals.
Pharmaceutical measures are effective but may not be available in the early stages of an outbreak.In health care settings, infected individuals may be cohorted or assigned to individual rooms. Protective clothing such as masks, gloves, and gowns is recommended when coming into contact with infected individuals if there is a risk of exposure to infected bodily fluids. Keeping patients in negative pressure rooms and avoiding aerosol-producing activities may help, but special air handling and ventilation systems are not considered necessary to prevent the spread of influenza in the air. In residential homes, new admissions may need to be closed until the spread of influenza is controlled. When discharging patients to care homes, it is important to take care if there is a known influenza outbreak.Since influenza viruses circulate in animals such as birds and pigs, prevention of transmission from these animals is important. Water treatment, indoor raising of animals, quarantining sick animals, vaccination, and biosecurity are the primary measures used. Placing poultry houses and piggeries on high ground away from high-density farms, backyard farms, live poultry markets, and bodies of water helps to minimize contact with wild birds. Closure of live poultry markets appears to the most effective measure and has shown to be effective at controlling the spread of H5N1, H7N9, and H9N2.