Influenza viruses are an important cause of respiratory infection in humans and
ID: 150415 • Letter: I
Question
Influenza viruses are an important cause of respiratory infection in humans and also infect other host species such as pigs and birds. The ongoing epidemic and pandemic threat of influenza virus infection to the human population is primarily caused by antigenic change and escape of the virus from immune mediated protection. Discuss the different mechanisms leading to these changes and provide an overview of the dynamics of infection and transmission between these different hosts.
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Explanation / Answer
Influenza epidemics are of two types. Yearly epidemics are caused by both type A and type B viruses.
Antigenic shift and antigenic drift are the two different mechanisms responsible for producing the strains that cause these two types of epidemics.
Antigenic shift: A major change in one or both of the surface antigens, a change that yields an antigen showing no serologic relationship with the antigen of the strains prevailing at the time is called antigenic shift. Antigenic shift has been demonstrated in type A influenza virus only. Influenza A virus causes epidemics and occasionally, pandemics. Pandemics are caused by the virus strains undergoing antigenic shift. Antigenic shift variants appear less frequently, about every 10 or 11 years. It is demonstrated that pandemic strains are the recombinant strains, originated from some animal or bird reservoir, either spreading to humans directly by host range mutation or as a result of a recombination between human and nonhuman strains. The pandemic strains also show the capability to spread rapidly among the population. Section IV Chapter 61 The completely novel antigens that appear during antigenic shift are acquired by genetic reassortment. The donor of the new antigens is probably an animal influenza virus. Type A viruses have been identified in pigs, horses, and birds, and animal influenza viruses possessing antigens closely related to those of human viruses. Fourteen distinct HA and nine NA antigens are known. Since continued surveillance of animal influenza viruses in recent years has failed to discover new antigens, these may represent the full variety of major influenza virus surface antigens (subtypes).
Antigenic drift: Repeated minor antigenic changes, on the other hand, generate strains that retain a degree of serologic relationship with the currently prevailing strain. This is called antigenic drift. The epidemics are caused by influenza A virus undergoing antigenic variations due to antigenic drift resulting from mutations and selections. Antigenic drift variants occur very frequently, virtually every year. This is responsible for emergence of the strains that cause yearly influenza epidemics. When persons are reinfected with drift viruses, the serum antibody responses to the surface antigens that are shared with earlier strains to which the person has been exposed are frequently stronger and of greater avidity than are the responses to the new antigens. This phenomenon, which has been called “original antigenic sin” is sometimes useful in serologic diagnosis. Antigenic drift represents selection for naturally occurring variants under the pressure of population immunity.