Featured Infectious Disease: Polio

In the first decade of the new century, polio, a deadly and crippling infectious disease, may well be eradicated from the earth by immunization. Polio has already largely been relegated to the history books in the United States—although many people who had contracted polio in childhood suffer the muscle pain and weakness of postpolio syndrome.

Consistent efforts to eradicate the disease, spearheaded by the World Health Organization and supported by the national immunization programs of most governments, are working very effectively. In the past 10 years, polio has declined by 90 percent.

What is polio?

Polio is a highly contagious infectious disease caused by polioviruses that can attack the central nervous system. The infection can be mild and almost unnoticed or severe enough to cause muscle paralysis and death. Most infected persons experience only mild symptoms, such as a sore throat, headache, fever, and intestinal upset, and recover completely. Some 10 percent of infected patients develop more serious symptoms, including a high fever, severe neck and back pain, or partial or complete paralysis.

The virus attacks the motor areas of the central nervous system. By destroying the nerve cells in the spinal cord, the virus can cause muscles activated by those nerves to become paralyzed. If the virus damages nerves high up in the spinal cord, it can also affect the respiratory muscles and compromise a person's ability to breathe. The result is death by respiratory failure in 2 to 10 percent of paralytic polio cases.

When did it start?

Polio has existed for thousands of years, but it wasn't until 1908 that two scientists showed that paralytic polio is a contagious viral disease. For much of this century, and especially before the discovery of the polio vaccine in the 1950s, polio was a much-feared illness, causing major epidemics in 1910 and 1916 and in the 1930s, 1940s, and 1950s. In 1952, more than 57,600 Americans contracted polio, the highest number on record in a single year. Today, some 10 to 20 million people worldwide, including many children in the developing world, remain paralyzed from the virus.

Who gets the disease?

Polio is primarily a disease of infants and children, although teenagers and older people (like President Franklin Delano Roosevelt) can become infected as well. In addition to those who contract the disease from the virus, about one out of every 300,000 to 500,000 people who are vaccinated will get the disease from the oral poliovirus vaccine they received to protect them from the disease.

Where is the disease found?

Although polio has been virtually eliminated in the United States—the Western Hemisphere has been declared polio-free since 1994—and has been rapidly declining worldwide, persons in many developing countries are still at risk. The World Health Organization says that 27 countries reported polio transmission as of late 1998. The disease exists in South Asia, particularly India, Pakistan, and Afghanistan, and Western and Central Africa, particularly Nigeria and the Democratic Republic of Congo. The virus also persists in Turkey and Kurdish-occupied areas of Iraq, where international health workers have had trouble obtaining permission to vaccinate.

How does it spread?

Spread by eating contaminated food or drinking contaminated water, the poliovirus, once safely inside the body, heads for the lymphoid tissue of the throat and small intestine. In temperate climates, the spread of poliovirus is cyclical; winter is the low season for polio and summer, the high season. In tropical climates, however, polio remains a year-round threat.

Ironically, the advanced state of public hygiene in most industrialized countries contributed to the century's epidemics. Infants or very young children became infected when open sewers were rampant, but their disease was so mild that many parents did not realize their children had polio. This "silent" infection provided lasting immunity. With the advent of indoor plumbing and other modern sanitary conditions, children were not exposed to the poliovirus in infancy and did not develop immunity. As a result, they were vulnerable to disease in late childhood and adulthood, when it posed a much more serious threat.

Is there a treatment?

No antiviral therapy exists that can "cure" polio. Even if the poliovirus were eliminated in the body, damage to the nervous system is irreparable with current medical technology. Vaccines can prevent polio, but only if a person has been immunized before exposure to the virus. In 1953, Dr. Jonas Salk developed a chemically inactivated virus that immunized against the three types of poliovirus. This inert virus is given by injection and requires multiple doses.

In 1957 Dr. Albert Sabin developed an oral vaccine—the first vaccine not injected with a needle—made from live but weakened polioviruses. The oral vaccine against the disease is highly effective and inexpensive and usually provides lifelong immunity to the virus. It does, however, carry a very small chance of infecting the patient with the very disease it is meant to cure or of passing on the virus to those not vaccinated. Today, to help reduce the risk of vaccine associated paralytic polio, the U.S. Centers for Disease Control recommends vaccinating children with two doses of inactivated vaccine and two doses of oral vaccine.

Polio References

Deatly, A.M., J.W. Coleman, G. McMullen et al. "Poliomyelitis in intraspinally inoculated poliovirus receptor transgenic mice." Virology 255:221-227, 1999.

Dove, A.W., and V.R. Racaniello. "The polio eradication effort: should vaccine eradication be next?" Science 277:779, 1997.

Haller, A.A., S.R. Stewart, and B.L. Semler. "Attenuation stem loop lesions in the 5´ noncoding region of poliovirus RNA: neuronal cell-specific translation defects." Journal of Virology 70:1467-1474, 1996.

Halstead, L.S. "Post-polio syndrome." Scientific American 278::42-47, 1998.

Hull, H.F. and R.B. Aylward. "Ending polio immunization." Science 277:780, 1997.

Nugent, C.I., K.L. Johnson, and K. Kirkegaard. "Functional coupling between replication and packaging of poliovirus replicon RNA." Journal of Virology 73:427-435, 1999.

Schlein, L. "Hunting down the last of the poliovirus." Science 279:168, 1998.

Shiroki, K., T. Isoyama, T. Ishii et al. "Intracellular redistribution of truncated La protein produced by poliovirus 3Cpro-mediated cleavage." Journal of Virology 73:2193-2200, 1999.

Wilson, D.J. "A crippling fear:experiencing polio in the era of FDR." Bulletin of Historical Medicine 72(3):464-495, 1998.

© 2013 Howard Hughes Medical Institute. A philanthropy serving society through biomedical research and science education.
4000 Jones Bridge Road Chevy Chase, MD 20815-6789 | (301) 215-8500 | email: webmaster@hhmi.org