Enteroviruses: Polio

Abstract

Poliomyelitis (polio), the much-feared crippling epidemic disease of the twentieth century, is on the verge of eradication. Before introduction of the inactivated poliovirus vaccine (IPV) in 1955 and the oral poliovirus vaccine (OPV) in 1961, polio had nearly a worldwide distribution, with widespread circulation of all three serotypes of the etiologic agent, poliovirus. Effective immunization programs in developed countries stopped endemic poliovirus circulation by the early 1970s, but circulation continued virtually unabated for many more years in most developing countries, continuing to threaten the majority of the world’s children with lifelong paralysis. As the result of intensive immunization activities launched by the World Health Organization in the Americas in 1985 and worldwide through the Global Polio Eradication Initiative (GPEI) in 1988, polio, like smallpox before it, is becoming a fading memory. Circulation of indigenous wild type 2 poliovirus ceased in 1999, wild type 3 poliovirus was last detected in November 2012, and unbroken circulation wild type 1 poliovirus has continued in parts of only three countries. Polio case counts have declined by >99% worldwide, from an estimated 350,000 cases in 1988 to 223 cases reported in 2012. However, progress has been uneven, with repeated setbacks after 2000 as wild polioviruses from endemic reservoirs reinfected polio-free countries and emerging circulating vaccine derived polioviruses sparked outbreaks in settings of declining population immunity to poliovirus. Nonetheless, the feasibility of global polio eradication has been repeatedly demonstrated in the most challenging settings on earth, including areas of conflict, extreme poverty, crowding, poor hygiene and sanitation, very weak infrastructure, and in environmental conditions favoring intense virus transmission. Eradication of polio in once hyperendemic India in 2011, has set the stage for the final push to global polio eradication.

Polioviruses are members of the Enterovirus genus, comprising >100 serotypes, inhabitants of the intestinal tracts and/or the nasopharyngeal tissues of humans and other mammals. Polioviruses, for which humans are the only natural reservoir host, occasionally invade the central nervous system (CNS) and cause destruction of motor neurons in the spinal cord, resulting in acute flaccid paralysis. However, poliovirus invasion of the CNS occurs in <1% of infections, and represents a dead-end for transmission, which is sustained by the fecal-oral or respiratory routes. The abrupt appearance in the late nineteenth century of large polio outbreaks in Europe and North America generated intense interest in the disease and prompted intensive studies of poliovirus epidemiology, pathology, immunology, and virology, leading to the development and worldwide deployment of effective poliovirus vaccines and many groundbreaking contributions to public health, medical science, and basic research.

The GPEI, the largest public health program in history, has coordinated the vaccination of 2.5 billion children, saving >10 million people (mostly children <2 years of age) from lifelong paralysis and sparing the lives of >250,000 others. The GPEI has trained a new generation of highly experienced health professionals and volunteers who are poised to control many other vaccine-preventable diseases, and has developed a detailed endgame strategic plan to secure forever the many gains achieved by polio eradication.

The findings and conclusions in this chapter are those of the author and do not necessarily represent the views of the Centers for Disease Control and Prevention.