polio

polio An acute and sometimes devastating viral disease that affects the nervous system. Humans are the only natural hosts for poliovirus. The virus enters the mouth and multiplies in lymphoid tissues in the pharynx and intestine. Small numbers of virus particles enter the blood and go to other sites, where the virus multiplies more extensively. Another round of virus in the bloodstream leads to invasion of the spinal cord and brain, the target sites struck by the virus. When the central nervous system (CNS) is inflamed, the anterior horn cells of the spinal cord and the brainstem are especially affected. Polio is a minor illness in 80 to 90 percent of clinical infections; this is termed the abortive type of polio. Abortive polio appears chiefly in young children and does not involve the CNS. Symptoms are slight fever, malaise, headache, sore throat, and vomiting 3 to 5 days after exposure. Recovery occurs in 24 to 72 hours. As a major illness, polio may or may not cause paralysis. Symptoms usually appear without prior illness, particularly in older children and adults, 7 to 14 days after exposure. Symptoms are fever, severe headache, stiff neck and back, deep muscle pain, and sometimes areas of increased or altered sensation. There may be no further progression from this type of illness, which is similar to viral meningitis, or there may be loss of tendon reflexes and weakness or paralysis of muscle groups. Recovery is complete with the abortive and nonparalytic forms of polio, although a set of symptoms known as postpolio syndrome may appear many years later. In paralytic polio, about 50 percent of patients recover, with no residual paralysis; about 25 percent are left with mild disabilities, and the remaining 25 percent of patients have severe permanent disability. The ideal strategy with polio is clearly to prevent it by immunizing against poliovirus. Also known as infantile paralysis and poliomyelitis. See also postpolio syndrome.