Polioviruses enter the body orally, infect the throat and intestinal tract, and then invade the bloodstream. In most people, the immune system conquers the infection and recovery is complete. Only in a small percentage of people does the virus enter the nervous system and attack motor nerves. The viruses can infect a cell only if the surface of that cell possesses specific receptors to which the virus can attach. This specificity of receptor sites helps explain the fact that polioviruses selectively infect motor nerve cells of the brain and spinal cord, while sparing many other kinds of cells. The infected cell is destroyed when the mature virus is released.

Even though poliomyelitis transmission no longer occurs in the United States, there are about 300,000 survivors ofthe disease, most of whom recovered years ago. Some of them develop muscle pain, increased weakness, and muscle degeneration 15 to 50 years after they had acute poliomyelitis. This condition, called the post-polio syndrome, sometimes involves muscles not obviously affected by the original illness. The late progression of muscle weakness is not due to recurrent multiplication of polio viruses. During recovery from acute poliomyelitis, surviving nerve cells branch out to take over the functions of the killed nerve cells. The late appearance of symptoms is probably due to the death of these nerve cells that have been doing double duty for so many years.

26.3 Viral Diseases of the Nervous System

PERSPECTIVE 26.1 Bye Bye Polio!

Poliomyelitis was first recognized as a distinct disease early in the nineteenth century, although based on ancient Egyptian inscriptions it probably existed long before that.The first polio epidemic was reported in 1887 in Sweden. For the next 30 years, polio epidemics were mostly confined to the Scandinavian countries, Canada, New Zealand, Australia, and the northeastern United States. Polio was a terrifying threat, especially in economically advanced nations. Franklin Delano Roosevelt, the losing vice presidential candidate in 1920, came down with the disease the next year, at age 39. Roosevelt later was elected president of the United States (1933 to 1945) and played an important role in the eventual defeat of the disease.

The National Foundation for Infantile Paralysis was established in 1937, and one of Roosevelt's law partners, Basil O'Connor, became its volunteer president. Eddie

Cantor, a famous comedian, developed the idea of a "March of Dimes" to defeat polio, and hundreds of thousands of volunteers collected dimes to support the effort. A major breakthrough occurred in 1949, when Dr. John Enders and his colleagues at Harvard University succeeded in growing polioviruses in monkey kidney cell cultures. John Enders,Thomas Weller, and Frederick Robbins were awarded the Nobel Prize in Medicine in 1954 for this work. Building on their finding, Dr. Jonas Salk perfected a vaccine consisting of formalin-inactivated poliovirus in the same year. His vaccine was quickly and widely employed, and the incidence of paralytic poliomyelitis plummeted. At about this same time, Dr. Albert Sabin, a Russian immigrant and, like Dr. Salk, a graduate of New York University medical school, selected mutant poliovirus strains that did not attack nervous tissue but infected the intestine, could spread from person to person, and stimulated antibody formation against the wild, virulent virus. Orally administered attenuated vaccines were quickly developed and proved more effective than the inactivated virus vaccines.

The results of programs using these vaccines was dramatic. In 1952, just before the development of the inactivated vaccine by Salk, 57,879 cases of polio were reported in the United States.Ten years later, the CDC reported only 910 cases, and the last transmission of wild poliovirus in the United States occurred in 1979. In 1994, the entire Western Hemisphere was declared free of poliomyelitis. Global eradication of poliomyelitis using attenuated vaccine is currently under way, and if all goes well, the world should be certified free of poliomyelitis by the year 2005. ■ attenuated vaccine, p. 422

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