There are four types of influenza (flu) viruses: A, B, C, and D. Wild waterfowl, including gulls, terns, and shorebirds, and wild waterfowl, such as ducks, geese, and swans, are considered reservoirs (hosts) of birds. influenza A virus.
Influenza A virus subtypes
Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: hemagglutinin (HA) and neuraminidase (NA). There are 18 known HA subtypes and 11 known NA subtypes. In birds, 16 HA and 9 NA subtypes have been identified. (Two additional subtypes, H17N10 and H18N11, have been identified in bats.) Many different combinations of HA and NA proteins are possible. For example, an “A(H7N2) virus” designates a subtype of influenza A virus that has an HA 7 protein and a NA 2 protein. Similarly, an “A(H5N1)” virus has an HA 5 protein and a NA 1 protein.
All known influenza A virus subtypes can infect birds, except for the A(H17N10) and A(H18N11) subtypes, which have only been found in bats. Only two subtypes of the influenza A virus, A(H1N1)pdm09 and A(H3N2), currently circulate among people. Influenza A viruses have been detected and are known to circulate in seven different animal species or groups, including humans, wild waterfowl, domestic birds, pigs, horses, dogs, and bats.
In many other animal species, avian influenza A viruses have been reported to cause occasional infections, but do not spread regularly between them (eg, cats and seals). Equine (equine) A(H3N8) influenza virus routinely circulates and can cause illness in horses, and canine (dog) A(H3N2) influenza virus routinely circulates and can cause illness in dogs.
Influenza A virus lineages
Avian influenza A viruses that infect birds have evolved in different genetic lineages depending on the geographic locations where they were first detected. These different lineages can be distinguished by studying the genetic makeup of these viruses. For example, avian influenza A viruses first detected in birds in Asia can be recognized as genetically different from avian influenza A viruses first detected in birds in North America.
These broad lineage classifications can be further narrowed down by genetic comparisons that allow researchers to group the most closely related viruses together. Host, time period, and geographic location are often used in the lineage name to help better delineate one lineage from another.
Avian influenza A viruses of high and low pathogenicity
Avian influenza A viruses are classified into the following two categories: low pathogenic avian influenza A (LPAI) viruses and highly pathogenic avian influenza A (HPAI) viruses. HPAI and LPAI are defined and explained below:
Low Pathogenic Avian Influenza (LPAI): Low Pathogenic Avian Influenza viruses do not cause signs of disease or disease is mild in chickens/poultry (such as ruffled feathers and a drop in egg production). Most avian influenza A viruses are low pathogenic and cause few signs of disease in infected wild birds. In poultry, some low pathogenic viruses can mutate into highly pathogenic avian influenza viruses.
Highly Pathogenic Avian Influenza (HPAI): Highly pathogenic avian influenza viruses cause severe illness and high mortality in infected poultry. Only some A(H5) and A(H7) avian influenza viruses are classified as HPAI A viruses, while most A(H5) and A(H7) viruses circulating among birds are LPAI A viruses. by HPAI A(H5) or A(H7) viruses can cause diseases affecting multiple internal organs with up to 90-100% mortality in chickens, often within 48 hours.
However, ducks can become infected without any signs of disease. HPAI A(H5) and A(H7) virus infections in poultry can also spread to wild birds, resulting in the further geographic spread of the virus as those birds migrate. While some wild bird species can be infected with some HPAI A(H5) or A(H7) virus subtypes without appearing ill, other HPAI A(H5) and A(H7) virus subtypes can cause severe illness and mortality in some wild birds. infected wild birds. as well as in infected poultry.
Both HPAI and LPAI viruses can spread rapidly through poultry flocks. The HPAI and LPAI designations do not refer to or correlate with the severity of illness in cases of human infection with these viruses; both LPAI and HPAI A viruses have caused mild to severe illness in infected humans. There are genetic and antigenic differences between the influenza A virus subtypes that generally infect only birds and those that can infect both birds and people.