Polio
Poliomyelitis, commonly shortened to polio, is an infectious disease caused by the poliovirus. Approximately 75% of cases are asymptomatic; mild symptoms which can occur include sore throat and fever; in a proportion of cases more severe symptoms develop such as headache, neck stiffness, and paresthesia. These symptoms usually pass within one or two weeks. A less common symptom is permanent paralysis, and possible death in extreme cases. Years after recovery, post-polio syndrome may occur, with a slow development of muscle weakness similar to what the person had during the initial infection.
Polio occurs naturally only in humans. It is highly infectious, and is spread from person to person either through fecal–oral transmission, or via the oral–oral route. Those who are infected may spread the disease for up to six weeks even if no symptoms are present. The disease may be diagnosed by finding the virus in the feces or detecting antibodies against it in the blood.
Poliomyelitis has existed for thousands of years, with depictions of the disease in ancient art. The disease was first recognized as a distinct condition by the English physician Michael Underwood in 1789, and the virus that causes it was first identified in 1909 by the Austrian immunologist Karl Landsteiner. Major outbreaks started to occur in the late 19th century in Europe and the United States, and in the 20th century, it became one of the most worrying childhood diseases. Following the introduction of polio vaccines in the 1950s, polio incidence declined rapidly., only Pakistan and Afghanistan remain endemic for wild poliovirus.
Once infected, there is no specific treatment. The disease can be prevented by the polio vaccine, with multiple doses required for lifelong protection. There are two broad types of polio vaccine; an injected polio vaccine using inactivated poliovirus and an oral polio vaccine containing attenuated live virus. Through the use of both types of vaccine, incidence of wild polio has decreased from an estimated 350,000 cases in 1988 to 30 confirmed cases in 2022, confined to just three countries. In rare cases, the traditional OPV was able to revert to a virulent form. An improved oral vaccine with greater genetic stability was developed and granted full licensure and prequalification by the World Health Organization in December 2023.
Signs and symptoms
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 that does not involve the central nervous system, sometimes called abortive poliomyelitis, and a major illness involving the CNS, which may be paralytic or nonparalytic. Adults are more likely to develop symptoms, including severe symptoms, than children.In most people with a normal immune system, a poliovirus infection is asymptomatic. In about 25% of cases, the infection produces minor symptoms which may include sore throat and low fever. These symptoms are temporary and full recovery occurs within one or two weeks.
In about 1 percent of infections the virus can migrate from the gastrointestinal tract into the central nervous system. Most patients with CNS involvement develop nonparalytic aseptic meningitis, with symptoms of headache, neck, back, abdominal and extremity pain, fever, vomiting, stomach pain, lethargy, and irritability. About one to five in 1,000 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. The weakness most often involves the legs, but may less commonly involve the muscles of the head, neck, and diaphragm. Depending on the site of paralysis, paralytic poliomyelitis is classified as spinal, bulbar, or bulbospinal. In those who develop paralysis, between 2 and 10 percent die as the paralysis affects the breathing muscles.
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.
Etymology
The term poliomyelitis derives from the Ancient Greek , meaning "grey", , referring to the grey matter of the spinal cord, and the suffix -itis, which denotes inflammation, i.e., inflammation of the spinal cord's grey matter. The word was first used in 1874 and is attributed to the German physician Adolf Kussmaul. The first recorded use of the abbreviated version polio was in the Indianapolis Star in 1911.Cause
Poliomyelitis does not affect any species other than humans. The disease is caused by infection with a member of the genus Enterovirus known as poliovirus. This group of RNA viruses colonize the gastrointestinal tract – specifically the oropharynx and the intestine. Its structure is quite simple, composed of a single sense RNA genome enclosed in a protein shell called a capsid. In addition to protecting the virus' genetic material, the capsid proteins enable poliovirus to infect certain types of cells. Three serotypes of poliovirus have been identified – wild poliovirus type 1, type 2, and type 3 – each with a slightly different capsid protein. All three are extremely virulent and produce the same disease symptoms. WPV1 is the most commonly encountered form, and the one most closely associated with paralysis. WPV2 was certified as eradicated in 2015 and WPV3 certified as eradicated in 2019.The incubation period ranges from three to six days for nonparalytic polio. If the disease progresses to cause paralysis, this occurs within 7 to 21 days.
Individuals who are exposed to the virus, either through infection or by immunization via polio vaccine, develop immunity. In immune individuals, IgA antibodies against poliovirus are present in the tonsils and gastrointestinal tract and 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.
A rare condition with a similar presentation, nonpoliovirus poliomyelitis, may result from infections with enteroviruses other than poliovirus.
The oral polio vaccine, which has been in use since 1961, contains weakened viruses that can replicate. On rare occasions, these may be transmitted from the vaccinated person to other people; in communities with good vaccine coverage, transmission is limited, and the virus dies out. In communities with low vaccine coverage, this weakened virus may continue to circulate and, over time may mutate and revert to a virulent form. Polio arising from this cause is referred to as circulating vaccine-derived poliovirus or variant poliovirus in order to distinguish it from the natural or "wild" poliovirus.
Transmission
Poliomyelitis is highly contagious. 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. It is seasonal in temperate climates, with peak transmission occurring in summer and autumn. These seasonal differences are far less pronounced in tropical areas. Polio is most infectious between 7 and 10 days before and after the appearance of symptoms, but transmission is possible as long as the virus remains in the saliva or feces. Virus particles can be excreted in the feces for up to six weeks.Factors that increase the risk of polio infection include pregnancy, being very old or very young, immune deficiency, and malnutrition. Although the virus can cross the maternal-fetal barrier 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.
Pathophysiology
Poliovirus enters the body through the mouth, infecting the first cells with which it comes in contact – the pharynx 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, 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.Known as viremia, the presence of a virus in the bloodstream enables it to be widely distributed throughout the body. Poliovirus can survive and multiply within the blood and lymphatics for long periods of time, sometimes as long as 17 weeks. In a small percentage of cases, it can spread and replicate in other sites, such as brown fat, the reticuloendothelial tissues, and muscle. This sustained replication causes a major viremia, and leads to the development of minor influenza-like symptoms. Rarely, this may progress and the virus may invade the central nervous system, provoking a local inflammatory response. In most cases, this causes a self-limiting inflammation of the meninges, the layers of tissue surrounding the brain, which is known as nonparalytic aseptic meningitis. Penetration of the CNS provides no known benefit to the virus, and is quite possibly an incidental deviation of a normal gastrointestinal infection. The mechanisms by which poliovirus spreads to the CNS are poorly understood, but it appears to be primarily a chance event – largely independent of the age, gender, or socioeconomic position of the individual.