Polio

Poliomyletis is exclusively a human disease, the only source of the virus being humans. It can be either the patient or the symptom less carrier. The disease is worldwide in distribution.

The epidemiology is found to be common in all countries, but paralysis seem to occur only in infants (infantile paralysis). In India, the disease is equally distributed in urban as well as rural areas.

The causative agent is the Polio virus. It is a spherical particle consisting of 4 viral proteins (VP1 – VP4). The genome is a single stranded RNA.

It is resistant to ether, chloroform, bile, intestinal enzymes and detergents. It can survive in feces for weeks at room temperature.

Heating at 55oC for 30 minutes, formaldehyde, chlorination and lyophilization kills the virus.

Natural infection occurs only in humans. Infection can be induced in chimpanzees and monkeys. Rodents, chicks are used for its isolation and primate monkey kidney cells are used for vaccine production and diagnostic culturing methods.

The virus is transmitted through the fecal- oral route through ingestion. Inhalation or entry through conjunctiva of the droplets of respiratory secretions. This may act as methods of spread in case of close contact with the patients in the early stages of infection.

Method of spread in the body:

1. The virus multiplies in the epithelial cells of the alimentary canal and lymphatic tissue starting from the tonsils.
2. Then it spreads to the lymph nodes, enters the bloodstream creating minor viremia.
3. It undergoes further multiplication in the reticuloendothelial system, thereby entering the bloodstream again causing major viremia.
4. Then further carried to the spinal cord and the brain.
5. In the central nervous system, the virus multiplies selectively in the neurons, destroying them.
6. The earliest change is the degeneration of Nissl bodies (the chromatids undergoes lysis).
7. Nuclear changes follow.
8. When the degeneration becomes irreversible, the cell undergoing necrosis is lysed by body’s own macrophages.
9. Lesions appear in the anterior portion of the spinal cord resulting in paralysis.

In 90-95% cases, the infection seem to be inapparent. The earliest manifestations include minor viremia with fever, headache, sore throat and weakness. This is the minor illness, which may subside or may lead to major illness. The fever starts again; with stiffness of neck, headache and features of meningitis.

Specimens like blood, CSF, throat swab and feces are taken for virus isolation. Serological diagnosis is often employed for the detection of antibody. Complement fixation test is used for demonstrating the antibodies formed soon after the onset of paralysis.

Passive immunization by the administration of human gammaglobulin is of little value. The live polio vaccine is administered orally, hence the name oral polio vaccine (OPV). OPV used in India contains Type I virus 10 lakh, type II virus 2 lakh and type II virus 3 lakh ID50 per dose (0.5ml).

The liquid vaccine is thermostabilised and acts only blow pH 7.0. In order to maintain this pH, they are kept in airtight containers. Their shelf life is four months at 4- 8^oC and -20^oC for two years. Improperly stored ones may not give proper results.

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