The disease is now limited to a few countries in the tropics. All of the known types of poliomyelitis (1,2 and 3) are prevalent although the virus strains responsible for paralytic illness in any area may vary, and at different periods in the same area one type or other may predominate. Large-scale epidemics may result if virulent wild-type virus (commonly type 1) is reintroduced into a community with breakdown in vaccine delivery and poor socioeconomic and environmental conditions.
In the tropics, a seasonal peak occurs in the hot and rainy season. In 1999, 7086 cases were reported worldwide, of which 2814 were in India.
during the period of acute febrile illness, and intramuscular injections some time before the acute episode, all seem to be provoking factors leading to paralysis. Tonsillectomy increases the risk of bulbar poliomyelitis. The mechanism of these various stresses is not clear.
The factor of greatest importance in determining the incidence of paralytic poliomyelitis is the state of immunity of the affected population. In many tropical countries where sanitation is primitive and living conditions are crowded and poor, conditions for the spread of poliovirus are good. Consequently, infants have the opportunity of coming into contact with all three types of poliomyelitis virus early in life, and few of them reach preschool age without having been infected with at least one strain, although, clinically, the infection is in most cases unapparent. Immunity is acquired early: serum antibody surveys carried out among children in many parts of the tropics have shown that by the time they are 3 years old 90% have developed antibodies against at least one type of poliomyelitis. In countries where the sanitary arrangements are good, the risk of contact with the virus at an early age is diminished and older persons are affected. Thus, the most significant difference between the occurrence of poliomyelitis in the well-developed countries of the temperate zone and the less-developed areas of the tropics is in the distribution of cases in the various age groups.
Humans are the reservoir of infection. The poliovirus is excreted in the stools of infected cases, convalescent patients and health carriers.
Poliomyelitis is a highly infectious disease and the alimentary tract is of prime importance as a portal of entry and exit of the virus, as it is with other enteroviruses. The virus is transmitted from person to person by the faecal-oral route or pharyngeal secretions, rarely by foodstuffs contaminated by faeces.
The incidence rates in males and females are similar. Trauma, excessive fatigue and pregnancy
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