Poliomyelitis, or polio, is an infectious disease caused by a virus that normally lives in the human digestive tract. About 90% of persons infected by the virus have no symptoms at all. In the other 10%, the polio virus causes an infection with symptoms ranging from a mild flu-like illness to paralysis of the lower limbs or death from paralysis of the muscles that control breathing.
The term poliomyelitis comes from the Greek words polio, meaning gray, and myelon, referring to the spinal cord. The term is accurate, as an important consequence of the disease is the involvement of the spinal cord.
There are three known types of polioviruses (called 1, 2, and 3), each causing a different strain of the disease. All are members of the viral family of enteroviruses, which are viruses that infect the gastrointestinal tract. Type 1 is the cause of epidemics and about 85% of cases of paralysis, which is the most severe manifestation of the infection.
When the poliovirus reaches the central nervous system, inflammation and destruction of the spinal cord motor cells (anterior horn cells) occurs, which prevents them from sending out impulses to muscles. Loss of impulse transmission causes the muscles to become limp or soft and they cannot contract. This condition is referred to as flaccid paralysis. The extent of the paralysis depends on where the virus strikes and the number of cells that it destroys. Usually, some of the limb muscles are paralyzed; the abdominal muscles or muscles of the back may be paralyzed, affecting the person's posture. The neck muscles may become too weak for the head to be lifted. Paralysis of the face muscles may cause the mouth to twist or the eyelids to droop. According to WHO, one of every 200 infections leads to paralysis. Of those, between 5% and 10% die because their breathing muscles become paralyzed.
Humans are the only natural host of polioviruses; these viruses are not transmitted by animals. Some people are more likely than others to develop the paralytic form of the disease if they do become infected. These include:
According to the Centers for Disease Prevention and Control (CDC), the incidence rate has been less than 0.01 cases per 100,000 people in the United States since 1965.
Polio is caused by a virus that enters the mouth through food or water that has been contaminated by fecal matter. It is an extremely contagious illness; anyone living with a recently infected person is likely to become infected too. Although people carrying the poliovirus are most contagious for 7–10 days before and after symptoms (if any) appear, they can spread the virus for weeks in their bowel movements.
Once inside the body, the polio virus takes between 6 and 20 days to incubate. It finds its way to the tissues lining the throat and the intestinal tract, where it multiplies rapidly. After about a week in the intestines, the virus travels to the tonsils and the lymph nodes, where it multiplies further and then enters the bloodstream. It can remain within the blood and lymphatic system for as long as 17 weeks. In a minority of cases, the virus enters the central nervous system from the blood and lymph. It then multiplies in and destroys the nerve cells (motor neurons) in the brain and spinal cord that control the movements of the muscles. The location and severity of the paralytic polio that results when the motor neurons are damaged varies with the part of the central nervous system that is affected.
Between 4 and 8% of acute polio infections are characterized by influenza-like symptoms. This type of infection is called abortive poliomyelitis. People with this form of polio infection experience sore throat and fever, nausea, vomiting, abdominal pain, constipation, or diarrhea. Abortive polio is difficult to distinguish from influenza or other viral infections. Patients recover completely in about a week.
About 10% of people infected with poliovirus develop severe headache and pain and stiffness of the neck and back. These symptoms are due to an inflammation of the meninges (tissues that cover the spinal cord and brain). This syndrome is called nonparalytic or aseptic meningitis. The term “aseptic” is used to differentiate this type of meningitis from those caused by bacteria. Patients with nonparalytic meningitis may experience a brief period of general illness followed by stiffness in the neck, back, or legs. They may also experience other abnormal sensations for a period of 2–10 days. As with abortive polio, patients with nonparalytic meningitis recover completely.
Between 1 and 2% of people infected with poliovirus develop the most severe form, paralytic polio. Some of these individuals may have 2–3 symptom-free days between the minor illness and the major illness, but often the symptoms appear without any previous minor illness. Symptoms again include headache and back and neck pain. The major symptoms, however, are due to invasion of the motor nerves, which are responsible for movement of the muscles.
Paralytic polio is usually divided into three types, depending on whether the paralysis affects the arms and legs (spinal polio; accounts for 79% of cases of paralytic polio); breathing, speaking, and swallowing (bulbar polio; 2% of cases); or the limbs as well as breathing and other functions (bulbospinal polio; 19% of cases). Bulbar polio is particularly likely to lead to death if the patient is not placed on a respirator because the virus affects the brain stem—the part of the brain that controls heartbeat as well as breathing and other vital functions.
The maximum state of paralysis in paralytic polio is usually reached within a few days of the onset of symptoms. The remaining unaffected nerves then begin the process of attempting to grow branches to compensate for the destroyed nerves. Often, the nerve cells are not completely destroyed. By the end of a month, the nerve impulses start to return to the apparently paralyzed muscle and by the end of six months, recovery is almost complete. In cases where the nerve cells are completely destroyed, however, paralysis is permanent.
TESTS. To confirm the diagnosis, samples of the patient's stool, spinal fluid, or throat mucus may be collected and sent to a laboratory for analysis to see whether the sample contains the virus itself. A blood sample early in the infection may also be analyzed for evidence of antibodies to the poliovirus.
PROCEDURES. A lumbar puncture is the procedure performed in order to obtain a sample of the patient's spinal fluid. A long, thin needle is inserted into the lower back between the vertebrae to withdraw spinal fluid. This test can be used to reveal an increased number of white blood cells and no bacteria (aseptic meningitis).
There is no drug that can cure polio as of 2012. Antibiotics are ineffective against any viral infection, including polio. Patients with abortive polio or nonparalytic meningitis do not usually need treatment other than resting at home.
Patients with paralytic polio may be placed on a respirator to help them breathe, particularly if they are diagnosed with bulbar polio. Other treatments include painkillers and hot packs for muscle aches, physical therapy to restore muscle strength, and occupational or speech therapy as needed. Physical therapy is the most important part of management of paralytic polio during recovery. Braces or special shoes may be recommended for some patients. A few patients may undergo surgery to restore limb function.
The overall prognosis for recovery from an acute attack of paralytic polio is generally good. Mortality is about 5–10%, mostly in elderly and very young patients; however, the death rate can reach 20–60% in cases of bulbar involvement. Half the patients with spinal polio recover fully; 25% have mild disabilities; and the remaining 25% are left with severe disabilities. Most patients recover from breathing problems, and only a small percentage of patients need long-term treatment on a respirator. Patients with muscle paralysis typically recover about 60% of their strength in the first 3–4 months of treatment.
About a quarter of patients who have recovered from paralytic polio develop a disorder called postpolio syndrome (PPS) between 10 and 40 years after the initial infection. PPS is not a re-infection although its cause is not completely understood as of 2009. PPS is marked by:
The World Health Organization has sponsored the initiative to eradicate polio. They work in conjunction with public health agencies and non-governmental organizations around the world, with their effort concentrated in countries where polio is still present. Because humans are the only host to the polio virus, it is possible that the virus can, like the smallpox virus, be completely eliminated by properly supported and coordinated vaccination efforts.
Polio can easily be prevented by administration of either the Salk vaccine, which contains an inactivated poliovirus, or the Sabin oral vaccine, which contains a weakened live virus. The Salk vaccine (also called the inactivated polio vaccine or IPV) is given as a series of four shots administered at 2 months, 4 months, between 6 and 18 months and a booster given between 4 and 6 years. This immunization contains no live virus, just the components of the virus that provoke the recipient's immune system to react as if the recipient were actually infected with the poliovirus without causing polio symptoms. The recipient thus becomes immune to infection with the poliovirus in the future. The United States switched exclusively to using IPV rather than oral polio vaccine in 1999. It is the only polio vaccine that can safely be given to people with weakened immune systems.
The Sabin vaccine (also called the oral polio vaccine or OPV) is given to infants and adults by mouth in several doses. OPV contains the live, but weakened, poliovirus, which make the recipient immune to future infections with poliovirus. OPV is less expensive to administer than IPV. It requires no injections, thus no sterile needles, and is easily administered to both children and adults. For these reason, it is often used in the developing world. It is not routinely given to people with weakened immune systems because it contains a live virus and can very rarely cause polio.
See also Endemic ; Meningitis ; Smallpox ; Vaccination ; Viruses .
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Rebecca J. Frey, PhD
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