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.
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 that 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 poliovirus 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 (natural) 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 of wild poliovirus infection still occur in Pakistan and Afghanistan and were last reported in Nigeria in 2018.
In addition to wild-type poliovirus, very rarely (about 1 in 2.4 million doses) the live poliovirus in the oral vaccine mutates. The mutated vaccine virus can spread from the person who got the vaccine to unvaccinated people, continuing to mutate and potentially causing polio. In some countries, the mutated vaccine virus was virtually the only cause of polio, so most of these countries (including the United States) stopped using the oral polio vaccine. However, some countries still use the oral polio vaccine because it helps get more people vaccinated. Thus, cases of polio also occur in countries that use the live oral vaccine and have a lot of people who are not immunized (which allows the virus to spread more). Most recently, poliovirus infection resulting from the vaccine has been reported in the Democratic Republic of Congo and other areas of Africa. Widespread immunization can stop the spread of both types of polio outbreaks, and travelers to certain countries may need to show proof of adequate vaccination.
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.
Most infections cause no symptoms. Only about 25 to 30% of infected people develop any symptoms.
Polio infections that do cause symptoms are categorized as
About 4% of people who have this serious form of polio typically develop a stiff neck and/or back and headache (aseptic meningitis) several days after the flu-like symptoms of abortive poliomyelitis. The symptoms last 2 to 10 days. People do not develop paralysis.
Fewer than 1% of people have this severe form of polio. In addition to aseptic meningitis, people who have this form also develop paralysis.
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.
Abortive poliomyelitis resembles other viral infections and is typically not diagnosed unless it occurs during a polio epidemic.
Nonparalytic poliomyelitis is suspected in people who have flu-like symptoms and stiff neck and/or back.
Paralytic poliomyelitis is suspected in people who have muscle or limb paralysis or weakness.
The diagnosis of nonparalytic poliomyelitis or paralytic poliomyelitis 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) to look for other disorders that affect the brain and/or spinal cord and to test the spinal fluid for poliovirus.
People who have abortive poliomyelitis or nonparalytic poliomyelitis recover completely.
About two thirds of people who have paralytic poliomyelitis have some permanent weakness and a few (about 4 to 6%) die, depending on what nerves and muscles are affected. The death rate is higher (about 10 to 20%) in those few people in whom polio affects the nerves that control blood pressure and breathing.
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 (see Postpolio Syndrome).
Two types of vaccine are available worldwide:
The live oral vaccine is a weakened strain of poliovirus that does not cause disease. The oral vaccine provides better immunity in a population because people temporarily pass the vaccine virus in their stool so other people in their community may also be exposed to the vaccine. However, as the vaccine virus passes from person to person, it can very rarely mutate to a disease-causing form of the virus and cause 1 case of polio for every 2.4 million doses given. Because polio has been eradicated in the United States, doctors recommend only the injected inactivated poliovirus 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 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.
Widespread immunization can stop the spread of both types of polio outbreaks, and travelers to certain countries may need to show proof of adequate vaccination.
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 (a machine that helps air get in and out of the lungs) may be needed if the muscles used in breathing are weakened. Often, the need for a ventilator is temporary.