Polio (also known as poliomyelitis) is a highly contagious disease caused by a virus that attacks the nervous system. Children younger than 5 years old are more likely to contract the virus than any other group.
According to the World Health Organization (WHO), 1 in 200 polio infections will result in permanent paralysis.
Poliovirus is a member of the enterovirus subgroup, family Picornaviridae. Picornaviruses are small, ether-insensitive viruses with an RNA genome.
There are three poliovirus serotypes (type1, type 2, and type 3); immunity to one serotype does not produce significant immunity to the other serotypes.
Poliovirus is rapidly inactivated by heat, formaldehyde, chlorine, and ultraviolet light.
Most people who get infected with poliovirus (about 72 out of 100) will not have any visible symptoms.
About 1 out of 4 people (or 25 out of 100) with poliovirus infection will have flu-like symptoms that may include:
These symptoms usually last 2 to 5 days, then go away on their own.
A smaller proportion of people (much less than one out of 100, or 1-5 out of 1000) with poliovirus infection will develop other, more serious symptoms that affect the brain and spinal cord:
Paralysis is the most severe symptom associated with polio, because it can lead to permanent disability and death. Between 2 and 10 out of 100 people who have paralysis from poliovirus infection die, because the virus affects the muscles that help them breathe.
Even children who seem to fully recover can develop new muscle pain, weakness, or paralysis as adults, 15 to 40 years later. This is called post-polio syndrome.
Note that “poliomyelitis” (or “polio” for short) is defined as the paralytic disease. So only people with the paralytic infection are considered to have the disease.
Poliovirus only infects people. It enters the body through the mouth and spreads through:
You can get infected with poliovirus if:
An infected person may spread the virus to others immediately before and up to 2 weeks after symptoms appear.
There are three wild types of poliovirus (WPV) – type 1, type 2, and type 3. People need to be protected against all three types of the virus in order to prevent polio disease and the polio vaccination is the best protection. Type 2 wild poliovirus was declared eradicated in September 2015, with the last virus detected in India in 1999. Type 3 wild poliovirus was declared eradicated in October 2019. It was last detected in November 2012. Only type 1 wild poliovirus remains. There are two vaccines used to protect against polio disease, oral polio vaccine and inactivated poliovirus vaccine.
Some people who develop symptoms from the poliovirus contract a type of polio that doesn't lead to paralysis (abortive polio). This usually causes the same mild, flu-like signs and symptoms typical of other viral illnesses.
Signs and symptoms, which can last up to 10 days, include:
· Sore throat
· Back pain or stiffness
· Neck pain or stiffness
· Pain or stiffness in the arms or legs
· Muscle weakness or tenderness
This most serious form of the disease is rare. Initial signs and symptoms of paralytic polio, such as fever and headache, often mimic those of nonparalytic polio. Within a week, however, other signs and symptoms appear, including:
· Loss of reflexes
· Severe muscle aches or weakness
· Loose and floppy limbs (flaccid paralysis)
Polio has been around since ancient times. This ancient Egyptian tomb painting shows a man with a withered leg unable to bear weight without use of a walking stick. This means that most muscle fibers are replaced with scarring (muscle-wasting) that is permanent.
If someone had polio as a child or young adult but had kept or recovered some or all movement of weakened arms or legs, even to the point of being athletic afterward, they can risk becoming weaker in late adulthood. That is post-polio syndrome (PPS), a condition that can affect polio survivors decades after they recover from their initial poliovirus infection. Some PPS patients become wheelchair-bound when they had not been before.
Post-polio syndrome is a cluster of disabling signs and symptoms that affect some people years after having polio. Common signs and symptoms include:
· Progressive muscle or joint weakness and pain
· Muscle wasting (atrophy)
· Breathing or swallowing problems
· Sleep-related breathing disorders, such as sleep apnea
· Decreased tolerance of cold temperatures
Poliovirus can be transmitted through direct contact with someone infected with the virus or, less commonly, through contaminated food and water. People carrying the poliovirus can spread the virus for weeks in their feces. People who have the virus but don't have symptoms can pass the virus to others.