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Poliomyelitis, often called polio or infantile paralysis, is an acute viral infectious disease spread from person to person, primarily via the fecal-oral route.[1] The term derives from the Greek polio (p?????), meaning “grey”, myelon (µ?e???), referring to the “spinal cord”, and -itis, which denotes inflammation.[2] 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.[3] 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.[4]
Poliomyelitis was first recognized as a distinct condition by Jakob Heine in 1840.[5] Its causative agent, poliovirus, was identified in 1908 by Karl Landsteiner.[5] 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.[6] 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. The polio vaccines developed by Jonas Salk in 1952 and Albert Sabin in 1962 are credited with reducing the global number of polio cases per year from many hundreds of thousands to around a thousand.[7] Enhanced vaccination efforts led by the World Health Organization, UNICEF and Rotary International could result in global eradication of the disease.[8]
Poliomyelitis is caused by infection with a member of the genus Enterovirus known as poliovirus (PV). This group of RNA viruses prefers to inhabit the gastrointestinal tract.[1] PV infects and causes disease in humans alone.[3] Its structure is very simple, composed of a single (+) sense RNA genome enclosed in a protein shell called a capsid.[3] 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.[9] All three are extremely virulent and produce the same disease symptoms.[3] PV1 is the most commonly encountered form, and the one most closely associated with paralysis.[10]
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.[11] Infection or vaccination with one serotype of poliovirus does not provide immunity against the other serotypes, and full immunity requires exposure to each serotype.[11]
Poliomyelitis is highly contagious and spreads easily by human-to-human contact.[11] In endemic areas, wild polioviruses can infect virtually the entire human population.[12] It is seasonal in temperate climates, with peak transmission occurring in summer and autumn.[11] These seasonal differences are far less pronounced in tropical areas.[12] 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.[13] Virus particles are excreted in the feces for several weeks following initial infection.[13] 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,[10] a mode especially visible in areas with good sanitation and hygiene.[11] 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.[10]
Factors that increase the risk of polio infection or affect the severity of the disease include immune deficiency,[14] malnutrition,[15] tonsillectomy,[16] physical activity immediately following the onset of paralysis,[17] skeletal muscle injury due to injection of vaccines or therapeutic agents,[18] and pregnancy.[19] Although the virus can cross the placenta during pregnancy, the fetus does not appear to be affected by either maternal infection or polio vaccination.[20] Maternal antibodies also cross the placenta, providing passive immunity that protects the infant from polio infection during the first few months of life.[21]
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.[22] 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 illnesses.[4]
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