Polio (poliomyelitis, infantile paralysis) is a highly contagious, sometimes fatal, viral infection that affects nerves and can cause permanent muscle weakness, paralysis, and other symptoms.
Polio is caused by a virus and is usually spread by consuming contaminated food or water or touching a contaminated surface and then touching the mouth.
Many people who have been infected have no symptoms, and most of the others have only mild symptoms.
Serious symptoms include fever, headache, a stiff neck and back, deep muscle pain, and sometimes weakness or paralysis.
The diagnosis is based on symptoms and the results of a stool culture.
Some children recover completely, whereas others have permanent weakness.
Routine vaccination can prevent the infection.
There is no cure for polio.
Polio is caused by poliovirus, an enterovirus, which is spread by swallowing food or water contaminated with stool from an infected person or by touching a contaminated surface, then touching the mouth. Sometimes polio is spread through saliva from an infected person or droplets expelled when an infected person sneezes or coughs. People become infected when they inhale airborne droplets or touch something contaminated with the infected saliva or droplets.
The infection usually begins in the intestine. It may then spread to the parts of the brain and spinal cord that control the muscles.
In the early 20th century, polio was widespread throughout the United States and elsewhere. Today, because of extensive vaccination, polio outbreaks have largely disappeared in developed countries, and most doctors have never seen a new polio infection. The last case of wild poliovirus infection in the United States occurred in 1979. The Western Hemisphere was certified polio-free in 1994. A global polio eradication program is under way, but cases still occur in sub-Saharan Africa (mainly Nigeria) and southern Asia (mainly the tribal areas of Pakistan and Afghanistan). Part of the reason polio has not been eliminated worldwide is the civil strife and military activity in the Middle East.
Unimmunized people of all ages are susceptible to polio. In the past, polio outbreaks occurred mainly in children and adolescents, because many older people had already been exposed to the virus and developed immunity.
Polio has a mild form and a severe form (called major poliomyelitis).
Only about 25 to 30% of infected people develop any symptoms. Nearly all people simply have fever, mild headache, sore throat, vomiting, and a general feeling of illness (malaise). These symptoms develop 3 to 5 days after exposure to the virus.
Fewer than 1% of people have more serious symptoms (major poliomyelitis), including the paralysis that many people associate with polio. Major poliomyelitis is more likely to occur in older children and adults.
The symptoms, which usually appear 7 to 21 days after infection, include fever, severe headache, a stiff neck and back and deep muscle pain. Sometimes areas of skin develop odd sensations, such as pins and needles or unusual sensitivity to pain.
Depending on which parts of the brain and spinal cord are affected, the disease may progress no further, or weakness or paralysis may develop in certain muscles. Typically, paralysis affects the muscles in the arms and legs, making them limp and unable to contract (called flaccid paralysis).
The person may have difficulty swallowing and may choke on saliva, food, or fluids. Sometimes fluids go up into the nose, and the voice may develop a nasal quality. Sometimes the part of the brain responsible for breathing is affected, causing weakness or paralysis of the chest muscles. Some people are completely unable to breathe.
When polio is mild, it resembles other viral infections and is typically not diagnosed unless it occurs during a polio epidemic.
A doctor suspects the more severe form of polio based on its symptoms. Diagnosis is confirmed by identifying poliovirus in a stool sample or throat swab or by detecting high levels of antibodies to the virus in the blood.
Doctors usually also do a spinal tap (lumbar puncture—see Figure: How a Spinal Tap Is Done) to look for other disorders that affect the brain and/or spinal cord and to test the spinal fluid for polio.
People with the milder form of polio completely recover.
About two thirds of people with severe form have some permanent weakness. Some people, even those who apparently have recovered completely, develop a return or worsening of muscle weakness years or decades after an attack of polio. This condition (postpolio syndrome) often results in severe disability (see Postpolio syndrome).
Between 4 and 20% of people with the severe form die.
Polio vaccine is included among the routine childhood immunizations (see Vaccinating Infants and Children). The vaccine is effective in more than 95% of children.
Two types of vaccine are available worldwide:
The live oral vaccine provides better immunity in a population but can mutate and cause polio in about 1 out of every 2.4 million children. Because polio has been eradicated in the United States, doctors recommend only the injected vaccine for children in this country. The oral vaccine is no longer available in the United States but is used in other parts of the world.
A first vaccination of people older than 18 is not routinely recommended because the risk of acquiring polio as an adult is extremely low in the United States. Because polio still exists in certain areas, adults who have never been vaccinated or have not been given all the recommended doses and who are traveling to an area where polio is still a health risk should receive the full series of 3 doses of injected vaccine. Ideally, people are given at least 2 doses—4 to 8 weeks apart—before travel. Those who have been completely vaccinated should still be given one dose of injected vaccine. Local and state health departments have information about which areas have polio, as does the Centers for Disease Control and Prevention (CDC: Polio Vaccine Guidance for Travelers).
Polio cannot be cured, and available antiviral drugs do not affect the course of the disease.
Treatments include rest, pain relievers, and drugs to lower fever.
A ventilator may be needed if the muscles used in breathing are weakened. Often, the need for a ventilator is temporary.