Classification

Influenza A and Influenza B Viruses

Influenza A virus (usually known as type A, though more properly a species, since it shares no antigens with influenza B) is divided into subtypes, all of which share related nucleoprotein and matrix proteins but differ in their hemagglutinin and/or neuraminidase. So far, 14 subtypes of HA (H1-H14) and 9 of NA (N1-N9) have been described in birds, some of which have been found in various combinations in mammals including humans. Strains arising naturally by antigenic drift are codified as follows: type (A or B), animal species (omitted if human), place of isolation, number of the isolate, year of first isolation, followed in parentheses by the H and N subtypes. For exam pie, the three strains chosen for incorporation in the Australian vaccine for the 1993 winter were A/Texas/36/91 (H1N1), A/Shanghai/24/90 (H3N2), and B/Panama/45/90.

Influenza C Virus

Influenza C virus comprises a distinct genus. Its genome contains only seven RNA segments. A single envelope glycoprotein, known as HEF, functions as hemagglutinin, fusion protein, and an esterase which has been postulated to cleave the cellular receptor (9-O-acetyl-N-acetyIneuraminic acid) to facilitate entry/uncoating. Influenza C virus shares no antigens with influenza A or B. It infects children quite commonly but does not cause significant disease. A similar virus has been isolated from swine in China.

Tick-borne Orthomyxoviruses

A surprising recent discovery is that a little-known group of tick-borne arboviruses infecting humans and livestock in Africa, Europe, and Asia is genetically related to influenza virus. The group has been allocated to an unnamed genus within the family Orthomyxoviridac. The minus sense RNA genome comprises only six or seven segments (for Thogoto and Dhori viruses, respectively), total 10 kb. The acariviruses display no antigenic relationship to influenza virus and are not known to cause disease in humans.

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