Influenza Commonly Known as Flu

Influenza is an acute infectious disease of the respiratory tract. It occurs in sporadic, epidemic, and pandemic forms. It occurs in animals and birds in nature. Ordinarily, nonhuman influenza viruses do not cause human infections.

Fowl plague (avian influenza) is a septicemic condition. Influenza has also been isolated from horses, whales and seals. Aquatic birds, especially, are found to be the natural reservoirs of influenza viruses.

The Influenza virus is typically spherical and filamentous. It is an RNA virus, and can be deactivated by heating at 50^oC for 30 minutes. It maintains viability at 0- 4^oC, and on fomites such as blankets for about a week. Freeze drying preserves it for years.

Ether, formaldehyde, phenol, salts of heavy metals and other chemical disinfectants can destroy its infectivity. Iodine proves to be effective in particular.

Hemagglutination is a characteristic feature of the virus. Certain peplomers (glycoprotein) on the viral surface brings about this activity.

Hemagglutination is reversed after the detachment of the virus from the cells. It is brought about by the presence of the enzyme, neuraminidase, on the surface of the virus.

The route of entry of the virus is the respiratory tract. The neuraminidase, smoothens the progress of the infection by reducing the viscosity of the mucus film, lining the respiratory tract. This exposes the cell surface receptors for virus adsorption.

The ciliated cells of the respiratory tract are the main sites of viral infection. These cells are damaged and discarded, making the basal cells of the trachea and bronchi to be exposed. This makes the respiratory tract vulnerable to bacterial invasion.

The disease is usually confined only to the respiratory tract. Very rarely, the virus had been isolated from spleen, liver, and kidneys.

The incubation period of the disease is 1- 3 days. The severity of the disease varies considerably. In some cases, it may be mild and in some, it may be fatal like pneumonia.

In the typical clinical illness, the onset accompanies;

• Fever
• Headache
• Generalized weakness

These are followed by respiratory symptoms. Abdominal pain and vomiting may be present. The uncomplicated illness subsides within 7 days.

Some important complications include;

• Pneumonia, which may be due to bacterial infection, or by the virus itself.
• Cardiac problems
• Neurological involvement like encephalitis
• Gastrointestinal symptoms (gastric flu)

In young children, Reye’s syndrome is visualized, characterized by degenerative changes in the brain, liver and kidneys. Once attacked by influenza, about 2 years of immunity is being provided.

Diagnosis of the disease is mainly by;

1. Demonstration of the viral antigen:

• Immunofluorescence
• Reverse transcriptase polymerase chain reaction, etc., has been made use of.

2. Virus isolation:

• Throat gargling is taken in the first 2- 3 days from the patient. Eggs or monkey kidney cells are used for culturing.

3. Serological diagnosis:

• Complement fixation test, using RNP antigen of influenza virus types A, B, and C
• Hemagglutination inhibition test
• Enzyme neutralization test for neuraminidase antibody detection
• Radial immunodiffusion for identifying the antibodies against the RNP antigen, hemagglutinin and neuraminidase.

Amantadine and rimantidine are useful in the treatment of influenza. They help to improve the symptoms, but may develop rapid resistance against them. Zanamivir, blocking the neuraminidase, effectively prevents influenza.

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