Classification of Influenza Viruses

Influenza viruses are single stranded RNA viruses belonging to the family Orthomyxoviridae.

The Orthomyxoviridae are divided into two genera:

• Influenza A and B viruses 
• Influenza C viruses           

Type A Influenza viruses

Further classified on the antigenic nature of their surface glycoproteins: 

• Haemagglutinin (HA) – the larger protein is an essential virulence factor
• Neuraminidase (NA) – the smaller protein is responsible for the transport of the virus into and out of cells.           

15 different HA’s (H1-H15) and 9 different NA’s (N1-N9) have been identified.

Influenza A subtypes isolated from different species

Subtype	Human	Swine	Horse	Birds
H1	+	+		+
H2	+			+
H3	+	+	+	+
H4				+
H5	+			+
H6				+
H7	+		+	+
H8				+
H9	+			+
H10-15				+

Each sub-type has a combination of one particular H and one particular N antigen. A subtype may have differences in the H antigen that results in different strains of each subtype. For example:

Subtype/combination	Strains/variants	Species
H1N1	PR/8/34	Human
	SW1a/15/30	Swine