POLIO

Poliomyelitis is an acute viral infection which ranges in severity from a nonspecific illness to paralysis with permanent disability. Worldwide, WHO estimates that some 140,000 new cases of paralytic poliomyelitis occurred in 1992. The cumulated number of children and adults with paralysis due to poliomyelitis is estimated at 10 to 20 million persons.

Mode of transmission
Humans are the sole reservoir for poliovirus. Wild polioviruses are spread directly or indirectly from person to person. Virus dissemination is facilitated by poor sanitation. In all countries, children under two years of age create a microenvironment of less than optimal hygiene within the family and within daycare settings, readily facilitating fecal-oral and oral-oral (mouth-fingers-mouth) transmission. Feces can serve as a source of contamination of water, milk, or food, and houseflies can passively transfer poliovirus from feces to food.

Clinical description
Wild poliovirus enters through the mouth, attaches to receptors on the epithelium of the throat and intestine, and replicates inside these cells. Newly synthesized poliovirus is shed from infected cells; it can be cultured from the pharynx for the first week after onset of paralysis and from feces for several weeks and sometimes months after onset. From these sites the virus spreads to cervical and mesenteric lymph nodes. Poliovirus enters the blood stream via the lymphatics. Virus from the blood stream can invade the central nervous system unless sufficiently high levels of neutralizing antibodies are present to block it. Within the central nervous system, the virus spreads along nerve fibers and in the process of its intracellular multiplication it destroys motor neurons, resulting in flaccid paralysis. Sensory neurons are not affected. The majority of wild poliovirus infections are asymptomatic. In a total population of more than 80 000 persons aged 0 to 20 years, fewer than 1% developed paralysis. About one-quarter of children aged 0 to 14 years were infected subclinically, with somewhat higher rates in younger children. Among children aged 1 to 14 years, about 100 were subclinically infected for each paralytic case; among infants, about 200 were subclinically infected for each paralytic case.