Influenza A Virus - Variants and Subtypes

Variants and Subtypes

Influenza type A viruses are categorized into subtypes based on the type of two proteins on the surface of the viral envelope:

H = hemagglutinin, a protein that causes red blood cells to agglutinate.

N = neuraminidase, an enzyme that cleaves the glycosidic bonds of the monosaccharide, neuraminic acid

Different influenza viruses encode for different hemagglutinin and neuraminidase proteins. For example, the H5N1 virus designates an influenza A subtype that has a type 5 hemagglutinin (H) protein and a type 1 neuraminidase (N) protein. There are 17 known types of hemagglutinin and 9 known types of neuraminidase, so, in theory, 153 different combinations of these proteins are possible.

Some variants are identified and named according to the isolate they resemble, thus are presumed to share lineage (example Fujian flu virus-like); according to their typical host (example human flu virus); according to their subtype (example H3N2); and according to their deadliness (example LP, low pathogenic). So a flu from a virus similar to the isolate A/Fujian/411/2002(H3N2) is called Fujian flu, human flu, and H3N2 flu.

Variants are sometimes named according to the species (host) in which the strain is endemic or to which it is adapted. The main variants named using this convention are:

• Bird flu
• Human flu
• Swine influenza
• Equine influenza
• Canine influenza

Variants have also sometimes been named according to their deadliness in poultry, especially chickens:

• Low pathogenic avian influenza (LPAI)
• Highly pathogenic avian influenza (HPAI), also called deadly flu or death flu

Most known strains are extinct strains. For example, the annual flu subtype H3N2 no longer contains the strain that caused the Hong Kong flu.