Poliomyelitis, or polio is an infectious disease caused by the poliovirus, which affects your spinal cord and brainstem. Although in the vast majority of cases a poliovirus infection is harmless, if it makes its way into your brain or spinal cord it can cause paralysis, and even death. In the mid 20th-century, poliovirus infections reached epidemic proportions. Today the threat of polio has been largely eliminated in the developed world due to the introduction of vaccines. However, its continued existence is risky for people living in the most afflicted countries, with children under 5 years of age being particularly vulnerable to infection. That said, public health officials believe that complete eradication of polio may be possible, and in 1999, one of the three strains of the poliovirus was eliminated entirely.

Your nervous system

Your nervous system is made up of the brain, spinal cord, and nerves, with your brain and spinal cord being known as the central nervous system, or CNS. Your CNS controls your perception of the world. It receives information from all parts of your body, processes it, and sends instructions to other parts of your body so that you may respond to the highly varied stimuli that you are exposed to on a daily basis. The network of nerves that carries the information around your body, and to and from your CNS is called the peripheral nervous system.
Your nervous system contains millions of nerve cells, or neurons of which there are three types, sensory neurons, interneurons, and motor neurons. The sensory neurons are the ones that respond to stimuli, such as heat or light, or chemicals, both inside and outside of your body. These neurons transmit this information to the CNS, where interneurons decipher it and then pass instructions along motor neurons to the parts of your body where an action is required such as muscles or glands. Your CNS can instruct your body to make two main types of actions: voluntary actions that your brain controls consciously and involuntary actions that your brain does not consciously control. A reflex action is an involuntary action that provides a good example of how the nervous system works. When information is received by a sensory neuron, for example when you are poked with a pin, it travels to the CNS where it is processed. Your CNS then sends instructions via your motor neurons that tell your hand muscles to move away from the source of pain.
Image of the CNS
Your CNS is divided into two types of tissue, called grey matter and white matter. If we were to look at a cross section of the spinal cord, we would see that the grey matter appears in the shape of a butterfly. The grey matter in the ventral (or anterior) horn largely contains motor neurons that are responsible for the movement of the muscles you use to swallow, breath, and keep your blood circulating, as well as those in your trunk, arms and legs. The motor neurons in this area are particularly susceptible to poliovirus infection.
image of the brain showing white matter, gray matter, the dorsal horn and the ventral horn

How do you get polio?

You are most likely to get a poliovirus infection through contact with infected feces. However, you can also get it through direct contact with someone infected with the virus, who can be contagious without exhibiting any symptoms, and who can spread the virus via their feces, for weeks. In fact, polio is so contagious that anyone living with a recently infected person is likely to become infected too. Because poliovirus can survive for weeks outside the human body, it can also be transmitted through contaminated water and food. For these reasons, is most easily spread in communities with poor infrastructure, poor sanitation and crowded living conditions, and young children are at particularly high risk of infection. The risk of infection is also higher when your immune system is not fully functional, such as when you are very young, old or pregnant. Not surprisingly, other conditions that weaken your immune system such as HIV or having your tonsils removed, may also put you at increased risk of a polio infection.

How poliovirus attacks your body

The poliovirus typically enters your body through your nose or mouth and almost immediately infects the cells lining these cavities (your pharynx), as well as your intestines, where it begins to reproduce. After about a week, it can spread to your tonsils and other parts of your immune system, where it multiplies rapidly. Eventually poliovirus may break out into your bloodstream and if this happens, it is transported widely around your body. In most cases, your body eradicates poliovirus while it is still in your intestines, or your blood stream. However, in some people, the poliovirus is able to get into the CNS. Once there, it infects and replicates inside motor neurons, copying itself thousands of times over. When it is ready to emerge, it destroys the neurons and spreads to neighbouring uninfected cells. At this point, the disease is medically classified according to the neuronal damage that occurs, and the region of the CNS affected:
  • Spinal polio: this is the most common form of polio. It occurs when the poliovirus infects and kills motor neurons in the grey matter in the ventral horn of the spinal column. As the cells die, the muscles of the trunk and limbs are no longer able to receive signals from the CNS, so they become weak and begin to atrophy. In just a short period of time (a few days) they may become completely paralyzed.
  • Bulbar polio: this form of polio occurs occurs in a small number of cases, and is caused by the poliovirus infecting and killing neurons within the bulbar region of the brain stem. This weakens the muscles we use to speak, swallow and breathe.
Cross section of the brain, illustrated
  • Bulbospinal polio: About one in five people with paralytic polio have both bulbar and spinal infection. In these cases, the poliovirus also infects the upper part of the cervical spinal cord, causing paralysis of the diaphragm.

What are the symptoms of polio?

The vast majority of people who become infected with polio will not have any symptoms and won’t even know they have been infected. However, if the virus gets past your intestine into your bloodstream, you may develop symptoms of sub-clinical, or nonparalytic polio. The symptoms are usually mild flu-like symptoms such as fever, fatigue, headache, and vomiting, stiffness in the neck, and pain in the limbs.
If the poliovirus reaches your CNS, the symptoms become much more sinister, and you develop what is known as paralytic polio. Muscle weakness and paralysis usually progress rapidly, often accompanied by fever, muscle pain, loss of reflexes and floppy limbs. It is common for paralysis to affect only one side of your body, and the paralysis of legs and arms is often more severe closer to the spinal cord than in the fingertips and toes. Within a week or two of infection, the virus can do sufficient damage to your motor neurons, spinal cord, and brainstem to leave you permanently paralyzed. However, in most cases, muscle function will return to some degree, and many people who go on to develop paralytic polio will recover completely. That said, muscle weakness and paralysis lasting a year or more may be permanent.1^1
If the bulbar brain becomes infected, you are likely to experience other symptoms associated with the damage caused to the nerves in this region. These may include difficulties with swallowing and tongue movement that can lead to buildup of mucus in the airways, causing suffocation. You may also experience facial muscle weakness, double vision, and abnormal respiration, which may also be fatal.

How likely are you to get polio?

Polio is highly infectious and mainly affects children under 5 years of age.2^2 Although it has been eradicated in most parts of the world, it is still currently endemic in Nigeria, Afghanistan, and Pakistan. Other countries in the nearby region, and countries importing goods from these countries are at higher risk for cases of polio. Although the number of cases of polio has decreased hugely over the last 25 years, there are still around 400 reported cases of polio every year, and 1 in 200 cases ends in permanent paralysis.2^2
map showing where polio has not been eradicated.

How can you prevent yourself against polio?

The most effective way to prevent polio is with the polio vaccine, which protects you against all three strains of poliovirus. Both orally administered, live attenuated polio vaccines and inactivated polio vaccines given by injection are widely used internationally. It is 90% effective after two doses, 99% effective after three, and if administered with booster shots can protect you for life. The World Health Organization recommends the oral vaccine (three doses) plus at least one dose of inactivated polio vaccines for infants in most countries.3^3 The Americas, Western Pacific, and Europe are currently certified by the World Health Organization as polio free. If you were vaccinated against polio during your childhood, and are traveling from one of these areas to a polio-infected region, it is very advisable that you get a single adult booster dose 4-6 weeks before you leave, if you have not already had one.

How is polio diagnosed and treated?

Your doctor may make a preliminary diagnosis of poliovirus infection based on your medical history and symptoms. For example, if you have not been vaccinated and present with difficulty swallowing or breathing, abnormal reflexes, and a stiff neck, your doctor may suspect you have polio. To confirm this, he will likely ask you for a mucus sample (throat swab), a stool sample, or he may want to collect a sample of cerebrospinal fluid - the fluid within your CNS. The sample is then sent to a laboratory for culture, to see if the poliovirus is present.
There is currently no cure or treatment for poliovirus infection. Instead, your doctor will focus on strategies that help speed your recovery and prevent complications. These may include bed rest, pain medications, a portable ventilator to assist your breathing, moderate exercise to prevent your muscles from atrophying, and a healthy diet to support your immune system and other body functions.

Consider the following:

  • Post-polio syndrome*: If you have had paralytic or nonparalytic polio, you may experience polio-related symptoms years later (the average time is about 35 years, but onset may occur anytime from around 10 years to more than 60 years later).4^4 This is known as post-polio syndrome. It can happen suddenly or gradually, and is related to the damage the poliovirus did to your neurons when you first became infected. Common signs and symptoms include progressive muscle weakness and difficulty with coordination, general fatigue, muscle atrophy, sleep and breathing disorders, depression, and cognitive problems such as memory loss. It is thought that the symptoms may be related to the remaining healthy neurons being unable to maintain the production of new nerve roots, so that eventually denervation is greater than reinnervation.5^5
Image of normal nerves, nerves affected by acute polio, nerves in recovery, and nerves during post-polio syndrome

Attribution:

This article is licensed under a CC-BY-NC-SA 4.0 license. https://creativecommons.org/licenses/by-nc-sa/4.0/

Additional references:

  1. Centers for Disease Control and Prevention (CDC). (2011). Epidemiology and prevention of vaccine-preventable diseases. Washington, DC: Public Health Foundation.
  2. World Health Organization. Poliomyelitis. Fact sheet No. 114. Updated October 2014. Retrieved from: http://www.who.int/mediacentre/factsheets/fs114/en/
  3. World Health Organization. Poliomyelitis (Polio). International travel and health. Retrieved from: http://www.who.int/ith/vaccines/polio/en/
  4. Jubelt, B., & Agre, J. C. (2000). Characteristics and management of postpolio syndrome. Jama, 284(4), 412-414.
  5. Trojan, D. A., & Cashman, N. R. (1996). Current trends in post-poliomyelitis syndrome. Milestone Medical Communications.
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