Poliomyelitis.

Poliomyelitis. I INTRODUCTION Poliomyelitis, infectious viral disease, commonly called polio, that sometimes causes temporary or permanent paralysis. The infection chiefly affects children and young adults. More than 95 percent of polio infections produce no noticeable symptoms or mild symptoms that last only a few days. In the remainder the virus enters the nervous system and infects nerve cells that control muscles. It can then cause paralysis, most often of the legs. In its most dangerous form, the polio virus attacks the brain, creating complications that sometimes result in death. Because of polio's prevalence among young children, it was sometimes called infantile paralysis. Polio once ranked among the most dreaded diseases in much of the world. The disease most often struck in summer and early fall. Fearing infection, many people avoided beaches, public pools, theaters, fairs--any place of public gathering. Occasionally schools were closed until epidemics subsided. Paralysis was frightening. Especially terrifying was the thought of lying immobile in an "iron lung"--an artificial breathing device for polio patients who could no longer breathe by themselves. The fight against polio represents one of the great medical success stories of the 20th century. In much of the world, polio went from a feared disease to mostly a memory in little more than half a century. The introduction of vaccines that protect against polio in the 1950s, along with successful vaccination programs, virtually wiped out polio in developed countries. Vaccination efforts continue in developing countries of Asia and Africa. II HOW POLIO SPREADS AND DEVELOPS The poliovirus spreads in human feces. People become infected with the virus through contaminated food and water, especially in areas where sanitation and hygiene are poor. Improper sewage disposal, for example, can contaminate a water supply. Poliovirus typically enters the body through the mouth and proceeds through the digestive tract to the intestines. After multiplying in the body, the virus is shed in the feces, from which it can spread and cause further infections, especially when infected people do not wash their hands and touch food or other people. Adults can become infected by changing the diapers of an infected infant and then touching their mouth. The poliovirus multiplies in the tonsils and in intestinal tissue known as Peyer's patches, where cells of the body's lymphatic system are concentrated. When the virus passes into the body's lymphatic system, it stimulates the production of antibodies--immune-system defenders that work to destroy the viral intruders. From the lymphatic system, the virus typically invades the bloodstream. III FORMS OF POLIO Once the poliovirus enters the bloodstream, it causes one of three forms of polio that vary in severity. In more than 95 percent of polio cases, the infection produces only mild symptoms or none at all. Sometimes, however, the virus invades the nervous system, causing more severe forms of polio. A Abortive Poliomyelitis Abortive poliomyelitis is a mild form of the disease that lasts only from a few hours to a few days. If symptoms occur, they may include fever, headache, sore throat, fatigue, nausea, or vomiting--many of the symptoms typical of the flu. For the vast majority of people infected with the poliovirus, the illness gets no worse. B Nonparalytic Poliomyelitis Poliovirus infection of nerve cells sometimes results in nonparalytic poliomyelitis--that is, polio without paralysis. Patients with nonparalytic polio experience the fever and other symptoms of abortive poliomyelitis. In addition, they typically feel pain and stiffness in the neck and back. They also may develop aseptic meningitis--inflammation of the membranes that surround the brain and spinal cord. Symptoms from nonparalytic polio usually subside within a week without causing lasting damage. C Paralytic Poliomyelitis In about 1 or 2 percent of poliovirus infections, paralytic poliomyelitis, a disabling form of the disease, occurs. The virus infects motor neurons (nerve cells that send signals to muscles) in the spinal cord and damages or destroys them. The muscles that the neurons activate become painful and weak. Paralysis begins when the muscle is no longer able to move. Roughly 2 to 5 percent of infants who develop paralytic polio die. For adults the disease is even deadlier, causing death in 15 to 30 percent of cases. Which muscles are affected and the extent of the paralysis depend on the part of the spinal cord the poliovirus invades and the number of nerves affected. The legs or arms are most often affected, and one side or both sides of the body may be involved. The older the person is when polio strikes, the more likely extensive paralysis becomes. In some cases paralysis of muscles that control breathing occurs, requiring mechanical breathing assistance (see Artificial Respiration). In the most serious cases of paralytic polio, the virus attacks the brainstem, causing bulbar poliomyelitis. This type of polio can affect nerves that send signals to the ears, eyes, and the muscles controlling chewing and swallowing. Sometimes the virus affects the part of the brain that controls the rate of breathing and the heartbeat, resulting in death. IV DIAGNOSIS AND TREATMENT OF POLIO Doctors diagnose polio by isolating the virus in throat cultures, stool samples, or samples of cerebrospinal fluid (the fluid that surrounds the brain and spinal cord) taken from an infected person. Blood tests that identify antibodies to the poliovirus also confirm a diagnosis. As yet there is no cure for polio--no drug or other medical treatment can halt the destructive action of the disease. However, medical treatment can lessen the severity of symptoms and prevent complications. Simple treatments, including moist heat applied to affected muscles, can ease pain, as can pain-relieving drugs. Antispasmodic drugs can help patients who suffer muscle spasms (involuntary muscle contractions) as a consequence of nerve damage. Strict bed rest was once advised for patients with paralytic polio, but today the only limitations on a patient's movement are those imposed by the disease itself. An extremely dangerous complication of paralytic polio develops when nerve damage affects the muscles used in breathing. When polio patients lose the ability to breathe by themselves, they must rely on a mechanical device for artificial respiration. Today, a machine called a ventilator is used. It forces air into the lungs by way of a tube inserted into the upper airway, through the nose, mouth, or a slit in the trachea. Physical therapy helps patients with paralytic polio stretch and move affected muscles. This movement minimizes the atrophy, or shrinkage, of affected muscles and limbs and builds strength. Exercises can also retrain working muscles to compensate for other muscles that have permanently lost the ability to move. Rehabilitative therapy trains patients to use braces, crutches, and other devices that provide support and aid mobility. V PREVENTION OF POLIO Immunization with polio vaccine is the best way to prevent polio. Vaccines work by exposing the body's immune system to a microbial infection that is strong enough to provoke an immune response but not severe enough to result in full-blown illness. In response to the infection, the immune system produces antibodies to fight the infectious agent. Once the body has overcome the challenge of the vaccine-induced infection, the antibodies can recognize and quickly handle any subsequent invasion by the same agent (see Immunization). Scientists use two main strategies for producing a vaccine, and both methods have been successfully employed against polio. One strategy uses quantities of virus that have been inactivated, or killed. This form of the polio vaccine is administered by injection. The other vaccine strategy uses live virus that scientists grow in a laboratory and systematically weaken so that it no longer causes serious infection. This form of polio vaccine, known as oral polio vaccine, is administered by mouth, either in a drink or in a sugar cube or other food. The oral vaccine is easier to administer than the injected vaccine, particularly in remote areas that lack trained medical staff or proper hospital facilities. The live poliovirus used in the oral polio vaccine involves some slight risk. There is a chance that a dose may contain improperly weakened virus that is still capable of causing illness. In the 1990s the risk of contracting paralytic polio from oral polio vaccine was 1 in approximately 2.4 million doses of vaccine. Successful immunization programs virtually wiped out the disease in North America, Latin America, and Europe. But the struggle to eradicate polio continues in some developing nations. Furthermore, health officials stress that vaccination programs must be maintained in areas considered polio free. They warn that the poliovirus could easily become dangerous again among children who have not acquired immunity. This danger was chillingly demonstrated in 1979 when an outbreak of paralytic polio erupted among unvaccinated members of Amish communities in Pennsylvania and Maryland. A Post-Polio Syndrome Some people who had paralytic polio develop new symptoms years later, a condition known as post-polio syndrome (PPS). Anywhere from 25 to 50 percent of these paralytic polio survivors grapple with PPS decades after their original illness, according to estimates by the National Institutes of Health. The cause of PPS is unknown, and there is no treatment available. Common symptoms of PPS include lessened endurance, fatigue, or exhaustion, after even minor activity; weakness and pain in muscles, even in those that seemed unaffected by the original illness; and joint pain. The symptoms persist and gradually worsen. People with PPS may also experience sleep difficulties, including sleep apnea; difficulty swallowing; muscle twitches and other symptoms. For some people the symptoms are mild, while for others they become severe as the muscles atrophy (waste away). Although medical experts do not know the exact cause of PPS, there is a widely accepted theory to explain the loss of muscle function. According to this theory, after nerve cells are damaged or destroyed during the original illness, the surviving nerve cells seek to compensate for this loss by growing new nerve endings, or muscular connections, to restore muscle function. However, these new connections appear to wear out after years of overwork, producing the symptoms of PPS. Another possible source of the problem is that the undamaged limb or limbs have overcompensated for the damaged limb or limbs for many years. Post-polio syndrome is not life threatening, but it can limit mobility and interfere with daily activities. There is no treatment for PPS. Physical therapy can help strengthen muscles, and pain medications can ease pain. Most experts advise people with PPS to pace their exertion through the day and simplify their activities wherever possible. B History Polio has afflicted humans since ancient times. A carved stone tablet found in Egypt, dating from roughly 1500 BC, depicts a man whose withered, deformed leg and foot are characteristic of paralytic polio. Descriptions of cases consistent with polio also date from ancient Greek and Roman times. One of the first accurate descriptions of polio was made by British physician Michael Underwood. In 1789 he noted cases of "debility of the lower extremities, [which] usually attacks children previously reduced by fever." Around 1850 German physician Jacob von Heine named the condition infantile paralysis because the disease seemed to affect mainly young children. Until the 19th century, polio was a common infection that rarely caused paralysis in children. As the 1800s ended, however, the nature of the disease changed. Ironically, this change occurred as a result of gradual improvements in sanitation and plumbing. Poor sanitation previously had constantly exposed people to poliovirus and other fecal contaminants. Most people, therefore, developed natural immunity after early and usually harmless exposure to the virus. Better plumbing and sanitation broke the cycle of exposure and natural immunity. Thus, people tended to be exposed to the virus at a later age, when the effects of the disease were more serious. The first large-scale polio epidemics hit Scandinavia in 1887 and the United States in 1894. Widespread outbreaks occurred in the United States in 1916, when over 37,000 polio cases were reported in 26 states, resulting in 6,000 deaths. For many years thereafter, polio struck about 38,000 Americans annually, and about 21,000 of them experienced paralysis. To direct the fight against the disease, in 1938 U.S. President Franklin Delano Roosevelt (who had himself lost the use of both legs to polio) founded the National Foundation for Infantile Paralysis, or March of Dimes. Through contributions as small as a dime, the organization raised millions of dollars for polio research and the support of those who had the disease. It is now known as the March of Dimes Birth Defects Foundation. B1 Advances in Treatment In the early part of the 20th century, treatment of polio patients consisted of immobilizing affected limbs in splints and plaster. Australian nurse Elizabeth Kenny, known more familiarly as Sister Kenny, recognized that such treatment would further weaken muscular function. She developed a regimen of moist heat and early limb mobilization that proved effective in the recovery of polio patients. Sister Kenny campaigned forcefully and tirelessly on behalf of her unorthodox method. Gradually her regimen won support, even against the initially heavy resistance in the United States medical community. Her regimen became a standard treatment for polio. In the early 1920s American physician Philip Drinker invented an artificial respirator to treat patients who could no longer breathe by themselves. This device consisted of a large, airtight cylinder that soon acquired the nickname "iron lung." Patients lay on their backs inside the iron lung, with only their heads outside the machine. Motors attached to the cylinder rhythmically changed the air pressure inside, forcing air in and out of the patients' lungs. For many people, the contraption summed up the worst fears of confinement and immobility associated with paralytic polio. For countless patients, however, the iron lung provided lifesaving support until recovery and rehabilitation restored their ability to breathe unaided. B2 Identifying the Poliovirus By the early 1900s scientists knew that polio was contagious but had not yet identified the microbial agent that caused the disease. In 1908 Austrian pathologist Karl Landsteiner first isolated and identified poliovirus from the spinal fluid of a boy who had died from polio. At the same time, American pathologist Simon Flexner confirmed the viral origins of polio. He also developed useful means of growing the poliovirus for experimental purposes in laboratory animals. Once polio was recognized as a viral disease, scientists focused on developing a vaccine to prevent the disease. B3 Development of Polio Vaccines In 1949 a dramatic discovery further paved the way for development of polio vaccines. American virologists John F. Enders, Thomas H. Weller, and Frederick C. Robbins succeeded in growing poliovirus in laboratory cultures. Previously, researchers had believed that poliovirus would grow only in the nerve cells of a live animal. Enders, Weller, and Robbins demonstrated that the virus could be successfully grown in other types of tissue outside of a living animal. Their achievement instantly transformed polio research. Researchers no longer had to depend on the cumbersome, time-consuming process of infecting monkeys and other animals in the laboratory to grow and examine the virus. Poliovirus could be quickly grown, evaluated, and subjected to experimental manipulation in test tubes. Their work earned the three researchers the Nobel Prize in physiology or medicine in 1954. American physician Jonas Salk developed the first successful polio vaccine in the early 1950s. The Salk vaccine was a killed-virus type that contained poliovirus inactivated by formaldehyde. After preliminary field trials, Salk won approval from the United States government for large-scale testing of the vaccine in 1954. More than 650,000 children in 44 states received injections of the vaccine, and evaluation of the vaccine proved that it was safe and effective. When this news was announced on April 12, 1955, church bells rang in celebration in many towns and cities across the United States. As a result of widespread inoculation programs, the U.S. incidence of polio had dropped by 90 percent by 1960. From an annual average of 38,000 cases per year during the period between 1950 and 1955, the average fell to 570 cases for the years 1961 through 1965. Even as the Salk vaccine was first being widely administered, fierce competition continued among other scientists who were attempting to develop oral vaccines that used live, weakened virus. From 1957 to 1960 American virologist Hilary Koprowski tested orally administered vaccines in human trials in what is now Rwanda and Burundi in equatorial Africa. His test vaccines successfully prevented an outbreak of the disease. Around the same time American virologist Albert Sabin performed large-scale tests of his oral vaccine in Mexico and the Soviet Union, also with success. In 1961 the U.S. government approved the Sabin oral vaccine for widespread use. By the 1970s the more easily administered Sabin oral vaccine had replaced the Salk killed-virus vaccine as the most commonly used vaccine worldwide. By the early 1990s, the Western Hemisphere was declared polio-free, meaning that no new cases of polio had been reported for at least three years. Polio rates were still high elsewhere, however, with an estimated 350,000 cases of polio reported in developing countries in 1988. To address this problem, the World Health Organization (WHO) launched a massive vaccination program called the Global Polio Eradication Initiative. Under this 20-year initiative more than 2 billion children were immunized against polio, and the number of polio cases worldwide dropped by 99 percent. Polio remains a problem in only a few countries where people's resistance to vaccination has not been overcome. Outbreaks in these countries, however, can easily spread. Contributed By: Christopher King Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.