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Poliomyelitis is an acute viral infection caused by an RNA virus. It is primarily an infection of the human alimentary tract.
Epidemiological Determinants:
Agent Factors:
- Agent: The causative agent is polio virus. Most outbreaks of polio are due to type 1 virus. Poliovirus can survive for long periods in the external environment. In cold conditions, it can live in water for 4 months and in feces for 6 months. It is well adapted to the fecal-oral route transmission. However, the virus can be easily inactivated by autoclaving and a variety of physical and chemical methods.
- Reservoir of Infection: most is the only known source of infection. Most infections are subclinical. Mild and subclinical infections play an important role in the spread of infection. It is estimated that for every clinical case, there may be 1000 and 75 subclinical cases among children and adults respectively. There are no chronic carriers of disease.
- Source of Infection: Virus is found in the feces and oropharyngeal secretions of an infected person.
- Period of Infectivity: the cases are most infectious 7 to 10 days before and after onset of disease symptoms. In the feces virus is excreted for 2 to 3 weeks or as long as 3 to 4 months.
Host Factors:
- Age and Sex: Though the disease occurs in all age groups, children are more susceptible than adults, because of acquired immunity among adults. In India polio is essentially a disease of infancy and childhood. Six months to 3 years is the most vulnerable age of occurrence. Sex differences in polio have been noted in the ratio of 3 males to one female.
- Risk Factors: Several risk factors have been found to provoke the disease in individuals already infected with the virus. These include fatigue, trauma, intramuscular injections, operations like tonsillectomy undertaken during epidemics of polio and administration of immunization like DPT.
- Immunity: Maternal immunity received from mother will be lost during first six months of life, owing to the disappearance of antibodies. Once infected though the person becomes immune to a particular type of virus, re-infection with other types of polio virus cannot be ruled out. Neutralizing antibodies is widely recognized as an important index of immunity to polio after infection.
Environmental Factors:
Polio is likely to occur during rainy season. Nearly 60 per cent of the cases recorded in India are between June and September. Environmental sources of infection are contaminated water, food and flies. Polio virus survives for a long time in a cold environment. Overcrowding and poor sanitation aggravate the exposure to infection.
Mode of Transmission: Fecal-oral route: the main route of spread of infection in developing countries is fecal-oral route. The infection may spread directly through contaminated fingers. Indirectly spread can be observed through water, milk, other foods, flies and articles of daily use. Droplet infection: this may occur in the acute phase of the disease when the virus occurs in the throat. Close personal contact with infected person facilitates spread.
Incumbation Period: 7 to 14 days (ranges from 3 to 35 days)
Clinical Features: when an individual susceptible to polio is exposed to virus, one of the following occurs;
- Subclinical infection: there are no presenting symptoms
- Minor illness: it causes only a mild or limiting illness due to the presence of virus in blood. The patient recovers quickly.
- Non-paralytic polio: stiffness, and pain in the neck and back are the symptoms. The disease lasts for 2 to 10 days and recovery is rapid.
- Paralytic polio: the virus invades CNS and causes different degrees of paralysis. The prominent sign is asymmetrical flaccid paralysis. Fever at the time of onset of paralysis indicates polio. The other associated symptoms are malaise, anorexia, nausea, vomiting, headache, sore throat, constipation and abdominal pain. There may be stiffness of the back and neck muscles. After the acute phase, atrophy of the affected muscles leads to a life with residual paralysis which can be easily identified as poliomyelitis.
There is no proper treatment for polio. Good nursing care from beginning can prevent crippling. Physiotherapy can be initiated in the affected limb immediately. This helps weakened muscles to regain strength.
Prevention and Control: immunization is the sole effective means of preventing poliomyelitis. The children between 6 months onwards need to be immunized upto 5 years to protect against polio.
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