Polio, also known as infantile paralysis, or by it's full name poliomyelitis, a highly contagious disease that is caused by a virus. Contrary to widespread belief, poliomyelitis is not always a serious disease. The majority of people who contract polio are unaware of their infection. Many others have a minor illness, and only a relatively small number of people become more seriously ill. It is the serious form of the disease that may result in paralysis and even be fatal.
Poliomyelitis affects people in all parts of the world. Because it is so contagious, it often occurs in epidemics, but the epidemics are usually limited to small geographic areas. In temperate regions most cases of poliomyelitis occur in late summer and early fall, but in tropical climates the season for the disease is much longer. Although young children are most commonly affected, the number of cases affecting people more than 15 years old has been increasing during the past 25 years.
Although the poliomyelitis virus can cause disease in chimpanzees and monkeys, it occurs naturally only in man. It is not known how the virus is transmitted from one person to another. However, the virus is present in the intestinal tract and stool of an infected person, and it is believed that the disease may be spread by contaminated water and food. Flies have been suspected of helping to spread the contamination. Most people who become infected either have no visible illness or have only a minor illness that is often not recognized as poliomyelitis. However, they harbor the virus and can spread it to others, making the control of epidemics very difficult.
There is no specific treatment for poliomyelitis, and none is needed in mild cases. In more severe cases, heat treatment with hot wet packs or in the form of diathermy is given to reduce muscle pain. If the muscles involved in breathing are paralyzed or weakened, various machines, including the iron lung, are used to enable the person to breathe.
After recovery from the disease, paralyzed or weakened muscles are treated by physical therapy to strengthen and restore them so that they can function as well as possible. There is usually a gradual improvement in muscle function for the first year or two after recovery from poliomyelitis, but the damage that exists after that time is likely to be permanent. Some of the permanent effects of the disease can be corrected or improved by surgical operations.
Poliomyelitis is thought to be an ancient disease. There are Egyptian carvings of people who appear to show the aftereffects of polio, and an Egyptian skeleton dating from 3700 b.c. had bone malformations that probably resulted from polio. However, it was not until the late 1700's that accurate medical descriptions of the disease began to appear.
In 1908, Karl Landsteiner, an Austrian scientist who later became an American citizen, isolated the poliomyelitis virus. He and an associate, Hans Popper, transferred the disease from infected people to monkeys, making it possible for the first time to study the disease in experimental animals. However, monkeys are expensive and impractical laboratory animals for large-scale studies. Finally, in 1950 the American microbiologists John F. Enders, Frederick C. Robbins, and Thomas H. Weller succeeded in growing the virus on various kinds of cells in tissue culture. With this method, large quantities of the virus could be grown and studied. The intensive research work that fol lowed led rapidly to the development of the Salk and Sabin vaccines.
When a person becomes infected with the poliomyelitis virus, his body manufactures antibodies, or substances that neutralize the disease-causing organisms and bring about immunity. However, there are three different types of poliomyelitis viruses, and each must be neutralized by a different antibody. As a result, a person who has only one type of antibody is still susceptible to the other two types of virus.
Two basic types of vaccines that bring about immunity to all three types of poliomyelitis viruses have been developed. The first vaccine was developed in 1954 by the American scientist Jonas E. Salk. Salk vaccine contains inactivated viruses that cannot cause disease but can stimulate the body to produce antibodies. Three injections of the vaccine are needed, and additional injections may be required from time to time. Salk vaccine protects against paralytic polio.
In the late 1950's a vaccine that can be given orally was developed by the American physician Albert B. Sabin. This vaccine, which is believed to be even more effective than the Salk vaccine, contains living viruses that have been grown on monkey tissues. Sabin vaccine has a very weak capacity for causing disease, but a very strong capacity for stimulating the body to produce antibodies. It provides protection against both paralytic and nonparalytic polio.
After the Salk and Sabin vaccines became available, the number of cases of poliomyelitis was drastically reduced in many areas. However, epidemics still occur in areas where public health facilities are inadequate to provide the needed vaccines.
Forms and Symptoms
After the virus enters the body, from a few days to a month may elapse before the disease appears. The viruses multiply in the nose, throat, and intestinal tract and may then escape into the blood. During this stage of the illness, fever, headache, sore throat, and other cold symptoms appear, or a minor gastrointestinal illness, sometimes accompanied by grippelike symptoms, may develop. In the majority of cases the disease gets no worse. Sometimes, however, the virus may invade the nervous system, causing more severe forms of the disease.
One form, called nonparalytic poliomyelitis, causes stiffness of the back and neck in addition to the symptoms of the milder illness. Headache and backache may be severe. Nonparalytic poliomyelitis produces temporary abnormalities in the spinal fluid. However, the nerve cells are not damaged, and the disease usually subsides in a few days or a week without causing permanent damage.
Another, more severe form, called paralytic poliomyelitis, occurs when the virus attacks nerve cells. When it attacks nerves cells in the spinal cord, the muscles that are supplied by the involved nerves become painful, and muscular weakness and then paralysis may set in. The muscles affected and the degree of damage that occurs depends on the area of the spinal cord that has been invaded and on the number of nerve cells involved. Generally, the disease runs its course in about a week. After this the nerve cells may recover, and the muscles then will also recover. If the nerve cells are destroyed, however, permanent damage results.
When paralytic poliomyelitis attacks nerve cells in the brain, it is called bulbar poliomyelitis. Various nerves in the head and face, including those that supply the ears, the eyes, and the muscles controlling chewing and swallowing, may be affected. Sometimes the part of the brain that controls breathing and heartbeat is involved, and death may occur. Paralytic poliomyelitis may also be a combination of the spinal and bulbar forms.
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