Causes and Symptoms
Poliomyelitis, or polio, is a contagious disease affecting humans and some nonhuman primates. It is caused by three closely related strains of a human enterovirus. In its most serious manifestation, it attacks nerve tissue in the spinal cord and brain stem, resulting in paralysis. Polio was one of the most feared diseases in developed countries in the twentieth century. A few medical researchers suspected the connection between the severe neurological symptoms of poliomyelitis and the more typical enteric form of the disease in the first decade of the twentieth century. A complete and accurate picture of the etiology of polio, however, was not demonstrated and accepted until the 1930s, when improved techniques for detecting viruses and antiviral antibodies enabled scientists to trace the disease in all of its phases.
The virus responsible for poliomyelitis is present in large numbers in the intestines of infected individuals. It is excreted in feces, from which it is spread to uninfected individuals through contaminated water, food, hands, and eating utensils and by flies and other filth-loving insects. Once it has been ingested, the poliovirus multiplies in the cells lining the intestine and invades the lymphatic system, producing swelling in the lymph nodes surrounding the intestine and in the neck. The symptoms of the disease at this stage may escape notice altogether, or the infected person may experience fever and a sore throat. These symptoms subside after two or three days as the body’s immune system begins producing antibodies to overcome the virus. Most cases never proceed beyond this stage, termed the minor illness of polio.
In a minority of cases, after a period of several days in which a patient is asymptomatic, the minor illness is followed by the onset of neurological symptoms, signaling that the virus has invaded the
spinal cord. Symptoms include pain and stiffness in the spine, lethargy, general muscular weakness, and flaccid (that is, not accompanied by spasms) paralysis of muscles, particularly of the legs. Paralysis of the legs occurs because the virus preferentially attacks neurons in the front or anterior horns of the spinal cord, including the motor nerves controlling the legs, and often affects one side more than the other. In the most severe cases, viral infection spreads from the spinal cord to the brain stem, attacking neurons serving the diaphragm and esophagus. Without aggressive medical intervention in the form of an artificial breathing apparatus, paralysis of the diaphragm is fatal.
In the absence of brain-stem involvement, a body’s normal defenses usually overcome the viral infection. Since the body is unable to replace destroyed neurons, however, acute polio leaves the patient with permanent motor impairment ranging from mild muscular weakness to severe crippling disability. Aided by appropriate physical therapy during the recovery period, patients can usually regain some of the motor function lost during the active disease. As survivors of the polio
epidemics of the 1940s and 1950s reached middle age in the late twentieth century, a late phase of the disease called postpolio syndrome was recognized. After decades of apparent normality, muscles affected by the initial paralytic attack experience gradual loss of function without evidence of renewed viral activity.
The proportion of cases resulting in permanent paralysis varies with the age structure of the population affected. Typically, no more than 10 percent of patients who experience a major illness including neurologic symptoms suffer such paralysis. Under premodern conditions, the latter probably accounted for less than 1 percent of the total cases, because subclinical infection, minor illness, and paralytic polio usually occurred in early childhood. Maternal antibodies provided protection for newborns, while a child’s own immune system created antibodies following exposure. In this way, lifelong immunity to subsequent infection was acquired. With improving sanitation and an older susceptible population, however, this proportion gradually increased.
During an epidemic, polio is primarily spread by persons with mild and subclinical infections, who may be unaware that they are ill. Infectivity persists for two to three weeks after the onset of the intestinal disease. There is no evidence that lifetime carriers exist. It is virtually impossible to prevent the spread of an asymptomatic, fecally transmitted pathogen among young children in group settings by any behavioral means. Fortunately, vaccines have effectively eliminated polio as an epidemic disease in the developed world and in Latin America.
Treatment and Therapy The history of efforts to prevent and treat poliomyelitis illustrates the changing attitudes of the medical community toward disease and the methods by which a once-important pathogen was virtually eliminated. Only time will tell whether the spectacular inroads made by medical science against poliomyelitis are permanent. Persons with compromised immune systems are reminders that vaccines cannot completely protect all individuals. They prevent health professionals from becoming complacent with respect to any infectious disease.
At the end of the nineteenth century, when epidemics of poliomyelitis first began to surface, medical science had made a number of important advances in the understanding and treatment of disease. First and foremost, the role of microorganisms in infectious disease was well established. Although viruses were still poorly defined, the principle that they were transmissible agents was understood. Second, although physicians had few specific remedies at their disposal, they had abandoned most of the drastic, plainly harmful remedies of earlier eras.
As polio became more and more prominent in morbidity statistics and the public imagination, the biomedical community responded on three different fronts. The first was an attempt to prevent transmission by quarantine measures and clinical studies. Scientists attempted to clarify the actual mode of transmission and the natural occurrence of the virus. Second, they attempted to treat paralytic cases in the acute and recovery phases. Lastly, scientists tried to develop a vaccine.
Quarantine measures were never very successful at controlling polio epidemics. Isolating critically ill patients in a sterile environment and restricting travel on the part of their family members, as was done in 1916 in New York, failed to quarantine people with mild infections, who were the main transmitters of the disease. Although the poliovirus can be found in untreated sewage, this was not a major source of infection in the United States. Flies can transmit the virus mechanically and thus may act as vectors, but fly eradication campaigns failed to have any effect on polio occurrence. In the period when it was incorrectly thought that the poliovirus entered through the nose, nasal sprays were touted as offering protection.
In the 1920s, the search for a cure emphasized the use of blood serum from individuals who had recovered from the disease. Theoretically, the idea was a sound one that had been used successfully for other diseases, but it proved ineffective in the case of polio, because the level of antibodies in the serum was not sufficiently high to have a therapeutic effect. More important, by the time patients developed paralytic symptoms, their bodies were already producing antibodies. Despite disappointing results, serum therapy was used extensively for fifteen years.
In 1920, Philip Drinker of the Harvard School of Public Health introduced the so-called
iron lung, a respirator that mimicked the action of lungs by subjecting patients to fluctuations in air pressure. The iron lung gave some hope of survival to patients with paralysis of the diaphragm or lesions in the nervous centers of the brain that govern respiration. Its introduction was accompanied by misgivings that it would only serve to keep alive severely disabled patients who had no hope of survival outside a hospital. Such ethical concerns were justified, but artificial respirators also proved effective in temporarily treating acute cases of respiratory paralysis that subsided with time.
With respect to paralyzed limbs, advances in orthopedics in the early twentieth century allowed for surgical procedures that minimized twisting and deformity and for the design of braces that improved mobility. Observing that deformity could be lessened by bracing and immobilizing limbs at the onset of the paralytic form of the disease, doctors of the 1920s and 1930s had a tendency to encase polio victims, even those with little or no paralysis, in elaborate casts attached to pulley systems. Against this trend, Elizabeth Kenny, an Australian nurse, conducted what amounted to a crusade against immobilization and advocated active physical therapy in acute paralytic poliomyelitis.
In 1921, future president of the United States Franklin Delano Roosevelt was stricken with acute paralytic polio that left him with severe paralysis of both legs. Roosevelt later used his private fortune to establish a center for the rehabilitation of polio victims in Warm Springs, Georgia, where he had spent his convalescence. After he became president in 1933, Roosevelt became a leader in the fight against poliomyelitis. For several years, the principal charitable organization funding polio treatment and research in the United States was the president’s Birthday Ball Commission, the immediate forerunner of the National Foundation for Infantile Paralysis (NFIP), better known under the name of its main fund-raising effort, the March of Dimes. Basil O’Conner, a personal friend of Roosevelt, headed both agencies.
Since an attack of polio in any of its forms confers lifelong immunity, researchers from the 1920s onward increasingly concentrated their efforts on developing a vaccine. Vaccines rely on dead or nonvirulent strains of a pathogenic agent to induce an immune response in a host. To produce a polio vaccine, one must have large quantities of poliovirus, and the only known source of poliovirus prior to 1938 was spinal cord tissue from infected monkeys or humans. In 1935, Maurice Brodie conducted human vaccine trials with a formalin-inactivated virus from monkey spinal cord. At the same time, John Kohler conducted trials with a virus that he claimed had been inactivated by repeated passage through many generations of monkeys. Brodie’s vaccine was unsuccessful; Kohler’s achieved notoriety as the suspected cause of several cases of paralytic polio.
After World War II, the NFIP concentrated its efforts on funding the development of an effective polio vaccine. Thanks to the work of John Enders and others, a live poliovirus could be produced in tissue culture. Improved serological techniques enabled researchers to assess immunity in chimpanzees without sacrificing the animal. By 1950, a practical vaccination program was beginning to take shape under the direction of Jonas Salk
, who headed the development of a formalin-inactivated injectable vaccine.
In 1954, with the collaboration of the National Institutes of Health and the US Census Bureau, the NFIP conducted a massive nationwide test of this inactivated Salk vaccine, involving 1.8 million children in the first, second, and third grades. In 1955, the number of new cases (or incidence) of polio among inoculated children was significantly lower than among controls, demonstrating that the vaccine was effective in clinical practice. Thereafter, the inoculation of children against polio became routine, and the incidence per 100,000 people declined dramatically—from 40 in 1952, the last major epidemic year, to 20 in 1955. The number of new cases per 100,000 people was 5 in 1959 and fewer than 1 in 1961 and subsequent years. There have been no domestically acquired cases of paralytic polio in the United States since 1987.
Salk’s vaccine conferred only temporary immunity, requiring booster shots to be administered at yearly intervals. This made protection of the population cumbersome in industrialized countries and impractical in developing countries. The NFIP consequently turned its attention toward an effort, under the direction of Albert Sabin, to develop an orally administered attenuated viral preparation. The challenge was to develop a strain of virus that would multiply in the digestive system and stimulate antibody production but that could not attack the human nervous system. This effort was also supported by the World Health Organization (WHO). In 1957, an oral live virus vaccine was tested in Ruanda-Urundi (today Rwanda and Burundi). Between 1958 and 1959, field trials were conducted in fifteen countries, including the United States and the Soviet Union.
The Sabin oral vaccine confers longer-lasting immunity and is easier to administer, and therefore came to be used routinely to immunize children and adults against polio throughout the world. The WHO has exploited this advantage and undertaken a program of complete polio eradication. This program has been highly successful, and as of 2012, there were only three countries left in the world where polio was endemic (meaning not brought in from outside), down from more than 125 countries when the Global Polio Eradication Initiative was launched in 1988. The three remaining countries with endemic poliovirus are Nigeria, Pakistan, and Afghanistan. Six other countries in 2012 also reported isolated cases of polio, but they were either mild cases derived from the vaccine, or were brought into the country from outside. Altogether, 2012 saw fewer than 300 reported cases of polio worldwide.
Perspective and Prospects There is evidence that poliomyelitis has afflicted human beings from the beginning of time. There is an Egyptian tomb painting of a priest with a withered leg, and descriptions of individuals with polio-like diseases occur in Greek medical literature. In general, however, polio seems to have been a rare disease; there are no records of epidemics of paralytic polio before the second half of the nineteenth century. The symptoms of paralytic polio are so distinctive and devastating that it is unlikely cases were overlooked.
In the early nineteenth century, a number of physicians published descriptions of cases in which a fever in infants or very young children was followed by paralysis of the lower limbs. At that time, polio was unknown among older children and adults. As a consequence, the disease came to be known as infantile paralysis. The occurrence was infrequent and sporadic, although Charles Bell, a distinguished English neurologist, recorded an account of an epidemic affecting all the three- to five-year-old children on the isolated island of St. Helena around 1830.
Between 1880 and 1905, several localized outbreaks of epidemic poliomyelitis occurred in rural Scandinavia. In 1894, the United States suffered its first major outbreak, in Rutland County, Vermont. In contrast to earlier experiences, significant numbers of older children and young adults were affected. It was also puzzling to epidemiologists that the outbreaks should have occurred in isolated rural areas rather than in urban centers. Ivar Wickman, a Swedish epidemiologist who tracked the course of the severe Scandinavian epidemic of 1905, obtained evidence for abortive and nonparalytic cases, and postulated that they were important to the epidemiology of the disease. His results were not taken seriously until thirty years later.
In 1916, the northeastern United States suffered one of the most devastating epidemics in the history of poliomyelitis, with more than nine thousand acute cases in New York City alone. Public health authorities, disregarding evidence that acute cases represented less than 10 percent of actual cases, instituted draconian quarantine measures that were largely ineffective. More than 95 percent of those affected in the 1916 epidemic were under nine years of age. By 1931, the date of the next major epidemic in the Northeast, the proportion of victims younger than nine had declined to 84 percent; by 1947, it had further declined to 52 percent. Poliomyelitis had somehow been transformed from an uncommon endemic disease affecting only very young children to a sporadic, rural epidemic disease that affected primarily but not exclusively children. Finally, it had become a widespread epidemic disease affecting all age groups in both rural and urban environments.
In 1905, when Swedish researchers attempted to show the existence of subclinical poliomyelitis infections, there was only one way to demonstrate polio in an unequivocal, scientifically rigorous manner. It involved filtering material from a diseased person to remove bacteria, inoculating the filtrate into the brain of a susceptible monkey, and waiting for paralysis to develop. Cost and logistics precluded large-scale tests. The trials that were conducted often failed because of inadequate sterility. In 1939, Charles Armstrong succeeded in propagating one of the three poliovirus strains in rodents, greatly facilitating research. In 1948, Enders and his colleagues succeeded in growing the poliovirus in tissue culture. In the meantime, reliable techniques for identifying antibodies to specific pathogens had been developed. This development enabled epidemiologists to determine which individuals had the live poliovirus in their bodies and which had developed immunity.
A series of studies conducted in the early 1950s among Alaskan Inuits, urban North Americans, and Egyptian villagers dramatically demonstrated the normal epidemiological pattern of polio occurrence and progress in three very different populations. Among Inuits living in Point Barrow, Alaska, only people over twenty showed antibodies to the virus, as a result of a known and devastating epidemic in 1930. In Miami, Florida, the proportion of persons with antibodies rose from 10 percent at age two to nearly 80 percent in adulthood. In Cairo, nearly 100 percent of the population over the age of three proved to have antibodies.
Therefore, the following epidemiological picture emerged. Before 1900, sanitary conditions in most of the world approximated those in Cairo, and most people contracted polio before the age of three. The vast majority of infections were subclinical, and paralytic cases occurred only sporadically in infants. As sanitation improved in the United States and Europe, the chances of contracting polio as an infant decreased. Thus, a pool of susceptible individuals of mixed ages arose, and epidemics occurred. Like mumps, measles, and other childhood illnesses, polio is more likely to cause severe illness in an adult than in a young child. For this reason, paralytic polio became a more serious health problem in Miami than in Cairo. Epidemics occurred first in rural areas in the United States and Scandinavia, where sanitation was relatively good and people were somewhat isolated from major population centers that served as sources of infection.
Defenders of the use of
animals in biomedical research often cite the history of the conquest of poliomyelitis to support their point of view. From the earliest days of scientific poliomyelitis research until the discovery of tissue-culturing techniques for viruses in the 1940s, experimental work was dependent on monkeys. For many years, the only way of confirming that the virus was present was to inoculate a monkey with a suspected sample: If the monkey became paralyzed, the test was positive. Cultures were maintained through serial transfer from monkey to monkey, and the earliest vaccines were prepared from monkey spinal cord tissue.
The first successful tissue culture
experiments involved fetal intestinal tissue. The experiments depended on having an available source of a characterized viral strain originally isolated from a human but maintained through several generations of transfer through animals. Even after the maintenance and characterization of viral strains and the production of virus for vaccines had moved from animal laboratories to test tubes of cultured cells, the first tests of the safety and efficacy of vaccines were performed with primates. Virtually every step in the conquest of polio involved experimental procedures.
Although the fight against poliomyelitis has been spectacularly successful, it would be unwise to be complacent about a disease that still exists in parts of the world and that is selectively virulent under modern urban conditions in developed nations. The percentage of schoolchildren, particularly those living in poorer neighborhoods, who receive routine vaccinations against childhood diseases is decreasing in the United States. Because of the availability of safe drinking water, children are no longer likely to naturally acquire immunity from subclinical infections.
In about 1 in 20 million cases, individuals will develop polio after receiving a vaccine. A naturally acquired case of polio is now exceedingly rare. The number of such cases is less than the number of cases of polio as a result of adverse reactions to the vaccine. For this reason, many parents are not having their children immunized against polio. These well-intentioned people are putting their children at an unnecessary risk of contracting the disease.
Other diseases that were once thought to be virtually extinct, such as measles and tuberculosis, are experiencing a resurgence because of declining commitment to public and community health and increasing numbers of people with compromised immune systems. Until the polio eradication campaign is complete, there is no guarantee that polio will be excluded from the list of resurgent diseases.
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