III.

Forms of Polio

Once the poliovirus enters the bloodstream, it causes one of three forms of polio that vary in severity. In more than 95 percent of polio cases, the infection produces only mild symptoms or none at all. Sometimes, however, the virus invades the nervous system, causing more severe forms of polio.

A.

Abortive Poliomyelitis

Abortive poliomyelitis is a mild form of the disease that lasts only from a few hours to a few days. If symptoms occur, they may include fever, headache, sore throat, fatigue, nausea, or vomiting—many of the symptoms typical of the flu. For the vast majority of people infected with the poliovirus, the illness gets no worse.

B.

Nonparalytic Poliomyelitis

Poliovirus infection of nerve cells sometimes results in nonparalytic poliomyelitis—that is, polio without paralysis. Patients with nonparalytic polio experience the fever and other symptoms of abortive poliomyelitis. In addition, they typically feel pain and stiffness in the neck and back. They also may develop aseptic meningitis—inflammation of the membranes that surround the brain and spinal cord. Symptoms from nonparalytic polio usually subside within a week without causing lasting damage.

C.

Paralytic Poliomyelitis

In about 1 or 2 percent of poliovirus infections, paralytic poliomyelitis, a disabling form of the disease, occurs. The virus infects motor neurons (nerve cells that send signals to muscles) in the spinal cord and damages or destroys them. The muscles that the neurons activate become painful and weak. Paralysis begins when the muscle is no longer able to move. Roughly 2 to 5 percent of infants who develop paralytic polio die. For adults the disease is even deadlier, causing death in 15 to 30 percent of cases.

Which muscles are affected and the extent of the paralysis depend on the part of the spinal cord the poliovirus invades and the number of nerves affected. The legs or arms are most often affected, and one side or both sides of the body may be involved. The older the person is when polio strikes, the more likely extensive paralysis becomes. In some cases paralysis of muscles that control breathing occurs, requiring mechanical breathing assistance (see Artificial Respiration).

In the most serious cases of paralytic polio, the virus attacks the brainstem, causing bulbar poliomyelitis. This type of polio can affect nerves that send signals to the ears, eyes, and the muscles controlling chewing and swallowing. Sometimes the virus affects the part of the brain that controls the rate of breathing and the heartbeat, resulting in death.