Although around 90% of polio infections cause no symptoms at all, affected individuals can exhibit a range of symptoms if the virus enters the blood stream. In about 1% of cases the virus enters the central nervous system, preferentially infecting and destroying motor neurons, leading to muscle weakness and acute flaccid paralysis. Different types of paralysis may occur, depending on the nerves involved. Spinal polio is the most common form, characterized by asymmetric paralysis that most often involves the legs. Bulbar polio leads to weakness of muscles innervated by cranial nerves. Bulbospinal polio is a combination of bulbar and spinal paralysis.
Poliomyelitis was first recognized as a distinct condition by Jakob Heine in 1840. Its causative agent, poliovirus, was identified in 1908 by Karl Landsteiner. Although major polio epidemics were unknown before the late 19th century, polio was one of the most dreaded childhood diseases of the 20th century. Polio epidemics have crippled thousands of people, mostly young children; the disease has caused paralysis and death for much of human history. Polio had existed for thousands of years quietly as an endemic pathogen until the 1880s, when major epidemics began to occur in Europe; soon after, widespread epidemics appeared in the United States.
By 1910, much of the world experienced a dramatic increase in polio cases and frequent epidemics became regular events, primarily in cities during the summer months. These epidemics—which left thousands of children and adults paralyzed—provided the impetus for a "Great Race" towards the development of a vaccine. Developed in the 1950s, polio vaccines are credited with reducing the global number of polio cases per year from many hundreds of thousands to around a thousand. Enhanced vaccination efforts led by the World Health Organization, UNICEF, and Rotary International could result in global eradication of the disease.
The term poliomyelitis is used to identify the disease caused by any of the three serotypes of poliovirus. Two basic patterns of polio infection are described: a minor illness which does not involve the central nervous system (CNS), sometimes called abortive poliomyelitis, and a major illness involving the CNS, which may be paralytic or non-paralytic. In most people with a normal immune system, a poliovirus infection is asymptomatic. Rarely the infection produces minor symptoms; these may include upper respiratory tract infection (sore throat and fever), gastrointestinal disturbances (nausea, vomiting, abdominal pain, constipation or, rarely, diarrhea), and influenza-like illness.
The virus enters the central nervous system in about 3% of infections. Most patients with CNS involvement develop non-paralytic aseptic meningitis, with symptoms of headache, neck, back, abdominal and extremity pain, fever, vomiting, lethargy and irritability. Approximately 1 in 1000 to 1 in 200 cases progress to paralytic disease, in which the muscles become weak, floppy and poorly controlled, and finally completely paralyzed; this condition is known as acute flaccid paralysis. Depending on the site of paralysis, paralytic poliomyelitis is classified as spinal, bulbar, or bulbospinal. Encephalitis, an infection of the brain tissue itself, can occur in rare cases and is usually restricted to infants. It is characterized by confusion, changes in mental status, headaches, fever, and less commonly seizures and spastic paralysis.
Poliomyelitis is caused by infection with a member of the genus Enterovirus known as poliovirus (PV). This group of RNA viruses colonize the gastrointestinal tract — specifically the oropharynx and the intestine. The incubation time (to the first signs and symptoms) ranges from 3 to 35 days with a more common span of 6 to 20 days. PV infects and causes disease in humans alone. Its structure is very simple, composed of a single (+) sense RNA genome enclosed in a protein shell called a capsid. In addition to protecting the virus’s genetic material, the capsid proteins enable poliovirus to infect certain types of cells. Three serotypes of poliovirus have been identified—poliovirus type 1 (PV1), type 2 (PV2), and type 3 (PV3)—each with a slightly different capsid protein. All three are extremely virulent and produce the same disease symptoms. PV1 is the most commonly encountered form, and the one most closely associated with paralysis.
Individuals who are exposed to the virus, either through infection or by immunization with polio vaccine, develop immunity. In immune individuals, IgA antibodies against poliovirus are present in the tonsils and gastrointestinal tract and are able to block virus replication; IgG and IgM antibodies against PV can prevent the spread of the virus to motor neurons of the central nervous system. Infection or vaccination with one serotype of poliovirus does not provide immunity against the other serotypes, and full immunity requires exposure to each serotype.
Poliomyelitis is highly contagious via the oral-oral (oropharyngeal source) and fecal-oral (intestinal source) routes. In endemic areas, wild polioviruses can infect virtually the entire human population. It is seasonal in temperate climates, with peak transmission occurring in summer and autumn. These seasonal differences are far less pronounced in tropical areas. The time between first exposure and first symptoms, known as the incubation period, is usually 6 to 20 days, with a maximum range of 3 to 35 days. Virus particles are excreted in the feces for several weeks following initial infection. The disease is transmitted primarily via the fecal-oral route, by ingesting contaminated food or water. It is occasionally transmitted via the oral-oral route, a mode especially visible in areas with good sanitation and hygiene. Polio is most infectious between 7–10 days before and 7–10 days after the appearance of symptoms, but transmission is possible as long as the virus remains in the saliva or feces.
Factors that increase the risk of polio infection or affect the severity of the disease include immune deficiency, malnutrition, tonsillectomy, physical activity immediately following the onset of paralysis, skeletal muscle injury due to injection of vaccines or therapeutic agents, and pregnancy. Although the virus can cross the placenta during pregnancy, the fetus does not appear to be affected by either maternal infection or polio vaccination. Maternal antibodies also cross the placenta, providing passive immunity that protects the infant from polio infection during the first few months of life.
Poliovirus enters the body through the mouth, infecting the first cells it comes in contact with—the pharynx (throat) and intestinal mucosa. It gains entry by binding to an immunoglobulin-like receptor, known as the poliovirus receptor or CD155, on the cell membrane. The virus then hijacks the host cell's own machinery, and begins to replicate. Poliovirus divides within gastrointestinal cells for about a week, from where it spreads to the tonsils (specifically the follicular dendritic cells residing within the tonsilar germinal centers), the intestinal lymphoid tissue including the M cells of Peyer's patches, and the deep cervical and mesenteric lymph nodes, where it multiplies abundantly. The virus is subsequently absorbed into the bloodstream.
In around 1% of infections, poliovirus spreads along certain nerve fiber pathways, preferentially replicating in and destroying motor neurons within the spinal cord, brain stem, or motor cortex. This leads to the development of paralytic poliomyelitis, the various forms of which (spinal, bulbar, and bulbospinal) vary only with the amount of neuronal damage and inflammation that occurs, and the region of the CNS that is affected.
The destruction of neuronal cells produces lesions within the spinal ganglia; these may also occur in the reticular formation, vestibular nuclei, cerebellar vermis, and deep cerebellar nuclei. Inflammation associated with nerve cell destruction often alters the color and appearance of the gray matter in the spinal column, causing it to appear reddish and swollen. Other destructive changes associated with paralytic disease occur in the forebrain region, specifically the hypothalamus and thalamus. The molecular mechanisms by which poliovirus causes paralytic disease are poorly understood.
Early symptoms of paralytic polio include high fever, headache, stiffness in the back and neck, asymmetrical weakness of various muscles, sensitivity to touch, difficulty swallowing, muscle pain, loss of superficial and deep reflexes, paresthesia (pins and needles), irritability, constipation, or difficulty urinating. Paralysis generally develops one to ten days after early symptoms begin, progresses for two to three days, and is usually complete by the time the fever breaks.
Spinal polio is the most common form of paralytic poliomyelitis; it results from viral invasion of the motor neurons of the anterior horn cells, or the ventral (front) gray matter section in the spinal column, which are responsible for movement of the muscles, including those of the trunk, limbs and the intercostal muscles. Virus invasion causes inflammation of the nerve cells, leading to damage or destruction of motor neuron ganglia. When spinal neurons die, Wallerian degeneration takes place, leading to weakness of those muscles formerly innervated by the now dead neurons. With the destruction of nerve cells, the muscles no longer receive signals from the brain or spinal cord; without nerve stimulation, the muscles atrophy, becoming weak, floppy and poorly controlled, and finally completely paralyzed. Progression to maximum paralysis is rapid (two to four days), and is usually associated with fever and muscle pain. Deep tendon reflexes are also affected, and are usually absent or diminished; sensation (the ability to feel) in the paralyzed limbs, however, is not affected.
The extent of spinal paralysis depends on the region of the cord affected, which may be cervical, thoracic, or lumbar. The virus may affect muscles on both sides of the body, but more often the paralysis is asymmetrical. Any limb or combination of limbs may be affected—one leg, one arm, or both legs and both arms. Paralysis is often more severe proximally (where the limb joins the body) than distally (the fingertips and toes).
Paralytic poliomyelitis may be clinically suspected in individuals experiencing acute onset of flaccid paralysis in one or more limbs with decreased or absent tendon reflexes in the affected limbs that cannot be attributed to another apparent cause, and without sensory or cognitive loss.
A laboratory diagnosis is usually made based on recovery of poliovirus from a stool sample or a swab of the pharynx. Antibodies to poliovirus can be diagnostic, and are generally detected in the blood of infected patients early in the course of infection. Analysis of the patient's cerebrospinal fluid (CSF), which is collected by a lumbar puncture ("spinal tap"), reveals an increased number of white blood cells (primarily lymphocytes) and a mildly elevated protein level. Detection of virus in the CSF is diagnostic of paralytic polio, but rarely occurs.
Two types of vaccine are used throughout the world to combat polio. Both types induce immunity to polio, efficiently blocking person-to-person transmission of wild poliovirus, thereby protecting both individual vaccine recipients and the wider community (so-called herd immunity).
The first candidate polio vaccine, based on one serotype of a live but attenuated (weakened) virus, was developed by the virologist Hilary Koprowski. Koprowski's prototype vaccine was given to an eight-year-old boy on February 27, 1950. Koprowski continued to work on the vaccine throughout the 1950s, leading to large-scale trials in the then Belgian Congo and the vaccination of seven million children in Poland against serotypes PV1 and PV3 between 1958 and 1960.
The second inactivated virus vaccine was developed in 1952 by Jonas Salk at the University of Pittsburgh, and announced to the world on April 12, 1955. The Salk vaccine, or inactivated poliovirus vaccine (IPV), is based on poliovirus grown in a type of monkey kidney tissue culture (Vero cell line), which is chemically inactivated with formalin. After two doses of IPV (given by injection), 90% or more of individuals develop protective antibody to all three serotypes of poliovirus, and at least 99% are immune to poliovirus following three doses.
Subsequently, Albert Sabin developed another live, oral polio vaccine (OPV). It was produced by the repeated passage of the virus through non-human cells at sub-physiological temperatures. The attenuated poliovirus in the Sabin vaccine replicates very efficiently in the gut, the primary site of wild poliovirus infection and replication, but the vaccine strain is unable to replicate efficiently within nervous system tissue. A single dose of Sabin's oral polio vaccine produces immunity to all three poliovirus serotypes in approximately 50% of recipients. Three doses of live-attenuated OPV produce protective antibody to all three poliovirus types in more than 95% of recipients. Human trials of Sabin's vaccine began in 1957, and in 1958 it was selected, in competition with the live vaccines of Koprowski and other researchers, by the US National Institutes of Health. It was licensed in 1962 and rapidly became the only polio vaccine used worldwide.
Because OPV is inexpensive, easy to administer, and produces excellent immunity in the intestine (which helps prevent infection with wild virus in areas where it is endemic), it has been the vaccine of choice for controlling poliomyelitis in many countries. On very rare occasions (about 1 case per 750,000 vaccine recipients) the attenuated virus in OPV reverts into a form that can paralyze. Most industrialized countries have switched to IPV, which cannot revert, either as the sole vaccine against poliomyelitis or in combination with oral polio vaccine.
Patients with abortive polio infections recover completely. In those that develop only aseptic meningitis, the symptoms can be expected to persist for two to ten days, followed by complete recovery. In cases of spinal polio, if the affected nerve cells are completely destroyed, paralysis will be permanent; cells that are not destroyed but lose function temporarily may recover within four to six weeks after onset. Half the patients with spinal polio recover fully; one quarter recover with mild disability and the remaining quarter are left with severe disability. The degree of both acute paralysis and residual paralysis is likely to be proportional to the degree of viremia, and inversely proportional to the degree of immunity. Spinal polio is rarely fatal.
Without respiratory support, consequences of poliomyelitis with respiratory involvement include suffocation or pneumonia from aspiration of secretions. Overall, 5–10% of patients with paralytic polio die due to the paralysis of muscles used for breathing. The mortality rate varies by age: 2–5% of children and up to 15–30% of adults die. Bulbar polio often causes death if respiratory support is not provided; with support, its mortality rate ranges from 25 to 75%, depending on the age of the patient. When positive pressure ventilators are available, the mortality can be reduced to 15%.
Many cases of poliomyelitis result in only temporary paralysis. Nerve impulses return to the formerly paralyzed muscle within a month, and recovery is usually complete in six to eight months. The neurophysiological processes involved in recovery following acute paralytic poliomyelitis are quite effective; muscles are able to retain normal strength even if half the original motor neurons have been lost. Paralysis remaining after one year is likely to be permanent, although modest recoveries of muscle strength are possible 12 to 18 months after infection.
One mechanism involved in recovery is nerve terminal sprouting, in which remaining brainstem and spinal cord motor neurons develop new branches, or axonal sprouts. These sprouts can reinnervate orphaned muscle fibers that have been denervated by acute polio infection, restoring the fibers' capacity to contract and improving strength. Terminal sprouting may generate a few significantly enlarged motor neurons doing work previously performed by as many as four or five units: a single motor neuron that once controlled 200 muscle cells might control 800 to 1000 cells. Other mechanisms that occur during the rehabilitation phase, and contribute to muscle strength restoration, include myofiber hypertrophy—enlargement of muscle fibers through exercise and activity—and transformation of type II muscle fibers to type I muscle fibers.
In addition to these physiological processes, the body possesses a number of compensatory mechanisms to maintain function in the presence of residual paralysis. These include the use of weaker muscles at a higher than usual intensity relative to the muscle's maximal capacity, enhancing athletic development of previously little-used muscles, and using ligaments for stability, which enables greater mobility.
While now rare in the Western world, polio is still endemic to South Asia and Nigeria. Following the widespread use of poliovirus vaccine in the mid-1950s, the incidence of poliomyelitis declined dramatically in many industrialized countries. A global effort to eradicate polio began in 1988, led by the World Health Organization, UNICEF, and The Rotary Foundation. These efforts have reduced the number of annual diagnosed cases by 99%; from an estimated 350,000 cases in 1988 to a low of 483 cases in 2001, after which it has remained at a level of about 1,000 cases per year (1,606 in 2009). Polio is one of only two diseases currently the subject of a global eradication program, the other being Guinea worm disease. If the global Polio Eradication initiative is successful before that for Guinea worm or any other disease, it would be only the third time humankind has ever completely eradicated a disease, after smallpox in 1979 and rinderpest in 2010. A number of eradication milestones have already been reached, and several regions of the world have been certified polio-free. The Americas were declared polio-free in 1994. In 2000 polio was officially eliminated in 36 Western Pacific countries, including China and Australia. Europe was declared polio-free in 2002. As of 2006, polio remains endemic in only four countries: Nigeria, India (specifically Uttar Pradesh and Bihar), Pakistan, and Afghanistan, although it continues to cause epidemics in other nearby countries born of hidden or reestablished transmission.
The history of poliomyelitis (polio) infections extends into prehistory. Although major polio epidemics were unknown before the 20th century, the disease has caused paralysis and death for much of human history. Over millennia, polio survived quietly as an endemic pathogen until the 1880s when major epidemics began to occur in Europe; soon after, widespread epidemics appeared in the United States. By 1910, frequent epidemics became regular events throughout the developed world, primarily in cities during the summer months. At its peak in the 1940s and 1950s, polio would paralyze or kill over half a million people worldwide every year.
The fear and the collective response to these epidemics would give rise to extraordinary public reaction and mobilization; spurring the development of new methods to prevent and treat the disease, and revolutionizing medical philanthropy. Although the development of two polio vaccines has eradicated poliomyelitis in all but four countries, the legacy of poliomyelitis remains, in the development of modern rehabilitation therapy, and in the rise of disability rights movements worldwide.
The symptoms of poliomyelitis have been described by many names. In the early nineteenth century the disease was known variously as: Dental Paralysis, Infantile Spinal Paralysis, Essential Paralysis of Children, Regressive Paralysis, Myelitis of the Anterior Horns, Tephromyelitis (from the Greek tephros, meaning "ash-gray") and Paralysis of the Morning. In 1789 the first clinical description of poliomyelitis was provided by the British physician Michael Underwood—he refers to polio as "a debility of the lower extremities". The first medical report on poliomyelitis was by Jakob Heine, in 1840; he called the disease Lähmungszastände der unteren Extremitäten. Karl Oskar Medin was the first to empirically study a poliomyelitis epidemic in 1890. This work, and the prior classification by Heine, led to the disease being known as Heine-Medin disease.