Agent Factors Agent Influenza viruses are classified within the family of Orthomyxoviridae. There are three viral sub–types, namely influenza type A, type B and type C. These three viruses are antigenically distinct. There is no cross–immunity between them. Of importance are the influenza A and B viruses which are responsible for epidemics of disease throughout the world. Both influenza A and B viruses have two distinct surface antigens – the Hemoglutinin (H) and the Neuraminidase (N) antigens. The H antigen initiates infection following attachment of the virus to susceptible cells. The N antigen is responsible for the release of the virus from the infected cell.
The influenza A virus is unique among the viruses because it is frequently subject virus to antigenic variation, both major and minor. When there is a sudden, complete or major change, it is called a shift, and when the antigenic change is gradual, over a period of time, it is called a drift. Antigenic shift appears to result from genetic recombination of human with animal or avian virus, providing a major antigenic change.
This can cause a major epidemic or pandemic involving most or all age groups. Antigenic drift involves “Point mutation” in the gene owing to selection pressure by immunity in the host population. Antigenic changes occur to a lesser degree in the B group influenza viruses. Influenza C appears to be antigenically stable.
Since the isolation of the virus A in 1933, major antigenic changes have occurred twice – once in 1957 (H2N2) and then again in 1968 (H3N2). Strains occurring between 1946 and 1957 have been called H1N1 strains. The shift in 1968 involved only the H antigen. In 1977, a new antigenic type appeared in China and the USSR and the virus was identified as A (H1N1). Within a year, it had been isolated in countries all over the world. Curiously, this was an earlier virus which has appeared after a lapse of over 20 years. In the past, the emergence of a new, influenza A sub–type led to the prompt disappearance of the previously prevalent sub–type. In the 1977 episode, however, this did not happen. The prevailing A (H3N2) was not displaced. Dual infection with both viruses have also been reported. As of now, three types of influenza viruses – A (H1N1), A (H3N2) and B exist. Influenza viruses of the H1N1 sub–type have caused epidemics of the disease in two periods of this century – from about 1946 up until 1957, and from 1977 until the present.