Virus | Subtypes | Mutation | Spread

What are Subtypes?

Virus subtypes only exist among type A influenza viruses (which include bird flu) and classify each based on the type of hemagglutinins (abbreviated to HA) and neuraminidases (NA) the virus possesses.

The Total Number of Bird Flu Subtypes

To date, there exist 16 varieties of HA and 9 of NA, thus totaling 144 (9 by 16) possible combinations of influenza subtypes. Any of these that typically infect birds are categorized as avian influenza viruses. However, scientists predict that particular combinations of surface hemagglutinins and neuraminidases are incompatible with one another and will not form a virus subtype together.

Human Infecting Subtypes

Typically, only 3 varieties of HA have been found in human influenza viruses - H1, H2 and H3, while only 2 forms of NA - N1 and N2, have been found as part of human-infecting flu virus subtypes. However, the following avian influenza viruses, normally causes of illness in birds, have proven to be exceptions in (comparatively) rare cases:

H5N1: Hong Kong (1997), Southeast Asia (2003+)
H7N7: Netherlands (2003)
H7N2: New York (2003), Virginia (2002)
H7N3: British Colombia (2004)
H9N2: China (1999), Hong Kong (2003)



Experts have generalized that bird flu virus subtypes of H5, H7 and H9 have the most potential to trigger both human and bird infection. Since 9 varieties of NA exist, each of them may potentially be grouped with a H5, H7 or H9 type hemagglutinin to form a bird flu subtype. This amounts to 27 subtypes with the biggest potential of crossing the human infection barrier.

Currently, H5N1 produces the most critical conditions during human infection and has the highest death rate, while the three H7 subtypes trigger bird flu infections that cause minor to medium levels of illness. Any H9 subtypes known to cause disease in birds are all of low pathogenicity and the single H9N2 subtype that has produced human illness, also proved to produce relatively mild conditions in patients.

Bird Infecting Subtypes

Highly pathogenic subtypes that have infected bird species up until now include:

H5N1: in Scotland, 1959; in Hong Kong, 1997; in England, 1991 and throughout Southeast Asia from 2003+
H5N2: in USA, 1983; in Mexico, 1994-2003 and in Italy, 1997
H5N8: in Ireland, 1983
H5N9: in Ontario, 1966
H7N2: in North Korea and Japan, 2003
H7N3: in England, 1963; in Pakistan, 1995-2003; in Australia, 1992/1994 and in British Colombia, 2004
H7N4: in Australia, 1997
H7N7: in Australia, 1976/1985; in Germany, 1979; in England, 1979 and in the Netherlands, 2003

There have also been a variety of other subtypes not listed above, which have caused highly pathogenic outbreaks among ratites – including H3N2 (infectious to humans), H4N2, H4N6, H7N1, H9N2, H10N4 and H10N7. However, these subtypes were found to only trigger low pathogenic disease in domestic chickens.

Highly Pathogenic Subtypes and Low Pathogenic Subtypes

H5N1, H7N3 and H7N7 are presently the only subtypes of virus to have caused highly pathogenic forms of bird flu infection in humans. However, a low pathogenic H7N7 strain triggering minor illness was also found in bird flu patients during the H7N7 outbreak in the Netherlands. Other low pathogenic subtypes that have caused avian influenza in humans include H7N2 and H9N2.