Poliomyelitis is caused by the invasion of the central nervous by the pathogenic strains of an enterovirus (1)
- there are three strains of polio virus, all of which can cause paralysis
- type I is more virulent than types II and III.
- in the pre vaccine era, type I strain was responsible for 85% of paralytic disease (2)
- one case of polio can potentially infect up to 5 non-immune contacts. (2)
In paralytic poliomyelitis there is replication of the virus within the anterior horn cells of the spinal cord and motor nuclei of the brainstem, with subsequent death.
Factors increasing the likelihood of paralytic poliomyelitis include:
- paralytic infection is more common in adults than children. Amongst adults, the most commonly affected by paralytic poliomyelitis are pregnant women.
- infection, particularly of alum-precipitated vaccines such as whooping cough or diphtheria - paralysis often begins in injected arm
- in immune suppressed individuals the infection may develop from live vaccine
- physical exertion during incubation period may predispose to paralytic poliomyelitis
- tonsillectomy - may predispose to bulbar poliomyelitis. This procedure is contraindicated during a polio epidemic.
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