Poliomyelitis and polio

Poliomyelitis is a contagious infectious disease that is caused by three types of poliovirus. The viruses cause damage and destruction of cells in the nervous system. Paralysis can result in about 2% of those who contract the disease, which is called polio. Most people who contract polio either have mild symptoms or no symptoms at all.

Poliomyelitis has been part of human history for millennia. An Egyptian stone engraving depicting the debilitating effects of poliomyelitis dates from over 3,000 years ago. In that time, the occurrence of polio was rare, as sanitation was poor. The close proximity between people and raw sewage bestowed protective immunity against the polioviruses, which reside in the feces. As sewage treatment became better and indoor plumbing became widespread in the twentieth century, exposure to the virus became less and the protective immunity was less likely to develop in children. By the time the disease was first described in Britain in 1789 by Michael Underwood, outbreaks in children were occurring. From the latter decades of the nineteenth century through to the 1950s, polio epidemics occurred frequently. Children were most at risk and could be crippled from polio, or suffer muscle damage severe enough to require the assistance of iron lungs, early mechanical ventilators, because their lungs had been damaged to the point of incapacity.

The word poliomyelitis derives from the Greek word polio (grey) and myelon (marrow, indicating the spinal cord). It is the poliomyelitis effect on the spinal cord that is associated with the devastating paralysis of the severe form of the disease.

Poliovirus is a member of the enterovirus group of the family Picornaviridae. Poliovirus serotypes (an antigenic means of categorizing viruses) P1, P2, and P3 are the agents of poliomyelitis. The viruses are very susceptible to heat, ultraviolet light and chemicals such as formaldehyde and chlorine.

Poliovirus is spread most commonly via contact with feces. Only humans are involved in the transmission, as only humans harbor the virus. Typically, feces-soiled hands are not washed properly and then are put into or round the mouth. Spread of the virus by coughing or sneezing can also occur. The virus multiplies in the pharynx or the gastrointestinal tract. From there the virus invades adjacent lymph tissue, enters the blood stream and can infect cells of the central nervous system. When neurons of the brain stem are infected, the paralytic symptoms of poliomyelitis result.

About 95% of those who are infected by the poliovirus may not exhibit symptoms. In those people who do exhibit symptoms, about 4–8% exhibit a fever and flu-like malaise and nausea. Recovery is complete within a short time. These people can continue to excrete the virus in their feces for a time after recovery, and so can infect others. About 2% of those with symptoms develop a more sever form of nonparalytic aseptic meningitis . The symptoms include muscular pain and stiffness. In the severe paralytic poliomyelitis, which occurs in less than 2% of all polio infections, breathing and swallowing become difficult and paralysis of the bladder and muscles occurs. Paralysis of the legs and the lung muscles is common. This condition is known as flaccid paralysis.

The paralytic form of polio can be of three types. Spinal polio is the most common, accounting for 79% of paralytic polio in the Unites States from 1969 to 1979. Bulbar polio, which accounts for 2% of cases, produces weakness in those muscles that receive impulses from the cranial nerves. Finally, bulbospinal polio, which is a combination of the two, accounts for about 20% of all cases.

At the time of the development of the vaccines to polio, in the early 1950s, there were nearly 58,000 cases of polio annually in the United States, with almost 20,000 of these people being rendered paralyzed. Earlier, President Franklin Roosevelt committed funds to a "war on polio." Roosevelt was himself a victim of polio.

Jonas Salk developed a vaccine to the three infectious forms of the poliovirus (out of the 125 known strains of the virus) in the early 1950s. His vaccine used virus that had been inactivated by the chemical formaldehyde. An immune response was still mounted to the virus particles when they were injected into humans. The vaccine was effective (except for one early faulty batch) and quickly became popular.

Soon after the Salk vaccine appeared, Albert Sabin developed a vaccine that was based on the use of still-live, but weakened, polio virus. The vaccine was administered as an oral solution. While effective as a vaccine, the weakened virus can sometimes mutate to a disease-causing form, and the vaccine itself, rarely, can cause poliomyelitis (vaccine-associated paralytic poliomyelitis). As of January 2000, the Centers for Disease Control has recommended that only the Salk version of the polio vaccine be used.

There is still no cure for poliomyelitis. In the post-vaccine era, however, poliomyelitis is virtually non-existent in developed countries. For example, in the United States there are now only approximately eight reported cases of polio each year, mostly due to the vaccine-associated paralytic phenomenon. The last cases of poliomyelitis in the Unites States caused by wild virus occurred in 1979. Elsewhere, there are still bout 250,00 cases every year, mainly in the India subcontinent, the Eastern Mediterranean, and Africa. However, an ongoing vaccination campaign by the World Health Organization aims to eradicate poliomyelitis by 2010.

See also History of public health