Individual strains may also change in a way that allows them to evade the immune system. Antigenic drift (gradual changes over time, due to mutations) can eventually lead to a situation where a virus strain is no longer recognizable by the immune system; therefore, there is a gradual loss of immunity as this drift occurs. In a more severe situation, known as antigenic shift, two Influenza A viruses infect the same cell and exchange genetic material during replication. This reassortment can cause the sudden creation of a new virus strain, against which the body has no defenses 4 . Avian influenza viruses are typically classified as LPAI or HPAI, based primarily on the ability to cause disease in the laboratory setting. Young chicks are inoculated with avian influenza intravenously, and the severity of the resulting disease is assessed to determine whether a virus strain is LPAI or HPAI 4 . Avian influenza viruses can also be classified as HPAI based on sharing certain genetic factors with other known HPAI viruses 4 . Fully virulent HPAI strains have historically contained H5 or H7 as Zoonotic potential Clinical human cases of avian influenza are uncommon, but have been noted with some strains. To date, there have been no reports of human infection with the currently circulating H5N8 HPAI strain; however, this virus did evolve from a strain that is capable of infecting humans—Asian lineage H5N1 HPAI. Due to the documented spread of H5 avian influenza viruses to humans Geographic distribution Avian influenza viruses are found worldwide and maintained in wild bird populations. There are genetic differences between North American and Eurasian lineages, but genetic reassortment occurs when these lineages are found in the same areas (e.g., birds in Northeastern Asia and Alaska often have contact during the summer months) 4 . In developed countries, avian influenza is typically not found in commercial poultry due to biosecurity measures. Other domestic birds that are managed less intensively may be exposed to avian influenza 4 , including backyard poultry flocks, game birds, and Transmission Avian influenza virus is shed in the feces of infected birds, with feco-oral transmission as the primary route of infection 4 . Respiratory transmission may also occur, but is a less important mode of virus spread, occurring mostly with specific viral strains and land-based hosts 4 . Infected domestic birds typically shed LPAI for approximately 1 week after infection, with longer shedding periods of up to 2 weeks noted in a small number of birds 4 . Infected waterfowl may shed the virus for up to 3 weeks, while showing little or no clinical signs 4 . Based on these numbers, it is easy to understand how large numbers of virus particles may be shed by a single infected bird and the virus can be spread over wide areas. Fomites also play a major role in the spread of avian influenza and flies may serve as an important mechanical vector 4 . The virus can spread between commercial poultry farms on tools, vehicles, and workers’ clothing and shoes. Airborne spread may also Environmental persistence Influenza viruses remain infectious for 24 to 48 hours on nonporous surfaces and fomites, however, they can persist for extended periods in contaminated water 3 . One study demonstrated that avian influenza virus can survive up to 40 days in wet feces maintained at low temperatures, though virus survival decreases with increased temperatures and decreased moisture levels 7 . These factors make the virus highly contagious, remaining in the environment when excreted by waterfowl and easily spread by fomites.
their HA protein, although there is the possibility for emerging HPAI strains to not follow this pattern 4 . HPAI viruses cause mortality rates approaching 100% of infected domestic poultry 3 . LPAI is typically a less severe disease, causing only mild illness 4 . There is potential for LPAI strains to convert to HPAI due to reassortment; therefore, all H5 or H7 strains of avian influenza are now considered reportable diseases based on World Organization for Animal Health (OIE) guidelines 5,6 . Avian influenza has been found to affect a variety of bird species 6 . Aquatic birds are most commonly implicated as reservoirs of avian influenza, including members of the order Anseriformes (waterfowl, such as ducks, geese, and swans) and two families within the order Charadriiformes, the Laridae (gulls and terns) and Scolopacidae (shorebirds) 4 . Wild terrestrial birds (e.g., songbirds) are less likely to be affected with avian influenza, although they can become infected if exposed 4 . Poultry do not appear to serve as reservoir for infection of wild birds, as avian influenza strains adapted to poultry do not appear to cross back to wild birds 4 . with high exposure 4 , there is concern that humans in direct contact with infected poultry or their carcasses may be at risk of acquiring H5N8 HPAI. While avian influenza has been implicated as a source of some historical pandemics, person-to-person transmission of avian influenza viruses is rare 3,4 . zoo species. In undeveloped countries, avian influenza typically circulates in both wild and domestic birds. HPAI is currently considered to be endemic among poultry in a few Asian or Middle Eastern countries, with periodic outbreaks occurring among domesticated poultry in other areas 4 . In the United States, HPAI H5N8 is now considered to be endemic in three of the four major migratory flyways (Pacific, Central, and Mississippi). While the Atlantic flyway has not yet had documented cases of HPAI as of fall 2015, it is suspected that the virus may be already present in that population as well 2 . be possible between farms close to each other, under certain environmental conditions 4 . Humans and other mammals that become infected with avian influenza are typically individuals in close contact with infected chickens, either providing care or handling sick birds. Most of these infections are thought to be acquired by infected avian tissues or secretions coming in contact with the human respiratory tract or eye. Isolated reports also exist of humans and mammals infected by ingesting raw tissues of infected birds, but this appears to be a less common mode of transmission 4 . Once infected, humans and other mammals primarily shed the virus via the respiratory tract. There are isolated reports of humans and mammals shedding the virus in their feces, but this is thought to have little significance as a mode of transmission 4 . As previously stated, human-to-human transmission is rare. Avian influenza viruses can be removed from fomites and indoor environments relatively easily with the use of appropriate cleaning measures. Methods of killing avian influenza virus include heat (temperatures of 56°C for 3 hours or 60°C for 30 minutes), acidic conditions, oxidizing agents (including sodium dodecyl sulfate, lipid solvents, and B-propiolactone), and disinfectants (such as formalin or iodine compounds) 3 .
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