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Agent Factors of Influenza
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 virus The Influenza virus
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.

Reservoir of Infection
It has become increasingly evident that a major reservoir of influenza virus exists in animals and birds. Many influenza viruses have been isolated from a wide variety of animals and birds (e.g. swine, horses, dogs, cats, domestic poultry, wild birds, etc.) Some of these include the major H and N antigens related to human strains. There is increasing evidence that the animal reservoir provides new strains of the influenza virus by recombination between the influenza viruses of man, animals and birds.

Source of Infection
The source of infection usually is a case or sub–clinical case. During epidemics, a large number of mild and asymptomatic infections occur, which play an important role in the spread of infection. The secretions of the respiratory tract are infective.

Period of Infectivity
The virus is present in the nasopharynx a couple of days before and a couple of days after the onset of symptoms.

Host Factors of Influenza
Age and Sex
Influenza affects all ages and people of both sexes. In general, the attack rate is lower among adults. Children constitute an important link in the transmission chain. The highest mortality rate during an epidemic occurs among certain high–risk groups in the population such as old people (generally over 65 years of age), infants under 18 months, and persons with diabetes or chronic heart disease, kidney and respiratory ailments.

Human Mobility
This is an important factor in the spread of the infection.

Antibodies are important in immunity against influenza. The antibody to H neutralizes the virus while the antibody to N modifies the infection. Secondary antibodies develop in the respiratory tract after infection and consist predominantly of lgG. Antibodies must be present in sufficient concentrations at the superficial cells (the site of virus invasion) of the respiratory tract. This is possible only if the antibody titer is high in the blood or if the antibody is secreted locally. Antibodies appear in about seven days after an attack and reach a maximum level in about two weeks. After about 8 to 12 months, antibody levels drop to pre–infection levels.

Environmental Factors of Influenza
The seasonal incidence is striking, epidemics usually occur in the winter months in the northern hemisphere. In India, however, epidemics have often occurred in summer.

Overcrowding enhances transmission of the infection. The attack rates are high in closed population groups e.g. schools, institutions, ships, etc.