Influenza A virus

There are four types of influenza viruses: A, B, C and D. Waterbirds such as gulls, swallows, and waders, as well as ducks, geese, and swans are considered reservoirs (host organisms) for the virus. Type A avian influenza.

Influenza A virus subtype

Influenza A viruses are divided into subtypes based on two proteins on the virus surface: hemagglutinin (HA) and neuraminidase (NA). There are 18 known HA subtypes and 11 NA subtypes. In birds, 16 HA isoforms and 9 NA isoforms have been identified. (Two other isoforms, H17N10 and H18N11, have been detected in bats.) Many different combinations of HA and NA proteins are possible. For example, “A(H7N2) virus” refers to a subtype of influenza A virus that has an HA 7 protein and a NA 2 protein. Likewise, “A(H5N1) virus” has HA 5 protein and NA protein. 1.

All known influenza A virus subtypes can cause infection in birds, except for subtypes A(H17N10) and A(H18N11), which have been detected only in bats. Currently, only two subtypes of influenza A virus are circulating in the human population: A(H1N1) pdm09 and A(H3N2). Influenza A viruses have been detected circulating in seven different species or groups of animals, including humans, wild waterfowl, poultry, pigs, horses, dogs and bats. Avian influenza A viruses have been reported to cause occasional infections in many other animal species but are not regularly transmitted between them (e.g., cats and seals). Equine influenza A (H3N8) viruses are commonly spread among horses and can make horses sick, while canine influenza (dog) A (H3N2) viruses are commonly spread among dogs and can make horses sick.

Influenza A virus lineage

Avian influenza A viruses that infect birds have evolved into different genetic lineages depending on the geographic location where they were first discovered. These different lineages can be distinguished by studying the genetic makeup of these viruses. For example, an avian influenza A virus first detected in birds in Asia could be considered genetically different from an avian influenza A virus first detected in birds in North America. These general lineage classifications can be narrowed down through genetic comparisons, allowing researchers to group the most closely related viruses. Host organism, period, and geographical location are often used in lineage names to distinguish one lineage from another.

People rarely become infected with avian influenza A viruses. There are 5 subtypes of avian influenza A virus known to cause human infection (viruses H5, H6, and H7), H9 and H10). The most common avian influenza A virus subtypes causing infection in humans are H5, H7 and H9 viruses. Specifically, A(H5N1) and A(H7N9) viruses are responsible for most avian influenza A virus infections in humans, and HPAI A(H5N6) viruses and A(H9N2) viruses are also responsible for human infections. Last few years. Human infections with other subtypes, such as A(H6N1), A(H10N3), A(H10N7) and A(H10N8), have been detected in a small number of individuals. Human infection with HPAI A(H7) virus has not been reported in the United States; however, there have been four laboratory-confirmed cases of human infection with LPAI A(H7N2) virus. For more information about human avian influenza A virus infections, see Human avian influenza A virus infections.

Avian influenza A(H3) virus

LPAI A(H3N8) viruses have been found in wild birds and poultry in Asia. China reported a small number of cases of H3N8 virus infection, resulting in 2 severe cases of pneumonia, 1 death, and 1 mild case.

Avian influenza A (H5) virus

There are nine subtypes of AA(H5) virus (A(H5N1), A(H5N2), A(H5N3), A(H5N4), A(H5N5), A(H5N6), A(H5N7), A(H5N8) and A(H5N9)).

Most A(H5) viruses detected in wild birds and poultry worldwide are LPAI viruses, but occasionally cases of HPAI A(H5) viruses are detected. Sporadic cases of human infection with A(H5) viruses, such as the HPAI A(H5N1) virus, have been associated with poultry outbreaks in many countries. Since 1997, cases of highly pathogenic H5N1 avian influenza have been reported in my country. 23 countries. In these cases, 50% result in severe pneumonia and death. Since 2014, two countries have reported human infection with HPAI A (H5N6) virus, with more than 40% of death cases. In 2021, human infection with HPAI A (H5N8) virus was reported. .

Avian influenza A(H6) virus

Outbreaks of LPAI A(H6) viruses in birds have not been notified internationally, so their true prevalence is unknown. However, LPAI A(H6) viruses have been found in a variety of wild waterfowl and poultry in Eurasia and the Americas. A(H6) virus subtypes include LPAI A(H6N1) and A(H6N2). In 2013, Taiwan reported the first known human case of LPAI A(H6N1) virus infection.

Avian Influenza A (H7) Virus

There are nine known subtypes of A(H7) viruses. (A(H7N1), A(H7N2), A(H7N3), A(H7N4), A(H7N5), A(H7N6), A(H7N7), A(H7N8) and A(H7N9)). Most A(H7) viruses detected in wild birds and poultry worldwide are LPAI viruses. Cases of human infection with avian influenza A (H7) viruses occur sporadically. The most common A(H7) virus associated with human infection is the avian influenza A(H7N9) virus, which was first discovered in China in 2013. Although human infection with the A(H7N9) virus is rare, it causes severe respiratory illness and death in approximately 40% of reported cases. In addition to AH7N9 viruses, human infections with AH7N2, AH7N3, AH7N4, and AH7N7 viruses have been reported, and these viruses mainly cause mild to moderate disease with symptoms such as conjunctivitis. and/or upper respiratory symptoms.

Avian influenza A (H9) virus

There are 9 known subtypes of A(H9) virus, namely A(H9N1), A(H9N2), A(H9N3), A(H9N4), A(H9N5), A(H9N6), A(H9N7), A (H9N8) and A(H9N9); All influenza A(H9) viruses detected in wild birds and poultry worldwide are LPAI viruses. A(H9N2) viruses have been detected in birds in Asia, Europe, the Middle East and Africa. Sporadic infections with avian influenza A(H9N2) virus have been reported in people with mild upper respiratory illness, although some infections have resulted in death.

Avian influenza A(H10) virus

A(H10) virus subtypes include A(H10N3), A(H10N4), A(H10N5), A(H10N6), A(H10N7) and A(H10N8). A(H10N4) virus was detected in minks in 1984, and A(H10N5) virus was detected in pigs in 2008. A(H10) virus subtypes capable of causing human infection include A(H10N3), A(H10N7) and A(H10N8). Egypt reported the first human case of A(H10N7) virus infection in 2004, and Australia reported a human case of A(H10N7) virus infection in March 2010. China reported its first human case of infection with the A(H10N8) virus. In December 2013, China reported its first human case of infection with the A(H10N3) virus. In June 2021, China reported its first case of human infection with influenza A H10N5 virus (co-infection with seasonal influenza A H3N2 virus). In January 2024, an H10 virus infection occurred in China. Most human infections with A(H10) virus are caused by contact with infected poultry.