Date of publication: May 1996
Influenza is one of the most significant viral infections of the human respiratory tract worldwide. The two most important features of influenza are its epidemic nature, with attack rates often reaching 10 to 40 percent over a five to six week period (Betts, 1995), and the mortality that results, in part from pulmonary complications (Lui, 1987). The disruption caused in the community with absenteeism from school, work and loss of productivity in industry add to the high economic cost of influenza (Schoenbaum, 1987).
The causal agents include the Influenza type A and type B viruses which belong to separate genera within the family Orthomyxoviridae.
Influenza type A virus was first isolated in 1933 (Smith et al, 1933). It causes regular seasonal epidemics. The Influenza A viruses are classified into subtypes on the basis of their external glycoproteins, the haemagglutinin (H) and neuraminidase (N) antigens. In humans, three haemagglutinin (H1, H2, H3) and two neuraminidase (N1, N2) types have been recognised. In animals, a total of 14 H and 9 N antigen types exist. Within a subtype each strain is further identified by the site, and year of isolation, eg, A/Port Chalmers/1/73 (H3N2). Immunity to these antigens, particularly the H antigen, is associated with protection against that influenza subtype.
The Influenza A virus genome (see Glossary) consists of RNA which is segmented and inherently unstable. The changes that occur result in antigenic variation, a phenomenon which helps to explain why influenza continues to be a major epidemic disease of humans.
´Antigenic drift´ is the emergence of a strain with a minor change occurring through mutation in the genes coding for the H or N antigens. This results in the emergence, almost annually, of viruses with a selective advantage and able to grow in the presence of antibody against previously prevalent strains.
´Antigenic shift´ is a major change in one or both of the antigens through a recombination event with another Influenza A virus, and is usually associated with worldwide epidemics or pandemic influenza. In the recent past, pandemics have occurred after the emergence of the 1957 Asian (H2N2) and the 1968 Hong Kong (H3N2) influenza viruses.
Influenza type B virus was first identified in 1939 (Francis, 1940) and causes regional outbreaks or epidemics, although less frequently than type A influenza viruses. Influenza B also undergoes ´antigenic drift´, but at a slower rate than Influenza A. This virus does not have an animal source.
Influenza type C virus belongs to a third genus within the Orthomyxoviridae. It is widely distributed, but unlike Influenza A and B viruses, does not exhibit a regular pattern of seasonal epidemics, or undergo antigenic variation.
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