Polio

Polio (also called poliomyelitis) is a contagious, historically devastating disease that was virtually eliminated from the Western hemisphere in the second half of the 20th century. Although polio has plagued humans since ancient times, its most extensive outbreak occurred in the first half of the 1900s before the vaccination created by Jonas Salk became widely available in 1955.

At the height of the polio epidemic in 1952, nearly 60,000 cases with more than 3,000 deaths were reported in the United States alone. However, with widespread vaccination, wild-type polio, or polio occurring through natural infection, was eliminated from the United States by 1979 and the Western hemisphere by 1991.

Signs and Symptoms Polio is a viral illness that, in about 95% of cases, actually produces no symptoms at all (called asymptomatic polio). In the 4% to 8% of cases in which there are symptoms (calledsymptomatic polio), the illness appears in three forms:

1.a mild form called abortive polio (most people with this type may not even suspect they have it because their sickness is limited to mild flu-like symptoms such as mild upper respiratory infection, diarrhea, fever, sore throat, and a general feeling of being ill) 2.a more serious form associated with aseptic meningitis callednonparalytic polio (1%-5% show neurological symptoms such as sensitivity to light and neck stiffness) 3.a severe, debilitating form called paralytic polio (this occurs in 0.1%-2% of cases)

People who have abortive polio or nonparalytic polio usually make a full recovery. However, paralytic polio, as its name implies, causes muscle paralysis — and can even result in death. In paralytic polio, the virus leaves the intestinal tract and enters the bloodstream, attacking the nerves (in abortive or asymptomatic polio, the virus usually doesn't get past the intestinal tract). The virus may affect the nerves governing the muscles in the limbs and the muscles necessary for breathing, causing respiratory difficulty and paralysis of the arms and legs.

Two polio vaccines are used throughout the world to combat poliomyelitis (or polio). The first was developed by Jonas Salk and first tested in 1952. Announced to the world by Salk on April 12, 1955, it consists of an injected dose of inactivated (dead) poliovirus. An oral vaccine was developed byAlbert Sabin using attenuated poliovirus. Human trials of Sabin's vaccine began in 1957 and it was licensed in 1962

Because there is no long termcarrier state for poliovirus in immunocompetent individuals, polioviruses have no non-primate reservoir in nature, and survival of the virus in the environment for an extended period of time appears to be remote. Therefore, interruption of person to person transmission of the virus by vaccination is the critical step in global polio eradicationThe two vaccines have eliminated polio from most countries in the world, and reduced the worldwide incidence from an estimated 350,000 cases in 1988 to 1,652 cases in 2007