Polio (Poliomyelitis)
Polio (Poliomyelitis)
Disease History, Characteristics, and Transmission
Introduction
Polio, short for poliomyelitis, is a highly infectious viral disease that can cause rapid paralysis of the limbs and the muscles used in breathing. Also known as infantile paralysis, polio mainly affects children under the age of five, although it can also affect adults.
First described at the end of the eighteenth century, there have been many epidemics of polio both in the United States and in other countries around the world. Once an effective vaccine was introduced in 1955, however, the number of cases of polio began to fall dramatically. In 1988, the World Health Organization (WHO), along with other charities and organizations, launched the Global Polio Eradication Initiative. This relies on ensuring that all children are vaccinated against polio. The initiative has led to a dramatic drop in the number of cases of polio around the world—from around 350,000 in 1988 to fewer than 2,000 in the year 2005.
Disease History, Characteristics, and Transmission
The word poliomyelitis comes from polio, the Greek word for gray, and myelon, the Greek word for marrow (indicating the spinal cord). It is the effect of the poliovirus on the spinal cord that causes the paralysis associated with this disease. There are three types of poliovirus—1, 2, and 3—all of which cause very similar infections. Polioviruses belong to the enterovirus group and are part of the picornavirus family of RNA viruses (that is, their genetic material is composed of RNA rather than DNA). John F. Enders (1897–1985), Thomas Weller (1915–), and Frederick Robbins (1916– 2003) of Harvard School for Public Health, first grew poliovirus in tissue culture in 1948, and were awarded the Nobel Prize for this work in 1954.
Polio occurs mainly in the summer and fall seasons in temperate climates, but has no seasonal pattern in tropical climates. The incubation time of poliovirus is 7–14 days, during which time it multiplies in the cells lining the intestines and the respiratory tract. In 90% of cases, the infection causes no symptoms at all. Another 5% will have relatively mild symptoms including headache, fever, fatigue, and vomiting. These symptoms are indistinguishable from those of many other viral illnesses and usually clear up within a week.
One to two percent of polio infections result in nonparalytic aseptic meningitis, which causes stiffness of the neck, back, and/or legs. This condition tends to clear up within a week or so and complete recovery is usual. In one percent of cases, however, poliovirus spreads from the intestines, through the blood, to the nervous system, where it can destroy nerve cells in the spinal cord and the base of the brain.
Polio infection in the nervous system leads to paralytic polio—the most feared type of the disease. The severity of paralytic polio depends on how many neurons are affected, but onset of paralysis can be very rapid. Commonly, a child might go to bed with minor symptoms and wake up being unable to walk.
There are three forms of paralytic polio. Spinal polio causes a flaccid (floppy) paralysis of one or more of the limbs, without any loss of feeling. Complete recovery is quite possible, however, and tends to happen within the first six months. Any weakness or paralysis remaining after a year is likely to be permanent and the patient is left with some degree of disability. Spinal polio accounted for 79% of all cases of paralytic polio during 1969 to 1979.
Another, less common, form of paralytic polio is socalled bulbar polio, where the poliovirus affects the cranial nerves in the upper spinal cord. This leads to paralysis of the pharnyx (back of the throat), vocal cords, and respiratory (breathing) muscles. Bulbar polio is the most dangerous form of the disease, killing 75% of those affected. Pure bulbar polio accounted for only 2% of cases in the 1969–1979 period. The rest were bulbospinal polio, which is a combination of the two forms.
The complications of paralytic polio include urinary tract infection and pneumonia, which are common in any condition where a patient is immobilized. The over-all death rate from paralytic polio is 2–5% for children and 15–25% for adults.
Most doctors will never see a case of polio. However, they may see a condition known as post-polio syndrome which affects 25–40% of those who had paralytic polio in childhood. The syndrome is characterized by fatigue, new muscle pain, and exacerbation of any existing weakness. It is more common with those left with residual disability by the original infection and among women. It is not clear what causes post-polio syndrome, but it may result from nerve damage occurring during the original recovery process. Post-polio syndrome has been shown not to be a re-activation of polio infection, and those affected are not infectious to others.
Poliovirus is transmitted by the fecal-oral route—that is, through consumption of contaminated food and water. It enters the body through the mouth and multiplies in the throat and the gastrointestinal tract, the latter being confirmed through laboratory examination of the feces of people with polio. People with no symptoms, or only mild symptoms, can still transmit the infection. They are most infections from 7–10 days before and the same period after the onset of symptoms (if these are present). The disease is highly infectious and, therefore, in communities where even one child remains unvaccinated, all are at risk of developing the disease.
Rarely, someone may contract polio—and pass it on to others—through vaccination. Vaccine-association paralytic polio (VAPP) occurs in around one person in two to three million following vaccination with the live vaccine.
WORDS TO KNOW
ATTENUATED STRAIN: A bacterium or virus that has been weakened, often used as the basis of a vaccine against the specific disease caused by the bacterium or virus.
ENDEMIC: Present in a particular area or among a particular group of people.
ERADICATION: The process of destroying or eliminating a microorganism or disease.
INACTIVATED VACCINE: An inactivated vaccine is a vaccine that is made from disease causing microorganisms that have been killed or made incapable of causing the infection. The immune system can still respond to the presence of the microorganisms.
LIVE VACCINE: A live vaccine uses a virus or bacteria that has been weakened (attenuated) to cause an immune response in the body without causing disease. Live vaccines are preferred to killed vaccines, which use a dead virus or bacteria, because they cause a stronger and longer-lasting immune response.
WILD VIRUS: Wild or wild-type virus is a genetic description referring to the original form of a virus, first observed in nature. It may remain the most common form in existence but mutated forms develop over time and sometimes become the new wild type virus.
Scope and Distribution
Children younger than five have always been most at risk of polio, although it can affect people of any age. For instance, American President Franklin D. Roosevelt contracted the disease in 1921 at the age of 39. Polio was first described in England in 1789, and was only a sporadic disease during the eighteenth and nineteenth centuries.
However, from the turn of the nineteenth century polio became an epidemic disease around the world, although the first outbreak in the United States had occurred as early as 1843. It is not clear why the incidence of polio changed in this way. It is possible that improvements in standards of hygiene may have led to a loss of natural immunity against the disease, with children being less likely to be exposed to poliovirus in contaminated food or water. Such exposure may have led to mild or even asymptomatic infection that had no impact on health, but conferred a natural immunity against further attacks of polio.
In the early years of the twentieth century, Scandinavia, the United Kingdom, and North America were especially affected by polio. According to data released by the Health Section of the League of Nations (the forerunner of the United Nations), of 178,328 cases of polio occurring between 1919 and 1934, 54.3% were in the United States and Canada. However, the peak year for polio in the United States was 1952, with over 57,000 cases, including 21,000 paralytic cases.
In England, there were over 1,000 cases of polio in 1928 and in 1938, but between these years, the annual average was around 640. Polio became more of a problem in Britain after the World War II, with almost 8,000 cases being reported in 1947. There was also a very severe outbreak in 1952 that affected 238 per 100,000 of the population in Copenhagen, Denmark. The majority of these cases were bulbar polio, the most dangerous form of the disease.
The introduction of the polio vaccine in the 1950s began to bring the disease under control. However, in 1988, when the WHO declared a war on polio, the disease was still endemic in 125 countries around the world. Today polio is endemic only in four countries— Afghanistan, Pakistan, India, and Nigeria.
Polio has now been eradicated in the western world and in the western Pacific, including China. There have been occasional outbreaks, including one in the Netherlands in 1993, in areas where parents refused to have their children vaccinated. There are also very occasional cases linked with the vaccine itself.
Treatment and Prevention
There is no treatment for polio. During the 1930s and 1940s, it was common to immobilize the paralyzed limbs with splints to protect them. This was controversial, however, and some doctors recommended exercise instead, to build up the weakened limbs.
The need to support the breathing of those with paralytic polio led to the development of the Drinker respirator or “iron lung” by physiologist Cecil Drinker (1887–1956) of the Harvard School of Public Health and his brother, chemical engineer Philip Drinker (1894–1972). Introduced in 1928, the iron lung was the forerunner of the modern intensive care unit, whose mission is to support the vital functions of dangerously ill patients. Unfortunately, the existence of the iron lung forced doctors to have to make difficult decisions. Invariably, there would be more patients than iron lungs, and so they would have to decide which ones to treat.
IN CONTEXT: SCIENTIFIC, POLITICAL, AND ETHICAL ISSUES
The eldest son of Orthodox Jewish-Polish immigrants, Jonas Salk (1914–1995) earned his medical degree in 1939. After his internship, and work on a flu vaccine, Salk devoted a considerable amount of his energies to writing scientific papers on a number of topics, including the polio virus. Some of these came to the attention of Daniel Basil O'Connor, the director of the National Foundation for Infantile Paralysis—an organization that had long been involved with the treatment and rehabilitation of polio victims. O'Connor eyed Salk as a possible recruit for the polio vaccine research his organization sponsored. When the two finally met, O'Connor was much taken by Salk—so much so, in fact, that he put almost all of the National Foundation's money behind Salk's vaccine research efforts.
Salk's first challenge was to obtain enough of the virus to be able to develop a vaccine in doses large enough to have an impact; this was particularly difficult since viruses, unlike culture-grown bacteria, need living cells to grow. The breakthrough came when the team of John F. Enders, Thomas Weller, and Frederick Robbins found that the polio virus could be grown in embryonic tissue—a discovery that earned them a Nobel Prize in 1954. Salk subsequently grew samples of all three varieties of polio virus in cultures of monkey kidney tissue, then killed the virus with formaldehyde. Salk argued that it was essential to use a killed polio virus (rather than a live virus) in the vaccine, as the live-virus vaccine would have a much higher chance of accidentally inducing polio in inoculated children. He therefore, exposed the viruses to formaldehyde for nearly 13 days. Though after only three days he could detect no virulence in the sample, Salk wanted to establish a wide safety margin; after an additional ten days of exposure to the formaldehyde, he reasoned that there was only a one-in-a-trillion chance of there being a live virus particle in a single dose of his vaccine. Salk tested it on monkeys with positive results before proceeding to human clinical trials.
Despite Salk's confidence, many of his colleagues were skeptical, believing that a killed-virus vaccine could not possibly be effective. His dubious standing was further compounded by the fact that he was relatively new to polio vaccine research; some of his chief competitors in the race to develop the vaccine—most notably Albert Sabin (1906–1993), the chief proponent for a live-virus vaccine—had been working for many years. As the field narrowed, the division between the killed-virus and the live-virus camps widened, and what had once been a polite difference of opinion became a serious ideological conflict. Salk and his chief backer, the National Foundation for Infantile Paralysis, were lonely in their corner. Salk failed to let his position in the scientific wilderness dissuade him and he continued, undeterred, with his research. To test his vaccine's
strength, in early 1952, Salk administered a type I vaccine to children who had already been infected with the polio virus. His results clearly indicated that the vaccine produced large amounts of anti-bodies. Buoyed by this success, the clinical trial was then extended to include children who had never had polio.
In May 1952, Salk initiated preparations for a massive field trial in which over 400,000 children would be vaccinated. The largest medical experiment that had ever been carried out in the United States, the test finally got underway in April 1954, sponsored by the National Foundation for Infantile Paralysis. More than one million children between the ages of six and nine took part in the trial, each receiving a button that proclaimed them a “Polio Pioneer.” At the beginning of 1953, while the trial was still at an early stage, Salk's encouraging results were made public in the Journal of the American Medical Association. Predictably, media and public interest were intense. On April 12, 1955, the vaccine was officially pronounced effective, potent, and safe in almost 90% of cases. The meeting at which the announcement was made was attended by 500 of the world's top scientists and doctors, 150 journalists, and 16 television and movie crews.
Just two weeks after the announcement of the vaccine's discovery, however, eleven of the children who had received it developed polio; more cases soon followed. Altogether, about 200 children developed paralytic polio, eleven fatally. For a while, it appeared that the vaccination campaign would be railroaded. However, it was soon discovered that all of the rogue vaccines had originated from the same laboratory in California. Following a thorough investigation, it was found that the lab used faulty batches of virus culture, which were resistant to the formaldehyde. After furious debate and the adoption of standards that would prevent such a reccurrence, the inoculation resumed. By the end of 1955, seven million children had received their shots, and over the course of the next two years more than 200 million doses of Salk's polio vaccine were administered, without a single instance of vaccine-induced paralysis. By the summer of 1961, there had been a 96% reduction in the number of cases of polio in the United States, compared to the five-year period prior to the vaccination campaign.
After the initial inoculation period ended in 1958, Salk's killedvirus vaccine was replaced by a live-virus vaccine developed by Sabin; use of this new vaccine was advantageous because it could be administered orally rather than intravenously, and because it required fewer “booster” inoculations.
The battle between Sabin and Salk persisted well into the 1970s, with Salk writing an op-ed piece for the New York Times in 1973 denouncing Sabin's vaccine as unsafe and urging people to use his vaccine once more.
Jonas Salk (1914–1995) introduced an inactivated polio vaccine (IPV) in 1955, following testing and trials. This had to be given by injection. Then Albert Sabin (1906–1993) developed an attenuated (weakened) live vaccine (oral polio vaccine, OPV) that could be given by mouth, which was obviously much more convenient. Early experience with the Sabin vaccine was troubling, with some healthy volunteers developing paralytic polio (VAPP) after exposure. For instance, in 1962, there were 62 cases reported from non-epidemic areas of the United States, all occurring within 30 days of vaccination. It is unlikely that these cases arose from natural infection.
But Sabin persevered with his trials and the oral vaccine was introduced in the early 1960s and was used widely in the United States and in many developing countries. More advanced versions of OPV are now used where polio continues to be a threat. However, although OPV has been responsible for the elimination of wild poliovirus infection from the United States, the tiny risk of VAPP has meant that it has also been responsible for 95% of paralytic polio cases (the rest being imported cases).
For this reason, OPV has now been phased out and replaced by IPV, of which two versions are currently approved for use in the United States. A child should receive three doses of IPV within the first 18 months of life and a fourth dose at, or before, school entry. Travelers to the few remaining countries affected by polio, and certain laboratory workers, may need to have a booster dose of IPV vaccine to protect them. But a routine vaccination of adults is not necessary.
Impacts and Issues
Polio became a huge problem in North America. It affected children and cost them their lives or, if left disabled, could threaten their ability to lead a full and productive life. Therefore, the public exerted massive pressure on the government and scientists to find a solution in the form of a vaccine. In 1952, the number of cases of polio in the United States reached an unprecedented 57,268, and the debate over whether to release the vaccine that was being developed intensified. Organizations like the March of Dimes had raised substantial sums of money for research and called for trials of polio vaccine to begin without further delay, while researchers urged caution as they sought to perfect their lab experiments first. Testing of the Salk vaccine did, in fact, begin—albeit on a small scale—in 1952.
However, as with any vaccine, there were challenges to be met in terms of meeting demand and safety requirements, when it came to doing large scale vaccination against polio. There were problems in 1954, for example, when batches of vaccine were found to cause polio in experimental monkeys and were accordingly declared unfit for human use. Despite this, large scale testing of vaccine was finally able to begin in 1954. When successful results from these trials were reported the following year, public excitement was such that church bells were set ringing in celebration in several towns.
The Salk vaccine reduced the rate of polio from 13.9 per 100,000 of the population in 1954 to 0.5 per 100,000 in 1961. However, some argued that the Sabin vaccine, which consisted of live, but attenuated, polio virus, might be even more effective. These arguments continued over the next few years. The two vaccines differ in that the former is made from killed poliovirus, while the latter is based on live virus. Put simply, a killed vaccine may be less effective, but may be safer. A live vaccine may, potentially, actually cause active disease if the virus it contains mutates into a form that could be infectious. However, a live vaccine may induce a more effective immune response and so confer better protection against the disease.
By 1985, the success of the polio vaccine gave the Pan American Health Organization the confidence to set the goal of eradicating polio from the Americas by 1990. Meanwhile, the WHO had declared the official eradication of smallpox in 1980 and decided to build on this success by launching the Global Polio Eradication Initiative in 1988. This was supported by Rotary International, the worldwide voluntary organization, the U.S. Centers for Disease Control and Prevention (CDC), and UNICEF, the United Nations Children's Fund.
The Initiative has had considerable success. Since 1988, the number of cases of polio worldwide has fallen by 99%. In 1994, the WHO Americas region, consisting of 36 countries, was declared polio-free, followed by the WHO Western Pacific region, consisting of 37 countries, in 2000, and the WHO European region (51 countries) in 2002.
This leaves four countries with endemic polio, down from 125 countries in 1988. Mass vaccination is the key to eradication of polio and, in 2005 alone, 400 million children received polio vaccine. Two billion children have been vaccinated since the campaign began, and it is estimated that five million have been saved from disabling paralysis.
However, the total eradication of polio remains a challenge. It can be difficult, even under normal conditions, to access children living in remote areas. Where war and poverty compromise or destroy a country's infrastructure, achieving vaccination goals is even more challenging. Currently, there are outbreaks in northern Nigeria, and a new outbreak in western Uttar Pradesh in India.
Primary Source Connection
The following report was issued by the World Health Organization (WHO) about a single-case outbreak of polio in China that occurred in October 1999. The government of China cooperated with the WHO and other international health agencies to contain the outbreak and determine whether the initial case was acquired from another world region where polio is endemic, or whether the virus was a “wild type” that emerged in the local area. Tracking the source of outbreaks is vital to eradication efforts.
Polio in China
18 JANUARY 2000
Disease Outbreak Reported
The following case report is from the WHO Polio Eradication Programme:
The case was first reported to the County EPS in Geizi Township, Xunhua County, Haidong Prefecture, Qinghai Province, on 13 October 1999, and reported to the Provincial EPS on the following day. The case was born on 13 June 1998, had onset of paralysis on 12 October, after a day of fever on 11 October. The parents took the boy to a local private clinic in a neighbouring township when a sudden onset of flaccid paralysis made him unable to stand or walk (both of which he had been capable of before). Two stool samples were taken, the first on 14 October and the second on 25 October. They were analyzed in the provincial laboratory. Both samples yielded poliovirus isolates, which were later typed and differentiated as P1 wild viruses at the national laboratory in Beijing. At the time that the second sample was taken five contacts were sampled, one of which, a four year old cousin of the infected child, was also positive for wild poliovirus. The case child was unregistered and had received zero doses of polio vaccine.
The case belongs to the Sala minority group, a Muslim group of Turkic speaking people whose ancestors migrated to Qinghai from the area of Turkmenistan about seven hundred years ago. There are around 80,000 Sala in China, 60,000 in Qinghai Province (nearly all of which live in Xunhua Sala Autonomous County) and nearly all of the remainder in neighbouring Gansu Province. Adult male Sala travel widely as traders and workers, within Qinghai province and outside to other provinces, including Gansu, Sichuan, Xinjiang, and particularly Tibet, even as far as the border area with Nepal.
Neither the case nor the direct family reported a history of travel outside the county in the two months prior to onset. No visit to the family by a traveler from outside the county was reported to occur during the same period. However, the family, including the case, attended a major festival of Sala people in the county capital during the period 25 to 28 September 1999. Up to 30,000 Sala are reported to have attended this gathering.
Despite intensive investigation in the area of the case, including searches of health facilities, no evidence of wide-scale circulation of wild poliovirus has yet been found. Surveillance quality including laboratory proficiency in Qinghai Province and in neighbouring provinces is in general good. Indications are therefore that the virus has been recently imported.
The Ministry of Health of China is actively collaborating with the global laboratory network including CDC Atlanta, NIID Tokyo and the national laboratories in India. Initial sequencing information on the wild poliovirus show a close similarity to viruses recently circulating in India. The virus is significantly different from those that have been circulating in China up to the last case in 1994. Further genomic sequencing work is proceeding.
A combined MOH/WHO/UNICEF/JICA mission visited Qinghai Province from 20–25 December 1999 to review the response to the case. Initial case response immunization has been carried out, achieving high coverage of the target group. Extensive additional activities are planned, including large scale immunization across several provinces, intensified surveillance, retrospective review of hospital records at all levels in several provinces, and active search for cases of acute flaccid paralysis.
World Health Organization.
“POLIO IN CHINA.” EPIDEMIC AND PANDEMIC ALERT AND RESPONSE (EPR), DISEASE OUTBREAK NEWS (JANUARY 18, 2000).
See AlsoChildhood Infectious Diseases, Immunization Impacts; Polio Eradication Campaign.
BIBLIOGRAPHY
Books
Gould, Tony. A Summer Plague: Polio and Its Survivors. New Haven: Yale University Press, 1997.
Web Sites
Centers for Disease Control and Prevention (CDC). “Pink Book—Poliomyelitis.” <http://www.cdc.gov/nip/publications/pink/polio.pdf> (accessed March 25, 2007).
World Health Organization. “Poliomyelitis.” September 2006. <http://www.who.int/mediacentre/factsheets/fs114/en> (accessed March 25, 2007).
World Health Organization. Global Polio Eradication Initiative. “2005 Annual Report.” May 2006. <http://www.polioeradication.org/content/publications/annualreport2005.asp> (accessed March 25, 2007).
Susan Aldridge