Polio is a contagious viral illness. In its most severe form, polio causes paralysis, difficulty breathing and sometimes 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. 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 — polio caused naturally, not by a vaccine containing live virus — occurred in the U.S. Today, despite a concerted global eradication campaign, wild poliovirus continues to afflict children and adults in developing nations, including Afghanistan, India, Nigeria and Pakistan.
The Centers for Disease Control and Prevention (CDC) advises that you take precautions to protect against polio if you're traveling to certain parts of the world where there is risk of polio. 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.
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 — there's some evidence that flies may spread the virus to food — 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 your body, it multiplies in your throat and intestinal tract and then travels to your central nervous system through your blood and lymph. As it moves along your nerve fibers, poliovirus damages or destroys the motor neurons that carry messages between your brain and your 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 you to regain some or all of your 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.
You're at greatest risk of polio if you 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 — pregnant women, the very old and very young — are especially susceptible to poliovirus. Polio, which once affected infants and children almost exclusively, now affects people of all ages.
These factors also increase your 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 your 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 your 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 you at risk of polio also make it more likely that you'll develop the paralytic form if you're infected. These risk factors include :
- A compromised immune system
- Mouth or throat surgery
- Injury or strenuous physical exercise after exposure to the virus
Although post-polio syndrome still isn't well understood, certain factors may increase your chances of developing this complex of symptoms decades after your initial infection :
- Severe polio infection. The more severe the initial infection, the more likely that you'll have signs and symptoms of post-polio syndrome.
- Age at onset of initial illness. If you acquired polio as an adolescent or adult, rather than as a young child, your chances of developing post-polio syndrome increase.
- Recovery. Paradoxically, the greater your recovery after acute polio, the more likely it is that you'll develop post-polio syndrome. This may be because greater recovery places additional stress on motor neurons.
- Physical activity. If you’ve had polio, engaging in physical activity to the point of exhaustion or fatigue may overwork already-stressed motor neurons.
Recent research results suggest that 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.
When to seek medical advice
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 your doctor for polio vaccination recommendations before traveling to a part of the world where polio may still occur or OPV is used.
Call your doctor if :
- Your child hasn't completed the series of polio vaccinations
- Your child experiences an allergic reaction after receiving polio vaccine
- Your child has problems other than a mild redness or soreness at the injection site
- You have questions about adult vaccination or other concerns about polio immunization
- You had polio years ago and are now experiencing unexplained weakness and fatigue
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 — a type of polio that doesn't lead to paralysis (abortive poliomyelitis). This usually causes the same mild, flu-like signs and symptoms — sore throat, fever, nausea, vomiting, and constipation or diarrhea — 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 :
- Back pain or stiffness
- Neck pain or stiffness
- Pain or stiffness in the arms or legs
- 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 :
- Neck and back stiffness
- 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 your spinal cord and may cause paralysis of the muscles that control breathing and those in your arms and legs. The muscles affected and the extent of paralysis depend on the part of the spinal cord and number of neurons involved. Although paralysis can occur in any combination of limbs — for instance, both legs and one arm — 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 you may recover some degree of muscle function. But if the neurons are completely destroyed, the paralysis is irreversible, although you still retain your sense of feeling, unlike after many spinal cord injuries.
- Bulbar polio. In this severe type of polio, the virus affects the motor neurons in your brainstem, where the centers of the cranial nerves are located. These nerves are involved in your ability to see, hear, smell, taste and swallow. They also affect the movement of muscles in your face and send signals to your heart, intestines and lungs. Bulbar polio can interfere with any of these functions but is especially likely to affect your 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 your 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 — between 10 and 40 years — 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
- Decreased tolerance of cold temperatures
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 — a colorless fluid that surrounds the brain and spinal cord — 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 you inhale foreign material — usually stomach contents — into your 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 your bladder through the urinary tract and can permanently damage your kidneys if not treated promptly.
- Kidney stones. Kidney stones usually form when your urine becomes too concentrated, causing minerals and other substances in urine to form crystals in your kidneys. Over time, these crystals may combine to form a small, hard mass, or stone. If a stone stays inside your 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 your 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 your heart (myocardium) becomes inflamed, leading to chest pain, an abnormal heartbeat or congestive heart failure, which occurs when your heart can't pump enough blood to meet your 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 your lungs, cor pulmonale occurs when the right side of your heart can't pump hard enough to compete with the lungs' increased blood pressure.
Because no cure for polio exists, the focus is on increasing comfort, speeding recovery and preventing complications. Supportive treatments include :
- Antibiotics for secondary infections (none for poliovirus)
- Analgesics for pain
- Portable ventilators to assist breathing
- Moderate exercise
- A nutritious diet
Although improved public sanitation and careful personal hygiene may help reduce the spread of polio, the most effective way to prevent the disease is with polio vaccine.
However, efforts to banish polio worldwide face major obstacles. Some communities in parts of the world, concerned about the safety of polio vaccine, have limited immunization efforts. In other areas, war and civil disorder prevent health workers from reaching vulnerable populations. In addition, travelers in unvaccinated populations may unwittingly carry the virus into previously polio-free zones.
Currently, most children in the United States receive four doses of inactivated poliovirus at the following ages :
- 2 months
- 4 months
- Between 6 and 18 months
- A booster shot, between ages 4 and 6 years when children are just entering school
In some states, the fourth shot isn't required if children receive the third shot on or after their fourth birthday.
IPV is 90 percent effective after two shots and 99 percent effective after three. It can't cause polio and is safe for people with weakened immune systems, although it's not certain just how protective the vaccine may be in cases of severe immune deficiency. The most common side effects are swelling and redness at the injection site.
IPV causes a serious allergic reaction in some children and adults. Because the vaccine contains trace amounts of the antibiotics streptomycin, polymyxin B and neomycin, it shouldn't be given to anyone who's sensitive to these medications.
Signs and symptoms of an allergic reaction usually occur within minutes to a few hours after the shot and may include :
- High fever
- Difficulty breathing
- Hoarseness or wheezing
- Rapid heart rate
- Unusual paleness
- Swelling of the throat
If your child experiences an allergic reaction after any shot, get medical help immediately.
Fewer shots for your child
Polio vaccine is normally given in conjunction with other vaccinations, including diphtheria, tetanus and acellular pertussis (DTaP); hepatitis B- Haemophilus influenzae type b (HBV-Hib); and pneumococcal conjugate vaccine (PCV). But your child may not need to receive all these injections separately.
A combination vaccine called Pediarix is available that reduces the number of injections given during the first two years of life. Pediarix combines DTaP, hepatitis B and polio into a single vaccine. It's available worldwide, with the exception of Canada. Side effects of Pediarix are the same as those of the individual vaccines administered separately.
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.
If you're unvaccinated or your vaccination status is undocumented, you should receive a primary polio vaccination series with IPV — two doses at four- to eight-week intervals and a third dose six to 12 months after the second dose.