Poliomyelitis (Polio)

How polio spreads

Poliovirus lives in the throat and intestines. It spreads mainly through:

• Fecal-oral route: contact with stool of an infected person, or contaminated water or food
• Respiratory droplets: less commonly, from coughs and sneezes

Infected people can spread the virus for days to weeks, even without symptoms, which is why polio can circulate silently. Children under 5 are most often affected in areas where it still spreads.

Symptoms

Paralytic polio

The virus attacks motor neurons in the spinal cord or brainstem. Paralysis typically:

• follows a minor illness, sometimes after a few symptom-free days
• comes on rapidly, over hours to days
• is asymmetric, often affecting one leg more than the other
• is flaccid (floppy), with reduced reflexes and severe muscle pain or spasms
• leaves sensation intact

Bulbar polio affects brainstem nerves, causing difficulty swallowing, speaking and breathing. Between 2 and 10% of people with paralytic polio die, usually from breathing muscle paralysis.

Some strength may return over months, but much paralysis is permanent.

Polio today

Wild poliovirus has been eliminated from most countries. As of recent years, wild poliovirus type 1 remains endemic only in Pakistan and Afghanistan. Outbreaks of vaccine-derived poliovirus also occur in under-vaccinated communities, because the weakened virus in oral polio vaccine (OPV) can, rarely, regain the ability to cause disease when it circulates for a long time. The US uses only inactivated vaccine, which cannot cause polio. In 2022 a case of paralytic vaccine-derived polio occurred in an unvaccinated adult in New York, highlighting the importance of vaccination.

Diagnosis

Polio is suspected in anyone with sudden flaccid weakness, especially if unvaccinated or recently exposed. Tests include:

• Stool samples (two, 24 hours apart) and throat swabs for poliovirus
• Cerebrospinal fluid analysis via lumbar puncture
• MRI of the spine
• Testing to distinguish it from Guillain-Barré syndrome, transverse myelitis, acute flaccid myelitis from other viruses, and other causes of paralysis

Every case of acute flaccid paralysis in a child is reported and investigated by public health authorities.

Treatment

There is no antiviral treatment for polio. Care focuses on:

• Rest during the acute phase
• Pain relief and warm packs for muscle spasms
• Breathing support, including mechanical ventilation if breathing muscles are affected
• Physical therapy to maintain movement and prevent deformities
• Braces, mobility aids and orthopedic surgery for long-term weakness

Post-polio syndrome

Post-polio syndrome affects up to 25 to 40% of polio survivors, typically 15 to 40 years after the original illness. Symptoms include:

• new or increasing muscle weakness, sometimes in previously unaffected muscles
• profound fatigue
• muscle and joint pain
• muscle wasting
• breathing and swallowing problems
• sleep problems and cold intolerance

It is thought to result from gradual failure of the enlarged motor neurons that compensated after the original infection. Management includes pacing activities, avoiding overuse, tailored exercise, mobility aids and breathing support if needed. It is not contagious. See mobility impairment for practical support.

Prevention: polio vaccine

In the US, children receive 4 doses of inactivated polio vaccine (IPV) at:

• 2 months
• 4 months
• 6 to 18 months
• 4 to 6 years

Adults who were never vaccinated or have an incomplete series should complete it. Fully vaccinated adults traveling to areas where polio is circulating, and some laboratory and healthcare workers, can receive a one-time lifetime IPV booster.

Many countries use oral polio vaccine (OPV), which is cheap, easy to give and blocks transmission in the gut, and is essential to global eradication efforts.

Frequently asked questions

What are the symptoms of polio?

Most people have no symptoms. Some have a mild illness with fever, sore throat, headache and tiredness. A small number develop meningitis with a stiff neck, and about 1 in 200 develop sudden, floppy paralysis, usually in the legs.

How is polio spread?

Polio spreads mainly through contact with the stool of an infected person, often via contaminated hands, food or water. It can also spread through droplets from coughs and sneezes. People without symptoms can spread it.

Is polio still around?

Yes, but rarely. Wild polio remains endemic in only a few countries, and outbreaks of vaccine-derived polio occur in communities with low vaccination rates. Cases have occasionally appeared in high-income countries among unvaccinated people.

Do adults need a polio booster?

Adults who completed the childhood series usually do not need more doses. A single lifetime booster is recommended for fully vaccinated adults at increased risk, such as travelers to areas with polio or certain lab and healthcare workers. Unvaccinated adults should complete the series.

What is post-polio syndrome?

Post-polio syndrome is new weakness, fatigue, muscle and joint pain, and sometimes breathing or swallowing difficulty that appears decades after polio. It is not a reinfection. Pacing, tailored exercise, mobility aids and breathing support help manage it.