Polio

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Definition

Poliomyelitis, also called polio or infantile paralysis, is a highly infectious viral disease that may attack the central nervous system and is characterized by symptoms that range from a mild nonparalytic infection to total paralysis in a matter of hours.

Description

There are three known types of polioviruses (called 1,2, and 3), each causing a different strain of the disease and all are members of the viral family of enteroviruses (viruses that infect the gastrointestinal tract). Type 1 is the cause of epidemics and many cases of paralysis, which is the most severe manifestation of the infection. The virus is usually a harmless parasite of human beings. Some statistics quote one in 200 infections as leading to paralysis while others state that one in 1,000 cases reach the central nervous system (CNS). When it does reach the CNS, inflammation and destruction of the spinal cord motor cells (anterior horn cells) occurs, which prevents them from sending out impulses to muscles. This causes the muscles to become limp or soft and they cannot contract. This is referred to as flaccid paralysis and is the type found in polio. 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 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. Life may be threatened if paralysis of the throat or of the breathing muscles occurs.

Man is the only natural host for polioviruses and it most commonly infects younger children, although older children and adults can be infected. Crowded living conditions and poor hygiene encourage the spread of poliovirus. Risk factors for this paralytic illness include older age, pregnancy, abnormalities of the immune system, recent tonsillectomy, and a recent episode of excessively strenuous exercise concurrent with the onset of the CNS phase.

— Linda K. Bennington, CNS

Dictionary: po·li·o·my·e·li·tis  (pō'lē-ō-mī'ə-lī'tĭs)

A highly infectious viral disease that chiefly affects children and, in its acute forms, causes inflammation of motor neurons of the spinal cord and brainstem, leading to paralysis, muscular atrophy, and often deformity. Through vaccination, the disease is preventable. Also called infantile paralysis.

[New Latin : Greek polios, gray + MYELITIS.]

Definition

Poliomyelitis is an infectious disease that is caused by a subgroup of viruses. The hallmark of the disease is the rapid development of paralysis. Poliomyelitis is also commonly called polio. Once a cause of widespread public health measures to control epidemics, polio is now on the brink of eradication.

Description

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 with resulting paralysis.

Poliomyelitis was first described in 1789, although it likely dates back many centuries prior to that time. Outbreaks occurred in Europe and the United States beginning in the early nineteenth century. For the next hundred years, outbreaks became a regular summer and fall event in northern regions. As time passed, the number of cases and people crippled by the infection rose. By 1952, more than 21,000 people in the United States were paralyzed after a bout of poliomyelitis.

The manufacture and widespread use of several vaccines beginning in the 1950s drastically reduced the number of cases of poliomyelitis. In the United States, the last reported case of polio acquired from a wild-type (original form of a naturally occurring) virus was in 1979.

Demographics

Humans are the only known carriers of the polio virus. Poliomyelitis most commonly affects children under the age of five. Several generations ago, the disease was much more common than it is now. Even in the 1950s, poliomyelitis was global in its occurrence. Many children in underdeveloped and developed countries, including the United States, were susceptible. With the successful development of vaccines and the implementation of global vaccination campaigns, the infection has been drastically reduced. As of 2004, only isolated pockets of disease remain. These hot spots include areas in Africa, India, and the eastern Mediterranean.

Males and females are equally susceptible to polio. Irreversible paralysis, usually in the legs, occurs in about one of every 200 polio infections.

Causes and symptoms

Poliomyelitis originates with a viral infection. Poliovirus is a member of a group of viruses designated as enteroviruses. The viruses contain ribonucleic acid (RNA) as their genetic material. More specifically, the various polioviruses belong in a group (or family) called Picornaviridae.

There are three types of poliovirus that are related to each other based on their recognition by the body's immune system. This sort of a relationship is called a serotype. The three poliovirus serotypes are P1, P2, and P3. Even though they are closely related immunologically, developing immunity to one serotype is no guarantee of protection from infection from the other two serotypes. Thus, vaccines are geared towards producing an immune response that will be protective against all three serotypes.

Enteroviruses can be found in the gastrointestinal tract and are not often dissolved by the acidic conditions. Thus, poliovirus can be swallowed and remain intact, capable of causing an infection. As the virus particles lodge at the back of the throat in the pharynx, or are swallowed and end up in the intestinal tract, the viruses can begin to multiply. Like all viruses, the multiplication requires a host cell, in this case, cells lining the throat and intestines.

Shortly after the virus enters a person, viral particles can be recovered from the throat and from feces. About one week later, the virus is not usually detectable in the throat. However, virus can continue to be excreted in the feces for several more weeks. During this time, symptoms of the disease do not develop. Thus, the virus can be unknowingly passed to others via the oral or fecal-to-oral route. This transmission is a common method of transfer of a variety of viral and bacterial infections in settings like daycare centers.

Subsequently, the poliovirus invades lymph tissue. From there, the virus can enter the bloodstream and infect cells of the central nervous system. This typically takes from six to 20 days after infection. Multiplication of the virus inside motor neurons in environments like the brain destroys the host cells and causes paralysis. The appearance of paralysis is rapid.

Up to 95% of polio infections do not produce any symptoms or damage. However, these individuals can still excrete the virus in their feces, and so are capable of infecting others. For every 200 people who escape the effects of poliomyelitis, about one person becomes paralyzed.

Approximately 4–8% of polio infections are minor, and consist of fairly nonspecific symptoms, including sore throat and fever, nausea, vomiting, abdominal pain, or constipation. Recovery is complete in about a week. Indeed, a person may not know the difference between this brush with polio and the flu. This condition is known as abortive poliomyelitis. There is no involvement of the central nervous system.

In 1–2% of infections, a condition called nonparalytic aseptic meningitis is produced. Nonspecific symptoms characterize this condition, followed several days later by stiffness in the neck, back, and/or legs. The symptoms last from 2–10 days. Recovery is complete.

Less than 1% of those who are infected with the poliovirus develop what is termed flaccid paralysis. Paralysis appears anywhere from one to 10 days after symptoms that include loss of reflexes, severe muscle aches, and muscle spasms in the arms, legs, or back. In children, the initial symptoms can begin to fade before paralysis appears.

Over the next few days, the paralysis becomes worse. For many people, muscle strength eventually returns. However, for those who still have weak muscles and/or paralysis a year later, the changes are likely permanent.

Types of paralytic poliomyelitis

There are three types of paralysis that can develop in poliomyelitis. The first is called spinal polio. This is the most common form of polio-related paralysis, and accounted for nearly 80% of all polio-related paralysis from 1969 to 1979. This type produces the classical image of a person whose legs have been paralyzed. The second type is known as bulbar polio. This type accounts for about 2% of known cases. Stiffness and paralysis typically occurs in the neck and head. The third type of polio-related paralysis is called bulbospinal polio. A combination of the previous two conditions, it accounts for nearly 20% of paralysis.

Postpolio syndrome

In almost half of those who contract polio in childhood, muscle pain and weakness reappears three or four decades later. Postpolio syndrome does not appear to be caused by a recurrence of the viral infection, as no virus can be detected in the feces. Rather, it may result from motor neurons damaged in the initial bout of polio that fail to operate properly decades later. The reason for the failure is not known.

Diagnosis

The diagnosis of poliomyelitis is based on the recovery of the virus from the throat or feces of a person. It is possible to isolate the virus from the cerebrospinal fluid, but this is uncommon. When the virus is recovered, specialized testing can be done to determine if the virus is wild type (that is, it has been acquired from the environment), or whether it is a vaccine type (polio vaccines utilize intact, but weakened viruses).

Another means of diagnosis relies on the detection of antibodies that have been produced by the virus. Since antibodies are produced as a part of the vaccination process, physicians focus on the increasing levels of antibodies over a short time as evidence that the body is battling an active viral infection.

Still another diagnostic test detects increased number of white blood cells and protein in the cerebrospinal fluid. This is a more general response to infections. Other conditions can present similar symptoms, and need to be ruled out when diagnosing poliomyelitis. These include Guillain-Barré syndrome, meningitis, and encephalitis.

Treatment team

The treatment team ideally consists of the family physician, neurologist, infectious disease specialist, physical therapists, occupational therapists, specialty nurses, and family members. In field conditions in developing countries, the treatment team may consist of a physician and direct caregivers only. World health agencies rapidly mobilize to provide care and vaccinations in order to contain isolated outbreaks in developing countries.

Treatment

Prevention is the watchword for poliomyelitis, and prevention consists of vaccination. There are two polio vaccines available; inactivated (Salk) poliovirus vaccine, and oral poliovirus vaccine.

The inactivated vaccine was devised by American physician Jonas Salk (1914–1995) in the 1950s. The vaccine contains all three serotypes of the poliovirus. The viruses, which are inactivated and incapable of causing an infection, are grown in a type of monkey kidney cell. When injected, the viruses stimulate an immune response that is protective. Initially, vaccine impurity was the cause of illness and death in some people who received the Salk vaccine. Refinement of the vaccine preparation eliminated these unwanted effects. Still, in the 1990s, a controversy arose regarding the vaccine as a suggested source of acquired immunodeficiency syndrome (AIDS), based on the known presence of the AIDS virus in monkey tissue cells. However, scrupulously conducted examinations ruled out this suggestion.

The oral vaccine was developed by Polish-born American physician Albert Sabin (1906–1993) in the late 1950s and was licensed for use in 1963. This vaccine has largely replaced the injected Salk vaccine. The vaccine also contains live, but weakened (attenuated) poliovirus.

A series of vaccinations given at two, four, six to 18 months, and four to six years produces a lifelong immunity to the three poliovirus serotypes. In regions where poliomyelitis is actively occurring, even a single dose of vaccine can provide adequate protection from infection during the outbreak.

In 2002, a new formulation of polio vaccine was approved for use in the United States. In addition to the polioviruses, the vaccine also bestows immunity to the virus that causes hepatitis B.

Recovery and rehabilitation

There is no cure for poliomyelitis. Some people can partially recover from paralysis, while the condition is irreversible in others. Physical and occupational therapies can be helpful in providing strengthening exercises and assistive devices for walking, but these are seldom available in remote areas of developing countries where polio outbreaks still occur.

Prognosis

Among those who are paralyzed by the viral infection, 5–10% overall die due to the paralysis of muscles used for breathing. For every 100 people who become paralyzed by the viral infection, two to five children and 15–30% of adults will die from polio.

Special concerns

Vaccination can produce reactions ranging from a transient and minor skin irritation and allergic reaction to some components of the oral vaccine to paralysis. The latter, termed vaccine-associated paralytic polio, is very rare. The condition is associated more with the injectable vaccine than with the vaccine given orally. Nonetheless, adults can be affected. From 1980–1998, 152 adults in the United States developed some degree of paralysis from polio vaccination.

The decision by Nigeria to suspend its vaccination program in 2001 contributed to a rise in the number of polio cases in the African country. Nigeria has since reinstated the vaccination program. The Nigerian experience points out that continued vigilance is necessary to keep poliomyelitis under control.

Since the widespread availability of vaccines, the number of cases of poliomyelitis worldwide has decreased by over 99% since 1988. That year, the estimated number of cases was more than 350,000. As of April 2003, the number of cases was reduced to 1,919. The dramatic reduction in the disease is attributed to a multinational worldwide vaccination effort that began in 1988. The program was spearheaded by organizations such as the World Health Organization.

The effort intensified during the first half of 2004, with the urgent distribution of polio vaccine to 250 million children in the world's remaining hotspots. As of April 2004, the number of polio cases worldwide caused by a wild-type virus was reduced to 89. World health officials aim to interrupt the transmission of all wild-type polio virus by the year 2005.

Resources

BOOKS

Bruno, Richard L. The Polio Paradox: Understanding and Treating "Post-Polio Syndrome" and Chronic Fatigue. New York: Warner Books, 2003.

Oshinsky, David, Polio: An American Story. New York: Oxford University Press, 2004.

Salgado, Sebastio, and Kofi Annan. The End of Polio: A Global Effort to End a Disease. Boston: Bulfinch, 2003.

PERIODICALS

Centers for Disease Control and Prevention. "Progress toward global eradication of poliomyelitis." Morbidity and Mortality Weekly Report (July 2003): 366–369.

OTHER

Dowdle, Walter, et al. "Preventing Polio from Becoming a Reemerging Disease." Panel Summary from the 2000 Emerging Infectious Diseases Conference in Atlanta, Georgia. CDC. April 23, 2004 (June 2, 2004). http://www.cdc.gov/ncidod/eid/vol7no3_supp/dowdle.htm.

World Health Organization. "Poliomyelitis." April 14, 2004 (June 2, 2004). http://www.who.int/mediacentre/factsheets/fs114/en/.

ORGANIZATIONS

World Health Organization. Avenue Appia 20, Geneva, Switzerland. + 41 22 791 2111; Fax: + 41 22 791 3111. info@who.int. http://www.who.int.

Brian Douglas Hoyle, PhD

An acute infectious viral disease which in its serious form affects the central nervous system and, by destruction of motor neurons in the spinal cord, produces flaccid paralysis. However, about 99% of infections are either inapparent or very mild. See also Animal virus; Central nervous system.

The virus probably enters the body through the mouth; primary multiplication occurs in the throat and intestine. Transitory viremia occurs; the blood seems to be the most likely route to the central nervous system. The severity of the infection may range from a completely inapparent through minor influenzalike illness, or an aseptic meningitis syndrome (nonparalytic poliomyelitis) with stiff and painful back and neck, to the severe forms of paralytic and bulbar poliomyelitis. In all clinical types, virus is regularly present in the enteric tract. In paralytic poliomyelitis the usual course begins as a minor illness but progresses, sometimes with an intervening recession of symptoms (hence biphasic), to flaccid paralysis of varying degree and persistence. When the motor neurons affected are those of the diaphragm or of the intercostal muscles, respiratory paralysis occurs. Bulbar poliomyelitis results from viral attack on the medulla (bulb of the brain) or higher brain centers, with respiratory, vasomotor, facial, palatal, or pharyngeal disturbances.

Poliomyelitis occurs throughout the world. In temperate zones it appears chiefly in summer and fall, although winter outbreaks have been known. It occurs in all age groups, but less frequently in adults because of their acquired immunity. The virus is spread by human contact; the nature of the contact is not clear, but it appears to be associated with familial contact and with interfamily contact among young children. The virus may be present in flies.

Inactivated poliovirus vaccine (Salk; IPV), prepared from virus grown in monkey kidney cultures, was developed and first used in the United States, but oral poliovirus vaccine (Sabin; OPV) is now generally used throughout the world. The oral vaccine is a living, attenuated virus.

A disease produced by a small viral organism that enters the body via the alimentary tract and produces upper pharyngeal, pharyngeal, and intestinal inflammation in its mentor form. In the more severe variety, a subsequent viremia is produced, with extension of the infection to the anterior pulp horn cells and ganglia of the spinal cord, producing a flaccid paralysis. In bulbar poliomyelitis the viral infection involves the medulla, resulting in impairment of swallowing, respiration, and circulation. It is now recognized that three types of viruses are responsible for the nonparalytic, paralytic, and bulbar varieties of poliomyelitis. Excellent immunization procedures have been provided by use of killed viruses (Salk) and attenuated mutant vaccines (Sabin).

Definition

Poliomyelitis, also called polio or infantile paralysis, is a highly infectious viral disease that may attack the central nervous system and is characterized by symptoms that range from a mild nonparalytic infection to total paralysis in a matter of hours.

Description

There are three known types of polioviruses (called 1, 2, and 3), each causing a different strain of the disease and all being members of the viral family of enteroviruses (viruses that infect the gastrointestinal tract). Type 1 is the cause of epidemics, and many cases of paralysis, which is the most severe manifestation of the infection. The virus is usually a harmless parasite of human beings. Some statistics quote one in 200 infections as leading to paralysis, while others state that one in 1,000 cases reach the central nervous system (CNS). When it does reach the CNS, inflammation and destruction of the spinal cord motor cells (anterior horn cells) occurs, which prevents them from sending out impulses to muscles. This causes the muscles to become limp or soft, and they cannot contract, a condition called flaccid paralysis and is the type found in polio. 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 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. Life may be threatened if paralysis of the throat or of the breathing muscles occurs.

Humans are the only natural host for polioviruses, and it most commonly infects younger children, although older children and adults can be infected. Crowded living conditions and poor hygiene encourage the spread of poliovirus. Risk factors for this paralytic illness include older age, pregnancy, abnormalities of the immune system, and a recent episode of excessively strenuous exercise concurrent with the onset of the CNS phase. As of 2004, the last naturally occurring polio case in the United States was diagnosed in 1979.

Causes and Symptoms

Poliovirus can be spread by direct exposure to an infected individual, and more rarely, by eating foods contaminated with waste products from the intestines (feces) and/or droplets of moisture (saliva) from an infected person. Thus, the major route of transmission is fecal-oral, which occurs primarily with poor sanitary conditions. The virus is believed to enter the body through the mouth with primary multiplication occurring in the lymphoid tissues in the throat, where it can persist for about one week. During this time, it is absorbed into the blood and lymphatics from the gastrointestinal tract where it can reside and multiply, sometimes for as long as 17 weeks. Once absorbed, it is widely distributed throughout the body until it ultimately reaches the CNS (the brain and spinal cord). The infection is passed on to others when poor hand washing allows the virus to remain on the hands after eating or using the bathroom. Transmission remains possible while the virus is being excreted and it can be transmitted for as long as the virus remains in the throat or feces. The incubation period ranges from three to 21 days, but cases are most infectious from seven to ten days before and after the onset of symptoms.

There are two basic patterns to the virus: the minor illness (abortive type) and the major illness (which may be paralytic or nonparalytic). The minor illness accounts for 80 to 90 percent of clinical infections and is found mostly in young children. It is mild and does not involve the CNS. Symptoms include a slight fever, fatigue, headache, sore throat, and vomiting, which generally develop three to five days after exposure. Recovery from the minor illness occurs within 24 to 72 hours. Symptoms of the major illness usually appear without a previous minor illness and generally affect older children and adults.

About 10 percent of people infected with poliovirus develop severe headache and pain and stiffness of the neck and back. This is due to an inflammation of the meninges (tissues which cover the spinal cord and brain). This syndrome is called aseptic meningitis. The term aseptic is used to differentiate this type of meningitis from those caused by bacteria. The patient usually recovers completely from this illness within several days.

About 1 percent of people infected with poliovirus develop the most severe form. Some of these patients may have two to three symptom-free days between the minor illness and the major illness, but the symptoms often 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. This viral invasion causes inflammation and then destruction of these nerves. The muscles, therefore, no longer receive any messages from the brain or spinal cord. The muscles become weak, floppy, and then totally paralyzed. All muscle tone is lost in the affected limb and the muscle becomes soft (flaccid). Within a few days, the muscle begins to decrease in size (atrophy). The affected muscles may be on both sides of the body (symmetric paralysis) but are often on unbalanced parts of the body (asymmetric paralysis). Sensation or the ability to feel is not affected in these paralyzed limbs.

When poliovirus invades the brainstem (the stalk of brain which connects the two cerebral hemispheres with the spinal cord, called bulbar polio), a person may begin to have trouble breathing and swallowing. If the brainstem is severely affected, the brain's control of such vital functions as heart rate and blood pressure may be disturbed, a condition that can lead to death.

The maximum state of paralysis is usually reached within just a few days. The remaining, unaffected nerves then begin the process of attempting to grow branches, which can compensate for the destroyed nerves. Fortunately, the nerve cells are not always 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. If the nerve cells are completely destroyed; however, paralysis is permanent.

Diagnosis

Fever and asymmetric flaccid paralysis without sensory loss in a child or young adult almost always indicate poliomyelitis. Using a long, thin needle inserted into the lower back to withdraw spinal fluid (lumbar puncture) will reveal increased white blood cells and no bacteria (aseptic meningitis). Nonparalytic poliomyelitis cannot be distinguished clinically from aseptic meningitis due to other agents. Virus isolated from a throat swab and/or feces or blood tests demonstrating the rise in a specific antibody is required to confirm the diagnosis.

Treatment

There is no specific treatment for polio except symptomatic. Therapy is designed to make the patient more comfortable (pain medications and hot packs to soothe the muscles), and intervention if the muscles responsible for breathing fail (for instance, a ventilator to take over the work of breathing). During active infection, rest on a firm bed is indicated. Physical therapy is the most important part of management of paralytic polio during recovery.

Prognosis

When poliovirus causes only the minor illness or simple aseptic meningitis, the patient can be expected to recover completely. Among patients with the major illness, about 50 percent recover completely. About 25 percent of such patients have slight disability, and about 25 percent have permanent and serious disability. Approximately 1 percent of all patients with major illness die. The greatest return of muscle function occurs in the first six months, but improvements may continue for two years.

Post-polio syndrome (PPS) is a condition that can strike polio survivors anywhere from 10 to 40 years after their recovery from polio. It is caused by the death of individual nerve terminals in the motor units that remain after the initial polio attack. Symptoms include fatigue, slowly progressive muscle weakness, muscle and joint pain, and muscular atrophy. The severity of PPS depends upon how seriously the survivors were affected by the first polio attack.

Prevention

There are two types of polio immunizations available in the United States, but since the year 2000, one is rarely used. A vaccine takes advantage of the fact that infection with polio leads to an immune reaction, which will give the person permanent, lifelong immunity from reinfection with the form of poliovirus for which the person was vaccinated.

The Salk vaccine (also called the killed polio vaccine or inactivated polio vaccine, IPV) consists of a series of three shots that are given just under the skin to children at the ages of two months, four months, and any time between six and 18 months. A fourth injection is given between the ages of four to six years as a booster. 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. The recipient thus becomes immune to infection with the poliovirus.

Since the year 2000, the Sabin vaccine (also called the oral polio vaccine or OPV) has been discontinued in the United States, although it is still being used in other countries. It contains the live, but weakened, poliovirus and because OPV uses the live virus, it has the potential to cause infection in individuals with weak immune defenses (both in the person who receives the vaccine and in close contacts). Approximately nine cases a year of vaccine related polio was associated with OPV in the United States. Although this is a rare complication, occurring in only one in 6.8 million doses administered and one in every 6.4 million doses from having close contact with someone who received the vaccine, the risk of having polio from OPV was greater than it was of naturally acquiring it.

Following the launching of the Global Polio Eradication Initiative, the number of cases fell 99 percent from an estimated 350,000 cases to less than 3,500 cases worldwide in 2000. At the end of 2000, the number of polio-infected countries was approximately 20, down from 125. The goal of the World Health Organization (WHO) is to have polio eliminated from the planet by the year 2005. The virus has still been identified in Africa and parts of Asia, so travelers to those areas may want to check with their physicians concerning booster vaccinations.

Resources

Books

Oshinsky, David. Polio: An American Story. Oxford, UK: Oxford University Press, 2004.

Periodicals

Alexander, L. N., et al. "Vaccine Policy Changes and Epidemiology of Poliomyelitis in the United States" Journal of the American Medical Association 292 (2004): 1696–1701.

Organization

International Polio Network. 4207 Lindell Blvd., Suite 110, St. Louis, MO 63108–2915. Web site: www.postpolio.org.

March of Dimes Birth Defects Foundation. National Office, 1275 Mamaroneck Avenue, White Plains, NY 10605. Web site: www.modimes.org/.

Web Sites

World Health Organization. Global Polio Eradication Progress 2004. Available online at www.polioeradication.org/.

[Article by: Linda K. Bennington, MSN,CNS]

Poliomyelitis, or infantile paralysis, is a highly infectious disease caused by three serotypes of polioviruses. These viruses belong to the Enterovirsus genus of the family Picornaviridae. The infection is transmitted from person to person and rarely produces clinical symptoms. Less than 1 percent of infections will result in paralysis. Death may result, however, especially if respiratory muscles are affected.

Although archeological findings suggest that paralytic poliomyelitis existed before the modern era, the importance of the disease was not recognized until the late nineteenth century. Annual outbreaks of poliomyelitis involving thousands of cases occurred during summer and early fall in various areas of the northern hemisphere during the first half of the twentieth century, making poliomyelitis the leading cause of permanent disability and the cause of numerous premature deaths. The Drinker respirator, also known as the "iron lung," allowed a rapid reduction of poliomyelitis mortality in the 1930s and 1940s.

A major breakthrough in poliomyelitis control took place in 1949, when John F. Enders, Frederick C. Robbins, and Thomas H. Weller developed a tissue culture system for polioviruses. The availability of cultured viruses opened the way to vaccine development. The first poliovirus vaccines were licensed for use in the United States in 1955. These vaccines, developed by Jonas Salk, consisted of formalin-inactivated viruses administered through injections. In 1963, a live oral vaccine, developed by Albert Sabin, was licensed. Within ten years of the introduction of vaccines, the number of poliomyelitis cases decreased by over 95 percent in the United States, and the last case induced by indigenous transmission of wild poliovirus in the United States was detected in 1979. Poliovirus vaccines also allowed rapid declines in disease incidence in Canada, most European countries, Australia, and New Zealand. In Cuba, a two-round mass vaccination campaign in 1962 interrupted poliovirus transmission and rendered the island free of polio.

Most developing countries did not benefit from effective poliomyelitis control before the development of national control programs in the late 1970s. Mass vaccination campaigns, introduced in the Americas during the early 1980s, proved to be an effective means of bringing poliomyelitis under control. The last case of poliomyelitis in the Americas was detected in Peru in 1991, and the western hemisphere was certified as poliofree in 1994.

In 1988, the World Health Assembly launched the Poliomyelitis Eradication Initiative, with a goal of terminating the circulation of wild polioviruses by the year 2000. This worldwide effort relies on three main strategies: high levels of vaccination through routine programs; supplementary vaccination in the form of national immunization days and local door-to-door immunization ("moppingup") campaigns; and surveillance and investigation of all cases that resemble acute poliomyelitis (acute flaccid paralysis). From 1988 to 1999, the global number of estimated poliomyelitis cases decreased from 350,000 to 20,000.

An important benefit of achieving the Poliomyelitis Eradication Initiative goal will be the discontinuation of poliovirus vaccination. Stopping vaccination will require certifying all areas of the world to be free of wild poliovirus. It will also be necessary to ensure that all infectious and potentially infectious material are contained in maximum safety facilities and to stockpile enough vaccines to respond to any outbreak that might occur should poliovirus be released intentionally or unintentionally. In this way poliomyelitis eradication would follow the path pioneered by smallpox eradication.

(SEE ALSO: Communicable Disease Control; Immunizations; Smallpox)

Bibliography

Centers for Disease Control and Prevention (2000). "Poliomyelitis Prevention in the United States: Updated Recommendations of the Advisory Committee on Immunization Practices (AICP)." Morbidity and Mortality Weekly Report 49(RR-5):1–22.

Robbins, F. C. (1999). "The History of Polio Vaccine Development." In Vaccine, 3rd edition, eds. S. A. Plotkin and W. A. Orenstein. Philadelphia: W. B. Saunders.

Sutter, R. W; Cochi, S. L; and Melnick, J. L. (1999). "Live Attenuated Poliovirus Vaccines." In Vaccine, 3rd edition, eds. S. A. Plotkin and W. A. Orenstein. Philadelphia: W. B. Saunders.

World Health Organization. Polio Eradication. Available at http://www.polioeradication.org.

— PATRICK L. F. ZUBER

For more information on poliomyelitis, visit Britannica.com.

Poliomyelitis, or infantile paralysis, was one of the most feared diseases of the twentieth century, especially for its ability to cripple children and adolescents. The disease is caused by one of three strains of intestinal virus that under certain conditions invades and damages or destroys the anterior horn cells of the spinal cord. Paralysis is caused when damaged or destroyed nerves can no longer enervate muscles. The virus is commonly spread through contaminated fecal material. Before modern sanitation the poliovirus was constantly present, and most individuals were infected as young children, thereby gaining protective antibodies. Epidemics occurred when modern sanitation interrupted virus circulation for several years, creating a large susceptible population. In epidemics, perhaps as many as 90 to 95 percent of those infected had inapparent cases and a flu-like illness with no paralysis. Four to eight percent of those infected had an abortive case with mild symptoms. Less than 2 percent of all infections reached the central nervous system and caused paralysis. Polio killed when the muscles involved in breathing were paralyzed, but the disease was rarely fatal.

The first sizable polio epidemic in the United States occurred in Vermont in 1894. Recurrent epidemics in Europe and North America sparked research on the disease, and in 1908 the Vienna immunologist Karl Landsteiner discovered the poliovirus. Simon Flexner, the director of the Rockefeller Institute for Medical Research in New York, soon isolated the virus in the United States and discovered polio antibodies in patients.

In 1916, New York and the Northeast experienced the nation's most severe polio epidemic, with 27,000 cases and 6,000 deaths. In New York City alone there were 8,928 cases and 2,407 deaths. This epidemic puzzled physicians and frightened citizens. Polio struck both infants and adults, individuals living in clean, sanitary conditions and those living in filth, the wealthy and the poor. Lacking effective cures and vaccines, public health officials conducted a campaign to clean up the city and eradicate the flies believed to carry the disease. Polio patients were quarantined in their homes or removed, sometimes forcibly, to hastily established isolation hospitals. For the next four decades epidemics of poliomyelitis struck some part of the nation every year.

In 1921, Franklin D. Roosevelt, who later served as governor of New York and president of the United States, developed polio while vacationing at the family home. Seeking to regain the use of his paralyzed legs, he discovered the therapeutic effects of warm mineral water at a failing resort in Warm Springs, Georgia. In 1926 he purchased the resort and, with the help of his law partner, Basil O'Connor, turned it into a model facility for polio rehabilitation, although Roosevelt himself never walked unaided.

During the 1920s and 1930s, scientists and physicians sought to better understand the disease and to find a cure or vaccine. The researchers James D. Trask, John R. Paul, and Dorothy Horstmann of the Yale University Poliomyelitis Study Unit confirmed polio's character as an intestinal disease when they isolated the virus in water supplies and sewage during epidemics. Australian scientists in 1931 discovered that there were at least two different strains of poliovirus. Scientists ultimately discovered a third strain and placed the poliovirus in the family of enteroviruses. In 1935 two American physicians, Maurice Brodie in New York and John A. Kolmer in Philadelphia, developed and tested polio vaccines. These vaccines, however, proved ineffective and may actually have caused cases of the disease.

Polio rehabilitation also advanced in the 1930s, especially at the Georgia Warm Springs Foundation established by Roosevelt. The President's Birthday Ball Commission began raising funds in 1934 to support rehabilitation and research and was succeeded in 1938 by the National Foundation for Infantile Paralysis headed by Basil O'Connor. The National Foundation's March of Dimes campaign raised over $600 million between 1938 and 1962. About 60 percent of this money assisted individuals with hospital and doctor bills, and about 11 percent was spent on grants to scientists. The iron lung, a large canister-shaped respirator that "breathed" for patients with paralyzed respiratory muscles was developed in the 1930s. In the 1940s, Elizabeth Kenny, an Australian nurse, introduced her unorthodox ideas for treating polio paralysis. In place of immobility and casts to prevent the contraction of paralyzed limbs, Kenny applied hot packs to sooth paralyzed muscles and movement in order to maintain flexibility and retrain paralyzed muscles. Although many doctors rejected her theories about the causes of polio paralysis, her therapeutic practices soon became commonplace.

In the late 1940s, John F. Enders, Thomas H. Weller, and Frederick C. Robbins, physicians at Harvard funded by the National Foundation, first grew poliovirus on tissue culture outside the body, necessary for a successful vaccine. They won the Nobel Prize in 1954 for their discovery. The National Foundation also funded a typing program to identify all the possible strains of the virus in anticipation of producing a vaccine against every variant.

This research occurred against the background of an increasing incidence of epidemics in the late 1940s and early 1950s. Nine of the ten worst years for polio occurred between 1945 and 1955, and the epidemic of 1952 was second only to 1916 in its severity. The improved sanitation of the postwar years, the move to the suburbs, and the baby boom ensured that many children and even adolescents were vulnerable. Those years were marked by summers of fear when parents kept their children out of swimming pools and movie theaters and warned them against drinking from water fountains. The fund-raising of the March of Dimes, the information disseminated by the National Foundation, and the heartbreaking personal narratives that appeared in popular magazines kept polio in the forefront of the nation's consciousness. The specter of the crippling paralysis of polio threatened the postwar American Dream of healthy, happy children.

The fund-raising efforts of the March of Dimes and the laboratory work of the physicians came together in the 1950s. Jonas Salk, a University of Pittsburgh physician, applied lessons he had learned working on influenza vaccine and developed a successful killed virus vaccine that could be mass produced. By the early 1950s he was conducting preliminary trials of the vaccine. In 1954 the National Foundation arranged for a large-scale field trial of the Salk vaccine conducted by Dr. Thomas Francis Jr. of the University of Michigan. Over 1.8 million children were enrolled as "polio pioneers" in the trial. On 12 April 1955, Francis announced that the Salk vaccine was both safe and effective in protecting vaccinated children from polio. The U.S. government licensed the vaccine the same day. Basil O'Connor had already ordered millions of doses from pharmaceutical companies in order to begin a mass vaccination immediately. Unfortunately, a few weeks later, a bad batch of vaccine produced by Cutter Laboratories resulted in more than 200 cases of vaccine-associated poliomyelitis, including eleven deaths. This was an isolated episode, and the vaccination of America's children soon continued. Even as many children were protected by the Salk vaccine, Albert Sabin, also supported by the National Foundation, worked on an attenuated poliovirus vaccine. An attenuated vaccine, in which the virus was live but significantly weakened, had several advantages. It induced a stronger, longer lasting immune response; immunity was achieved more quickly; and it could be given orally instead of being injected. After field trials in the United States, the Soviet Union, and elsewhere, the Sabin vaccine was licensed for use in 1962. These two vaccines virtually eliminated poliomyelitis in the United States by the early 1960s. Since then only a handful of cases have occurred annually, usually in new immigrants or as vaccine-associated cases.

The survivors of the postwar polio epidemics often spent long periods in rehabilitation hospitals before being fitted with braces, wheelchairs, and respirators that allowed them to return to families, school, and work. Because of the barriers they faced in attempting to live and work, polio survivors were often in the forefront of the disability rights movement that emerged in the 1970s. Activists like Ed Roberts, who was one of the founders of the Independent Living Movement, insisted that individuals with disabilities had a right to accessible living and working environments. In the 1980s, many polio survivors began experiencing increased pain, muscle weakness and even paralysis that physicians eventually identified as post-polio syndrome, an apparent effect of the overuse of nerves and muscles to compensate for the destruction caused by the initial infection. The oral polio vaccine also came under attack in the United States for causing eight to ten cases of polio every year. In 2000 the federal government recommended returning to a safer Salk-type killed virus. The Sabin vaccine, however, remained in use overseas as the World Health Organization tried to eradicate the poliovirus worldwide. Thus in the twenty-first century, poliomyelitis, which was so feared in the twentieth century, may become only the second infectious disease, after smallpox, to be eliminated as a threat to humans.

Bibliography

Black, Kathryn. In the Shadow of Polio: A Personal and Social History. Reading, Mass.: Addison-Wesley, 1996.

Gould, Tony. A Summer Plague: Polio and Its Survivors. New Haven: Yale University Press, 1995.

Paul, John R. A History of Poliomyelitis. New Haven: Yale University Press, 1971.

Rogers, Naomi. Dirt and Disease: Polio Before FDR. New Brunswick: Rutgers University Press, 1990.

Seavey, Nina Gilden, Jane S. Smith, and Paul Wagner. A Paralyzing Fear: The Triumph Over Polio in America. New York: TV Books, 1998.

Smith, Jane S. Patenting the Sun: Polio and the Salk Vaccine. New York: William Morrow, 1990.

An acute disease, and an infectious disease, caused by a virus, that brings about inflammation of certain nerve cells in the spinal cord. It can have a wide range of effects, from mild to severe, including paralysis, permanent disability, and death. In the United States, the disease has now largely vanished since the development of a vaccine against it. (See Sabin vaccine and Salk vaccine.)

Inflammation of the gray matter of the brain; also the name applied to the viral disease of humans and also known as polio.

• feline p. — see feline polioencephalomyelitis.
• p. suum — see porcine viral encephalomyelitis.

Poliomyelitis, commonly called polio, was first described in 1789, but not fully recognized until 1840. In the United States, it first became a feared disease when a small epidemic in 1894 struck Vermont, because it singled out children and often caused paralysis or death. A larger outbreak began in 1916, also striking the East Coast. Franklin D. Roosevelt, later president of the United States, was paralyzed from the waist down by the disease in 1921. Another large outbreak hit the West Coast in 1934. In the late 1940s and early 1950s, a major epidemic of the disease struck the entire United States, infecting tens of thousands each year.

As early as 1908, the cause of polio had been identified -- a virus. Jonas Salk, a medical doctor, had worked on the vaccine for the influenza virus during the 1940s. As the head of the Virus Research Laboratory at the University of Pittsburgh School of Medicine, he began the study of the virus causing polio. He confirmed the existence of three types of polio viruses and started growing them in cultures of monkey kidney tissue. He prepared vaccines by killing these viruses with formaldehyde. The killed viruses did not cause the disease, but did stimulate the production of antibodies that protect the human organism against the disease.

In 1952 Salk tested the vaccine on children who had already experienced polio and thus were immune to the virus. The vaccine increased the amount of antibodies in these children, elevating their immunity. The first trial on children who never had polio took place in 1954; it showed the vaccine to be safe and effective. Throughout the 1950s the incidence of polio was reduced drastically worldwide as a result of the Salk vaccine. By 1957 the number of new cases of the disease in the United States had been reduced from 35,000 in 1953 to 5600.

Albert Sabin, a medical doctor who also had studied the polio virus, prepared a different vaccine containing a live strain of a virus. This virus also stimulates the production of antibodies but is too weak to cause the disease itself. The Sabin vaccine, which (unlike the Salk vaccine) can be administered orally, was tested in large-scale trials in 1957 in the United States and other countries. It was adopted in the Soviet Union and Europe in 1959, and was approved in the United States in 1961. It has two advantages over the Salk vaccine -- it can be administered orally and, since it is alive and therefore infectious, it provides some immunity to persons who come in contact with the one who takes the vaccine. By 1964, two years after introduction of the Sabin vaccine, the number of new cases in the United States was reduced to 121 and by 1979 there were no new cases at all that originated in the United States.

Around the world, however, the majority of people were still unvaccinated, so as many as 350,000 cases a year were still occurring in the late 1980s. In 1988 the World Health Organization (WHO) initiated a drive to eradicate polio completely, as another virus-borne disease, smallpox, had been defeated by vaccination. By 2003 the disease had been eliminated (except for isolated cases) everywhere except India, Pakistan, and Nigeria, with fewer than 300 new cases in those nations. At that time, WHO aimed for complete eradication by the end of 2004.