What Is Cause Of Polio?

Polio is caused by the poliovirus, a highly contagious virus that attacks the nervous system. The virus spreads from person to person, usually through contact with the stool of an infected person or, less often, through contaminated water or food. Once inside the body, it multiplies in the throat and intestines before potentially traveling to the brain and spinal cord, where it can cause paralysis.

How Does the Poliovirus Spread?

The poliovirus spreads through the fecal-oral route. This means the virus passes from the stool of one person into the mouth of another person. This often happens when someone does not wash their hands properly after using the bathroom and then touches food or surfaces that others touch.

In areas with poor sanitation, contaminated water is a major source of spread. The virus can also spread through droplets when an infected person sneezes or coughs, though this is much less common than the fecal-oral route. The virus is extremely contagious, and an infected person can shed the virus in their stool for several weeks, even if they never develop symptoms.

What Happens Inside the Body After Infection?

The virus enters the body through the mouth. It first multiplies in the lining of the throat and the intestines. The immune system usually fights off the virus at this stage, which is why most people who get infected never know they had it.

In a small number of cases, the virus spreads from the intestines into the bloodstream. From there, it can travel to the central nervous system. The virus specifically attacks motor neurons, the nerve cells that control muscle movement. When these cells are destroyed, the brain can no longer send signals to the muscles, leading to weakness and paralysis.

Why Do Only Some People Develop Paralysis?

Most people infected with the poliovirus experience no symptoms at all. About 1 in 4 people will have flu-like symptoms such as sore throat, fever, fatigue, and headache. These symptoms usually appear 3 to 6 days after exposure and go away on their own.

Paralysis is rare. It occurs in fewer than 1 in 200 infections. The risk of paralysis increases with age, but the reasons for this are not fully understood. Some research suggests that older children and adults have a stronger inflammatory response to the virus, which causes more damage to the nervous system.

Paralysis can affect one leg, one arm, or both legs. In severe cases, the virus attacks the muscles used for breathing. When this happens, a person may need a ventilator, sometimes permanently.

What Is the Difference Between Wild Polio and Vaccine-Derived Polio?

There are two distinct types of poliovirus that cause disease today: wild poliovirus and vaccine-derived poliovirus. Understanding the difference matters because the prevention strategies are different for each.

Wild poliovirus is the naturally occurring virus. It has been eliminated from most of the world. As of recent years, it remains endemic in only a few countries, primarily Afghanistan and Pakistan. Type 2 and Type 3 wild polioviruses have been globally eradicated.

Vaccine-derived poliovirus comes from the oral polio vaccine. The oral vaccine contains a weakened live virus. In very rare cases, this weakened virus can mutate in the gut of a vaccinated person and regain the ability to cause paralysis. This mutated virus can then spread to unvaccinated people. This is not a risk with the inactivated polio vaccine, which is the only vaccine used in the United States.

How Is Polio Prevented?

Vaccination is the only effective way to prevent polio. There is no cure for the disease, and no antiviral medication can stop the virus once it reaches the nervous system. Treatment focuses on managing symptoms and preventing complications.

Two types of vaccines exist. The inactivated polio vaccine is given as an injection and contains a killed virus. It cannot cause polio. This is the standard vaccine used in the United States and many other countries. The oral polio vaccine is given as drops and contains a weakened live virus. It is easier to administer and is used in many parts of the world for mass vaccination campaigns.

The inactivated polio vaccine is highly effective. After the recommended series of doses, more than 99% of people develop protective antibodies. In the United States, children receive four doses of the inactivated polio vaccine at ages 2 months, 4 months, 6 to 18 months, and 4 to 6 years. Adults who were vaccinated as children and are at increased risk of exposure, such as travelers to high-risk areas, can receive a single lifetime booster dose.

Can Polio Come Back in the United States?

Polio was declared eliminated from the United States in 1979. However, the virus has not disappeared globally. As long as polio exists anywhere in the world, it can be imported into any country.

In 2022, a case of paralytic polio caused by vaccine-derived poliovirus was confirmed in an unvaccinated adult in New York. Wastewater surveillance also detected the virus in several nearby counties. This case was a reminder that polio is only one plane ride away and that vaccination coverage must remain high.

Herd immunity is critical. When a high percentage of the population is vaccinated, the virus cannot easily find susceptible people to infect. If vaccination rates drop, the risk of an outbreak rises. Public health officials monitor vaccination coverage closely because even a small drop can create conditions for the virus to circulate.

What Are the Long-Term Effects of Polio?

People who survive paralytic polio often face lifelong challenges. The paralysis is usually permanent, but the extent depends on which nerve cells were damaged. Some people recover significant muscle function over time, while others do not.

Decades after the initial infection, many survivors develop post-polio syndrome. This condition causes new muscle weakness, fatigue, and pain in muscles that were previously affected or even in muscles that seemed unaffected. The exact cause is not fully understood, but it is believed to be related to the long-term wear on motor neurons that survived the initial infection.

Post-polio syndrome is not a new infection. It is not contagious. It is a late effect of the original viral damage, and it can appear 15 to 40 years after the initial illness.