Poliomyelitis

What Is It?

Poliomyelitis, commonly called polio, is a highly contagious infection caused by the poliovirus. In a small percentage of infected people, the virus attacks nerve cells in the brain and spinal cord, particularly the nerve cells in the spinal cord that control muscles involved in voluntary movement such as walking. Destruction of these neurons causes permanent paralysis in one in 200 cases.

The infection is spread through direct contact with virus particles that are shed from the throat or in feces. The disease was virtually eradicated in the Western hemisphere after the introduction of the inactivated vaccine in 1955 and the oral, live vaccine in 1961. It is anticipated that polio will soon be eradicated worldwide. Vaccination campaigns have succeeded in reducing the number of countries where polio is endemic (where it occurs locally). In 1988, more than 120 countries contained endemic poliovirus. In 1998, 50 countries contained endemic polio, and in 2002, only six countries still had locally circulating infection. Today, 90 percent of all endemic polio is contained within India, Nigeria and Pakistan. However, in developing countries, some individuals remain unvaccinated, and poor sanitation and poor hygiene promote spread of the virus. People traveling to these areas of the world must have up-to-date immunizations.

Today polio occurs in developed countries only as a rare complication of immunization with the live polio vaccine. Paralytic poliomyelitis caused by the live vaccine has become even more rare recently because countries that have eradicated polio are switching to the inactivated polio vaccine, which never causes the disease.

Today poliomyelitis occurs in the United States only as a rare complication of immunization with the live polio vaccine. Approximately five to 10 cases are reported each year, half of them in vaccine recipients — usually infants receiving their first dose of the vaccine. Most other cases occur in people in close contact with immunized infants; children or adults with weakened immune systems are especially vulnerable.

Symptoms

There are two forms of polio illness:

• Minor poliomyelitis (also called abortive poliomyelitis), which occurs primarily in young children, is the most common of the two. The illness is mild and the brain and spinal cord are not affected. Symptoms appear three to five days after exposure to the virus, and include slight fever, headache, sore throat, vomiting, lack of appetite and a general feeling of illness and discomfort.
• Major poliomyelitis (also called paralytic and nonparalytic polio) is a more severe illness that develops approximately seven to 14 days after exposure to the virus. Symptoms include fever, severe headache, stiff neck and back, and deep muscle pain. Some people experience temporary abnormalities of skin sensation. Muscle spasms and a tendency to retain urine are common. Muscle weakness and paralysis may develop rapidly or gradually during the time fevers are occurring, but paralysis does not continue to get worse afterwards. The disease most commonly affects the muscles of the legs, but the areas affected depend upon what portion of the spinal cord is damaged, and can include the arms, abdominal muscles, chest, neck and throat muscles. Respiratory failure occurs if the virus infects the nerves in the spinal cord that control the respiratory muscles of the chest, or the parts of the brain involved in respiration.