Overview
Poliomyelitis (polio) is caused by the RNA virus, Poliovirus. Polioviruses are related to enteroviruses that cause common stomach and flu like illnesses and upper respiratory illnesses as well as rhinoviruses that cause the common cold. It is capable of infecting other primates but is regularly a human specific virus. There are 3 different serotypes that are antigenically variant meaning immunity developed to one serotype does not confer to the others. Infection with poliovirus grants lifetime immunity to that specific serotype although you can get reinfected with a different serotype later on. Considered a disease of childhood, polio mainly affects children under 5 years of age.
Transmission
How does Polio spread?
Poliovirus 1 is most common and as of today the only remaining serotype, Poliovirus 2 and 3 having been eradicated. It is transmitted fecal-orally, like cholera, however unlike cholera, which thrives with poor sanitation and hygiene, because polio infection results in lifetime immunity, in poor conditions the majority of individuals were exposed as children and never would get seriously sick. This means that it thrives in areas with partial sanitation measures, because people are not always regularly exposed as children, a greater amount of the population remains vulnerable and epidemics are more likely to occur.
The typical incubation period of 7-10 days but can be anywhere from 4-35 days.
People will start shedding virus in their stool 1-2 days prior to symptoms and continue to for at least 1-2 weeks but can be as long as 4-6.
Symptoms
What does Polio look like?
About 75% of infections with poliovirus are asymptomatic. About 25% of people infected will have flu-like symptoms including sore throat, fatigue, fever, tiredness, nausea, headache, and stomach pain that usually resolve after around 2-5 days.
Poliomyelitis is the disease that happens in less than 1% of cases, around 1 in 200 cases. Poliomyelitis is when poliovirus exists in your gut and travels through your central nervous system where it attacks your motor neurons. The word myelitis means the infection of the central nervous system. Paralysis occurs in around 1 in 200 cases, and around 2-10% of those with paralysis will die due to the paralysis of the diaphragm or intercostal muscles making it impossible to breathe. Most cases infect parts of the spinal cord causing loss of function in possibly a limb or an entire region inferior to where the neurons were damaged, but the fatal cases, besides from diaphragm paralysis, are due to the infection of the brainstem and the destruction of the cranial nerves.
Post-polio syndrome is a condition causing progressive muscle weakness, fatigue, and pain from joint degeneration, which occurs in about 25-40% of poliomyelitis cases anywhere from 15 to 40 years post infection. Even children who seem to fully recover can develop new muscle pain, weakness, or paralysis as adults. The better your recovery the greater the likelihood of developing Post-Polio syndrome.
Treatment
How is Polio treated?
There is no cure for polio but there are two available vaccines: the oral polio vaccine, and the inactivated polio vaccine. Treatments for polio focus on limiting and alleviating symptoms. Heat and physical therapy can be used to stimulate the muscles and antispasmodic drugs are used to relax the affected muscles. This can improve mobility but does not reverse permanent paralysis. Physical or occupational therapy can help with arm or leg weakness caused by polio and might improve long-term outcomes, especially if implemented early in the course of illness.