Poliomyelitis (polio) is a highly infectious disease caused by a virus. It invades the nervous system, and can cause total paralysis in a matter of hours. It can strike at any age, but affects mainly children under three (over 50% of all cases). The virus enters the body through the mouth and multiplies in the intestine. Initial symptoms are fever, fatigue, headache, vomiting, stiffness in the neck and pain in the limbs. One in 200 infections leads to irreversible paralysis (usually in the legs). Children whose legs are paralyzed by polio often require crutches, special braces or wheelchairs in order to move around. However, amongst those paralyzed, 5%-10% die when their breathing muscles become immobilized. Although polio paralysis is the most visible sign of polio infection, fewer than 1% of polio infections ever result in paralysis.

Polio mainly affects children under five years of age and can spread widely before cases of paralysis are seen. Most people infected with the poliovirus do not develop polio paralysis or other symptoms of polio infection. However one in 200 people do have symptoms and can become paralyzed. The virus enters the environment through feces of people infected then is passed to others especially in situations of poor hygiene. Immune and or partially immune adults and children can still be infected with poliovirus and carry the virus for long enough to take the virus from one country to another, infecting close contacts and contaminating sanitation systems. This could facilitate transmission especially in countries where sanitation systems are sub-standard.

The disease may infect thousands of people, depending on the level of sanitation, before the first case of polio paralysis emerges. Individuals can carry the virus in their intestines just long enough to transmit to others. WHO considers a single confirmed case of polio paralysis to be evidence of an epidemic – particularly in countries where very few cases occur.

Once established in the intestines, poliovirus can enter the blood stream and invade the central nervous system – spreading along nerve fibres. As it multiplies, the virus destroys nerve cells (motor neurons) which activate muscles. These nerve cells cannot be regenerated and the affected muscles no longer function. The muscles of the legs are affected more often than the arm muscles. The limb becomes floppy and lifeless – a condition known as acute flaccid paralysis (AFP). More extensive paralysis, involving the trunk and muscles of the thorax and abdomen, can result in quadriplegia. In the most severe cases (bulbar polio), poliovirus attacks the motor neurons of the brain stem – reducing breathing capacity and causing difficulty in swallowing and speaking. Without respiratory support, bulbar polio can result in death.

During the first half of the 20th century, no illness inspired more dread and panic in the United States than did polio. Sometimes called infantile paralysis, polio struck in the U.S. every summer and fall with virulent epidemics. At that time, people with polio affecting the respiratory muscles were immobilized inside “iron lungs” – huge metal cylinders that operated like a pair of bellows to regulate their breathing and keep them alive. Today, the iron lung has largely been replaced by the positive pressure ventilator; nevertheless, it is still in use in some countries. In 1952, when the polio epidemic was at its peak, 3,000 people died.

By the mid-1950s, mass immunization with the polio vaccine began to slow polio’s spread, and in 1979 the last case of wild polio u2014 polio caused naturally, not by a vaccine containing live virus u2014 occurred in the U.S. Today, the disease has been eliminated from most of the world, and only seven countries world-wide remain polio-endemic. This represents the lowest number of countries with circulating wild poliovirus. At the same time, the areas of transmission are more concentrated than ever – 98 percent of all global cases are found in India, Nigeria and Pakistan. Adults previously vaccinated with a primary polio vaccine series and who are traveling to areas where polio is occurring should receive a booster dose of inactivated poliovirus (IPV). Immunity following a booster dose of IPV lasts a lifetime.

Despite polio’s ability to cause paralysis and death, the vast majority of people who get the poliovirus don’t become sick and are never aware they’ve been infected with polio. Yet even without signs or symptoms of polio, they still shed the virus intermittently in their stool for several weeks and occasionally for months. In areas with poor sanitation, a single person with poliovirus can potentially infect hundreds of others before the first case of polio that causes paralysis appears.

A small number of people who contract poliovirus develop nonparalytic polio u2014 a type of polio that doesn’t lead to paralysis (abortive poliomyelitis). This usually causes the same mild, flu-like signs and symptoms u2014 sore throat, fever, nausea, vomiting, and constipation or diarrhea u2014 typical of other viral illnesses. Most people recover from abortive polio in less than a week.

About 5 percent to 10 percent of infected people develop nonparalytic aseptic meningitis, a viral infection of the outer covering (meninges) of the brain. Signs and symptoms, which generally last two to 10 days, include fever, headache, vomiting, diarrhea, fatigue, back pain or stiffness, neck pain or stiffness , pain or stiffness in the arms or legs as well as muscle spasms or tenderness

Fewer than 1 percent of people infected with poliovirus develop paralytic polio, the most serious form of the disease. Paralytic polio often begins with a fever. Five to seven days later, other signs and symptoms appear, including headache, neck and back stiffness, constipation and increased sensitivity to touch. The paralytic polio symptom that causes limbs to appear loose and floppy (acute flaccid paralysis) often comes on suddenly and usually is worse on one side of the body. Paralytic polio has historically been divided into several types, depending primarily on which part of the body is affected. These classifications aren’t rigid, and overlap may occur among the different forms. In the past, distinctions among polio types may have varied with the method and time of diagnosis.

  • Spinal polio. This most common form of paralytic polio attacks certain nerve cells (motor neurons) in the spinal cord and may cause paralysis of the muscles that control breathing and those in the arms and legs. The muscles affected and the extent of paralysis depends on the part of the spinal cord and number of neurons involved. Although paralysis can occur in any combination of limbs u2014 for instance, both legs and one arm u2014 children under age 5 are most likely to become paralyzed in a single extremity, while in adults, paralysis of both arms and legs is more common.
  • Sometimes the neurons are only damaged, in which case the patient may recover some degree of muscle function. But if the neurons are completely destroyed, the paralysis is irreversible, although the patient still retains the sense of feeling, unlike after many spinal cord injuries.
  • Bulbar polio. In this severe type of polio, the virus affects the motor neurons in the brainstem, where the centers of the cranial nerves are located. These nerves are involved in the ability to see, hear, smell, taste and swallow. They also affect the movement of muscles in the face and send signals to the heart, intestines and lungs. Bulbar polio can interfere with any of these functions but is especially likely to affect the ability to breathe, speak and swallow and can be fatal without respiratory support.
  • Bulbospinal polio. A combination of both bulbar and spinal paralytic polio, this form can lead to paralysis of the arms and legs and may also affect breathing, swallowing and heart function.

Affecting some people who have recovered from polio, post-polio syndrome is a cluster of disabling signs and symptoms that appears decades u2014 between 10 and 40 years u2014 after the initial illness. Common signs and symptoms include new muscle weakness in limbs that may or may not have been affected initially, general fatigue and exhaustion after minimal activity, muscle and joint pain, breathing or swallowing problems, sleep-related breathing disorders, such as sleep apnea and decreased tolerance of cold temperatures.

Several viruses are transmitted to humans through animals. But the poliovirus resides only in humans and enters the environment in the feces of someone who’s infected. Poliovirus spreads primarily through the fecal-oral route, especially in areas where sanitation is inadequate.

Poliovirus can be transmitted through contaminated water and food u2014 there’s some evidence that flies may spread the virus to food u2014 or through direct contact with someone infected with the virus or who has recently received an oral polio vaccine (OPV), which contains live virus. Polio is so contagious that anyone living with a recently infected person is likely to become infected too. Although people carrying the poliovirus are most contagious seven to 10 days before and after signs and symptoms appear, they can spread the virus for weeks in their feces.

Once poliovirus invades the body, it multiplies in the throat and intestinal tract and then travels to the central nervous system through the blood and lymph fluid. As it moves along the nerve fibers, poliovirus damages or destroys the motor neurons that carry messages between the brain and the muscles.

In polio, to compensate for the missing neurons, the remaining nerve cells sprout new branching fibers (axons). Depending on how many neurons have been damaged, this regeneration may allow the patient to regain some or all of their muscle function, but it also places added stress on the nerve cell body, which has to nourish the additional fibers. Over time, this stress may lead to the gradual deterioration of the new fibers and eventually to the neuron itself. Researchers have theorized that this may cause the recurrence of signs and symptoms of post-polio syndrome.

Those at greatest risk of polio haven’t been immunized against the disease. In areas with poor sanitation and sporadic or nonexistent immunization programs, the most vulnerable members of the population u2014 pregnant women, the very old and very young u2014 are especially susceptible to poliovirus. Polio, which once affected infants and children almost exclusively, now affects people of all ages.

These factors also increase the risk:

  • Travel to an area where polio is common or that has recently experienced an outbreak
  • Living with or caring for someone who may be shedding poliovirus
  • Contact with someone recently vaccinated by oral polio vaccine (if you’re unvaccinated)
  • Handling laboratory specimens that contain live poliovirus
  • A compromised immune system, such as occurs with HIV infection
  • Trauma to the mouth, nose or throat such as dental surgery or a tonsillectomy
  • Extreme stress or strenuous physical activity after being exposed to poliovirus, both of which can depress the immune system

It’s not known why some people infected with poliovirus develop paralysis and others don’t. But many of the same factors that put patients at risk of polio also make it more likely that they can develop the paralytic form if infected. These risk factors include a compromised immune system, pregnancy, mouth or throat surgery or injury or strenuous physical exercise after exposure to the virus,

Although post-polio syndrome still isn’t well understood, certain factors increase the chances of developing this complex of symptoms decades after the initial infection:

  • Severe polio infection. The more severe the initial infection, the more likely that patients will have signs and symptoms of post-polio syndrome.
  • Age at onset of initial illness. If the patient acquired polio as an adolescent or adult, rather than as a young child, the chances of developing post-polio syndrome increase.
  • Recovery. Paradoxically, the greater the recovery after acute polio, the more likely it is that that patient will develop post-polio syndrome. This may be because greater recovery places additional stress on motor neurons.
  • Physical activity. If a patient has had polio, engaging in physical activity to the point of exhaustion or fatigue may overwork already-stressed motor neurons.

Recent research results suggest that the risk of post-polio syndrome is not as great as had once been thought. A Mayo Clinic study in 2006 that followed polio survivors during a 15-year period found that the physical decline of most polio survivors generally reflected that of the general population and was likely due to aging alone, rather than to post-polio syndrome.

In the United States, Centers for Disease Control and Prevention (CDC) records show that the last case of wild polio occurred in 1979. Between 1980 and 1999, however, the CDC found that144 people developed paralytic polio from exposure to the live virus in the oral polio vaccine. For that reason, only vaccine containing inactivated virus has been used in the U.S. since 2000.

In countries that use OPV, paralytic polio risk to travelers is low, but is not zero. Be sure to check with the doctor for polio vaccination recommendations before traveling to a part of the world where polio may still occur or OPV is used. Doctors often recognize polio by symptoms such as neck and back stiffness, abnormal reflexes, and difficulty swallowing and breathing. To confirm the diagnosis, a sample of throat secretions, stool or cerebrospinal fluid u2014 a colorless fluid that surrounds the brain and spinal cord u2014 is checked for the presence of poliovirus.

Paralytic polio can lead to temporary or permanent muscle paralysis, disability, and deformities of the hips, ankles and feet. Although many deformities can be corrected with surgery and physical therapy, these treatments often aren’t options in developing nations where polio is still endemic. As a result, children who survive polio may spend their lives with severe disabilities.

Other complications of polio involve the lungs, kidneys and heart and may include:

  • Pulmonary edema. Potentially life-threatening, this occurs when increased pressure in the lungs’ blood vessels forces fluid into the air sacs, filling the lungs with fluid and preventing them from absorbing oxygen.
  • Aspiration pneumonia. This inflammation of the lungs and bronchial tubes results when foreign material is inhaled u2014 usually stomach contents u2014 into the lungs. Although many factors can cause aspiration pneumonia, it’s most often a complication of polio, which can impair muscles that control swallowing.
  • Urinary tract infections. These infections begin when bacteria enter the bladder through the urinary tract and can permanently damage the kidneys if not treated promptly.
  • Kidney stones. Kidney stones usually form when urine becomes too concentrated, causing minerals and other substances in urine to form crystals in the kidneys. Over time, these crystals may combine to form a small, hard mass, or stone. If a stone stays inside the kidney, it can become so large it blocks the flow of urine, which causes pressure and pain and the risk of kidney damage, bleeding and infection. Untreated smaller stones may cause ongoing urinary tract infection or kidney damage.
  • Intestinal obstruction (paralytic ileus). This is a blockage of the bowel that prevents food and fluid from passing through. Minor blockages may cause bloating and a distended abdomen, but more-serious obstructions can lead to life-threatening complications.
  • Myocarditis. In this condition, the thick muscular layer of the heart (myocardium) becomes inflamed, leading to chest pain, an abnormal heartbeat or congestive heart failure, which occurs when the heart can’t pump enough blood to meet the body’s needs. Blood clots also may form in the heart, leading to a stroke or heart attack.
  • Cor pulmonale. The result of prolonged high blood pressure in the veins or arteries in the lungs, cor pulmonale occurs when the right side of the heart can’t pump hard enough to compete with the lungs’ increased blood pressure.

Because no drug developed so far has proven effective, treatment is entirely symptomatic. Moist heat is coupled with physical therapy to stimulate the muscles and antispasmodic drugs are given to produce muscular relaxation. Other supportive treatments include antibiotics for secondary infections (none for poliovirus), analgesics for pain, portable ventilators to assist breathing, moderate exercise and a nutritious diet While some of these measures can improve mobility, it cannot reverse permanent polio paralysis.

In the U.S., adults aren’t routinely vaccinated against polio because most are already immune and the chances of contracting wild polio are minimal. However, certain adults at high risk of polio who have had a primary vaccination series with either IPV or OPV should receive a single booster dose of IPV. A single booster dose of IPV lasts a lifetime. Adults at risk include those who are traveling to parts of the world where polio still occurs or those who care for people who may be excreting wild poliovirus or poliovirus from live OPV.