The virus enters the body by way of the mouth, invades the bloodstream, and may be carried to the central nervous system, where it causes lesions of the gray matter of the spinal cord and brain. The illness begins with fever, headache, stiff neck and back, and muscle pain and tenderness. If there is involvement of the central nervous system, paralysis ensues. Of those patients who develop paralytic poliomyelitis, about 25% sustain severe permanent disability, another 25% have mild disabilities, and 50% recover with no residual paralysis. The disease is usually fatal if the nerve cells in the brain are attacked (bulbar poliomyelitis), causing paralysis of essential muscles, such as those controlling swallowing, heartbeat, and respiration. There is no specific drug for treatment. For reasons not clearly understood, some people who have had severe polio experience postpolio syndrome, a condition in which new weakness and pain occurs years later in previously affected muscles.
The incidence of poliomyelitis declined radically in the United States when a mass immunization program with the Salk vaccine, a preparation made from killed organisms and injected, was begun in 1955. A live-virus vaccine had earlier been developed (1948) by Hilary Koprowski, but it was never approved for use in the United States. By 1961 the Sabin vaccine, a preparation made from weakened living organisms and taken orally, was released for use. Since then the disease has been virtually eliminated in the Americas, Europe, and Australasia, but vaccination programs continue because of polio's existence in other parts of the world (mainly areas of South Asia and West Africa) and the ease of travel.
In 1988 the World Health Organization began a global vaccination campaign to eradicate the disease—which continued to paralyze hundreds of thousands of children each year—by 2000. Although the date of eradication was later pushed back to 2005 (and even later a set deadline was abandoned), there were by 2003 less than a thousand new cases of polio worldwide. In 2003–4, however, the campaign was slowed when Muslim states in N Nigeria refused to use vaccines they believed would sterilize women, leading to an increase in cases there and in neighboring countries and to outbreaks of the disease in 17 countries including Yemen and Indonesia. Since then there have been other outbreaks, from various sources, in some African nations, in Central Asia, in Syria (as a result of its civil war), and in Papua New Guinea. The last known case of type 2 poliomyelitis occurred in 1999, and that of type 3 in 2012. In 2016 the formulation of the vaccine was altered to remove the eradicated type 2 poliovirus. According to WHO, endemic transmission of polio, however, continues to occur only Afghanistan, Pakistan, and Nigeria.
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