Реферат: Polio Essay Research Paper Poliomyelitis infectious virus
Polio Essay, Research Paper
Poliomyelitis, infectious virus disease of the central nervous system, sometimes resulting in paralysis. The greatest incidence of the disease, also known as infantile paralysis, is in children between the ages of five and ten years. The disease was described in 1840 by the German orthopedist Jacob von Heine. In its clinical form it is more prevalent in temperate zones.
The virus usually enters the body through the alimentary tract and spreads along nerve cells to affect various parts of the central nervous system. The incubation period ranges from about 4 to 35 days. Early symptoms include fatigue, headache, fever, vomiting, constipation, stiffness of the neck, or, less commonly, diarrhea and pain in the extremities. Because nerve cells that control muscular movement are not replaced once they are destroyed, poliovirus infection can cause permanent paralysis. When nerve cells in respiratory centers, which control breathing, are destroyed, the victim must be kept alive by an iron lung (see Artificial Respiration). For every paralytic case of poliomyelitis, however, there may be 100 nonparalytic cases.
Because no drug developed so far has proved effective, treatment is entirely symptomatic. Use of moist heat coupled with physical therapy to stimulate the muscles was first initiated by the Australian nurse Elizabeth Kenny, and antispasmodic drugs are administered to produce muscular relaxation. In the convalescent stage, occupational therapy is used.
Three broad types of the virus have been identified: the Brunhilde (type 1), Lansing (type 2), and Leon (type 3) strains. Immunity to one strain does not furnish protection against the other two.
Poliomyelitis control was made possible when, in 1949, the American bacteriologist John Franklin Enders and his coworkers discovered a method of growing the viruses on tissue in the laboratory. Applying this technique, the American physician and epidemiologist Jonas Salk developed a vaccine prepared from inactivated poliomyelitis viruses of the three known types. After field trials in 1954 the vaccine was pronounced safe and effective, and mass inoculation began. The American virologist Albert Sabin subsequently developed a vaccine containing attenuated, live polio virus that could be given orally. This vaccine, called trivalent oral polio vaccine (TOPV), was licensed in 1963 and has replaced the Salk injectable vaccine as the standard immunizing agent in the U.S. As a result of routine immunization, outbreaks of paralytic poliomyelitis declined dramatically from 57,879 cases in 1952 to only a few each year.
The vulnerability of a population that was not immunized was demonstrated in 1979, when 16 cases of paralytic poliomyelitis occurred among Amish people in the United States and Canada who had not been vaccinated.