Type A Influenza Virus And Why The Biology Of This Virus Is A Threat To Human - Essay Example

TYPE A INFLUENZA VIRUS AND WHY THE BIOLOGY OF THIS VIRUS IS A THREAT TO HUMAN Influenza is an acute infectious disease of the respiratory tract which occurs in sporadic, epidemic and pandemic forms. The name 'influenza' is said to have been given by Italians during the epidemic of 1743, which they ascribed to manevolent influence of heavenly bodies or of inclement weather. The protein part of the envelope is virus coded but the lipid layer is derived from the modified host cell membrane, during the process of replication by budding.

Projecting from the envelope are two types of spikes (peplomers): hemagglutinin spikes which are triangular in cross section and mushroom shaped neuraminidase peplomers which are less numerous. A unique feature of the influenza virus is its ability to undergo antigenic variation. This is of great importance in the epidemiology of the disease. Antigenic variability is highest in influenza virus type A and less in type B, while it has not been demonstrated in type C. The internal RNP antigen and M protein antigen are stable but both the surface antigens, hemagglutinin and neuraminidase, undergo independent antigenic variations, which may be of two types - antigenic shift and antigenic drift.

Here, the new antigens, though different from the previous antigens, are related to them, so that they react with the antisera to the predecessor virus strains, to varying degrees. Antigenic drift is due to the mutation and selection, the process being influenced by the presence of antibodies to the predecessor strains in the host population. Antigenic drift accounts for the periodical epidemics of influenza. Antigenic shift, on the other hand, is an abrupt, drastic, discontinuous variation in the antigenic structure, resulting in a novel virus strain unrelated antigenically to predecessor strains.

Such changes may involve hemagglutinin, neuraminidase or both. Antibodies to predecessor viruses do not neutralize the new variants and can, therefore, spread widely in the population causing major epidemics or pandemics. The changes involved in antigenic shift are too extensive to be accounted for by mutation. Birds, particularly aquatic birds, appear to be the primary reservoir of influenza viruses and natural infection has been identified in several avian species. In birds it is usually an asymptomatic intestinal infection.

The cloaca of healthy wild birds is the best source of isolation of influenza viruses. All isolates from nonhuman hosts belong to type A. Influenza virus types B and C appear to be exclusively human viruses and natural infection with them has not been identified in animals or birds. Ordinarily, nonhuman influenza viruses do not cause human infection. But they may play an important role in the emergence of pandemic influenza.It is now known that wild aquatic birds carry a full repertoire of genes of all influenza strains, including old human pandemic strains, and that the viruses do not cause any disease in them or undergo any mutational changes.

Birds shed the viruses abundantly in feces, which contaminate lakes and ponds. Domestic birds like ducks can get infected from wild birds and carry the infection to pigs, which may be an important link in the chain, as they are susceptible to infection by both human and avian influenza strains. Recombination may take place in pigs and such hybrid strains may lead to human infection with potential pandemic spread. Influenza occurs sporadically, as epidemics or in pandemic form. It is for this reason that worldwide surveillance is maintained on influenza, under the auspices of the WHO (Ananthanarayan 2003).

ReferenceAnanthanarayan, R and C. K. Jayaram Panicker . Text book of Microbiology, Sixth Ed, Chennai:Orient Longman Limited, (2000).