Summary
Highlights
Poliomyelitis, or polio, is caused by the poliovirus, which can infect the spinal cord and lead to paralysis. The first major US outbreak in 1894 resulted in 18 deaths and 132 cases of permanent paralysis. Early on, the highly infectious nature of polio was not understood. In 1905, Ivar Wickman discovered its infectiousness and that some infected individuals had mild forms. By 1908, Karl Landsteiner and Erwin Popper identified the cause as a virus, though it wasn't visible until the 1950s with electron microscopes.
A significant polio epidemic in 1916 in New York City killed over 2,000 people and paralyzed thousands more. Poliovirus could paralyze not just limbs, but also the diaphragm and breathing muscles. This led to the 1929 development of artificial respirators, later known as iron lungs, which allowed patients whose respiratory muscles were paralyzed to breathe by mechanically forcing air in and out of their lungs. An iron lung is a sealed chamber where a patient's entire body, except their head, is enclosed to assist breathing.
Poliomyelitis is caused by three serotypes of polioviruses (PV1, PV2, and PV3), which are non-enveloped, single-stranded RNA viruses. Immunity to one serotype does not typically provide immunity to others. Polioviruses only infect humans and are very stable in the environment, often found in places like swimming pools. They typically enter the body orally via the fecal-oral route, infecting the throat and intestinal tract before invading the bloodstream.
Over 70% of infected people show no symptoms, while about a quarter experience mild, flu-like symptoms. The immune system usually overcomes the infection for most, leading to full recovery. This makes large outbreaks puzzling, as only about 1% of cases involve weakness or paralysis. In these rare cases, the virus attacks motor neurons in the spinal cord and brain, destroying infected cells. The term "poliomyelitis" specifically refers to cases involving paralysis. If even one case of poliomyelitis is seen, it suggests the disease is widespread in the population.
Those who develop serious symptoms may experience paresthesia (pins and needles), meningitis (inflammation of brain and spinal cord membranes), or paralysis. In some cases, muscle pain, weakness, or paralysis can recur 15 to 40 years after the initial infection, a condition known as post-polio syndrome.
Polioviruses are endemic in areas with crowding and poor sanitation. In these regions, newborns often have maternal antibody protection and are exposed to the virus early, developing lifelong immunity after a mild illness. In countries with good sanitation, polio immunity is less common, making outbreaks potentially devastating due to a higher likelihood of paralysis cases, as seen in the 1950s US. Treatment for poliovirus infections focuses on supportive care: rest, fluids, pain medication, and in severe cases, the use of an iron lung.
Two types of vaccines prevent polio: the inactivated poliovirus vaccine (IPV), given as an injection, and the oral poliovirus vaccine (OPV). Thanks to successful vaccination programs, the United States has been polio-free since 1979, with no year-round transmission, though there was one imported case in 1993. Globally, poliovirus 2 was eradicated in 1999, and poliovirus 3 has not been seen since 2012. As of 2020, poliovirus 1 affects only two countries, Pakistan and Afghanistan, with hopes for its complete eradication soon.