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What is the meaning of the numbers next to the “H” and “N” in avian influenza designations?

Avian influenza (AI) viruses are classified by a combination of two groups of proteins: hemagglutinin or “H” proteins, of which there are 16 (H1 to H16), and neuraminidase or “N” proteins, of which there are nine (N1 to N9).

Many different combinations of “H” and “N” proteins are possible. Each combination is considered a different subtype and can be further broken down into different strains.  AI viruses are further classified by their pathogenicity, low or high, based upon the ability of a particular virus strain to cause disease in chickens.

Highly pathogenic avian influenza viruses in poultry are usually H5 or H7 subtypes of Type A influenza, although low pathogenic forms of these H5 and H7 viruses also exist.

Learn more: USGS Avian Influenza