​​Poliovirus is a contagious virus that has affected humans for centuries. Before vaccines were developed, polio outbreaks led to widespread distress and left thousands of children and adults with life-long issues like paralysis.¹ Great efforts have been made in controlling the disease, however polio is still a concern in some parts of the world. To fully grasp how polio can be prevented and eradicated, it’s important to understand how the virus operates and how it causes the disease known as polio.

Introduction

Poliovirus is a virus that causes polio and is a member of a family of viruses called Picornaviridae, which belongs to a species called Enterovirus C. There are three types of wild poliovirus: wild poliovirus 1 was the cause of polio cases globally, wild poliovirus 2 and 3 have been eradicated since 2015.² Polioviruses are small in size, ranging from 25 to 30 nanometres. They have an outer coat, also called capsid, consisting of 4 proteins called VP1, VP2, VP3 and VP4. The way these proteins are arranged causes parts of them to stick out of the virus (called antigens). This gives the virus its characteristic look and provides sites for our immune system and specific drugs to recognise and target it.³

How poliovirus enters the body

Poliovirus enters the body through person-to-person contact and faeces-contaminated water, food or aerosols. Once the virus enters the body, it travels to the throat and small intestine where it begins to multiply.⁴ Since viruses can’t replicate on their own, they hijack our cells and use their machinery to multiply.⁵ After 3 to 5 days in the body, the virus can be shed in stool and can be recovered through mouth swabs.³

After replicating in the throat and small intestines, poliovirus then spreads through the bloodstream, allowing it to travel to different parts of the body. This stage is called viremia, and in most cases, a person does not experience symptoms, or if so, the symptoms are very mild. The viremia may lessen as our body’s immune system fights the poliovirus, therefore any symptoms experienced will stop. If viremia does not subside at this point, poliovirus can spread to the central nervous system via the bloodstream, including the brain. At this point, the virus can cause great damage and individuals begin to experience severe symptoms, such as paralysis of the limbs, in some cases, this can be fatal.⁴ Fortunately, up to 95% of cases are non-paralytic and only present as flu-like symptoms.

How poliovirus causes polio

Polio, also known as poliomyelitis or infantile paralysis, is a viral infection caused by poliovirus which affects the motor neurons of the central nervous system (CNS). In the past, polio has been a major of lifelong disabilities, paralysis and death around the world. With the introduction of a vaccine, it has been eradicated in Europe, with the last case found in 1998. However, in 2014 international spread of poliovirus occurred.⁶

Most people with polio do not have symptoms, some people get mild, flu-like symptoms such as:

• Fatigue
• Sore throat
• Headaches
• High temperature
• Vomiting
• Muscle pain
• Stiff neck

On rare occasions, people with polio experience more serious symptoms, affecting motor neurons and CNS, causing weakness in muscles or paralysis which can last over hours or days. Most people will fully recover from polio, some will be left with permanent disabilities.⁷

Types of polio

The disease is split into two types: spinal and bulbar, depending on which motor neurons are affected.⁶

Spinal polio

Spinal polio begins with symptoms similar to meningitis, followed by severe muscle pain and other sensory issues like tingling, numbness, muscle spasms and twitching. After one or two days, weakness and paralysis begin to set in. The paralysis usually causes limbs to become limp and tends to affect the body asymmetrically (affecting one side of the body more than the other). This type of paralysis is called acute flaccid paralysis, with paralysis in the lower limbs most commonly affected.⁶

Bulbar polio

Bulbar polio is a more severe form of polio as it affects the muscles responsible for important functions like swallowing and breathing. This can cause difficulty swallowing, a build-up of secretions in the throat and trouble breathing. In rare cases, it can also cause encephalitis, which is inflammation of the brain.⁶

Polio treatment

There is currently no cure for polio. However, treatment focuses on managing symptoms and supporting normal bodily functions, which reduces the risk of developing long-term issues. Such treatment options are bed rest, painkillers, support for breathing and regular exercises to prevent further issues with the muscles and joints. If individuals develop long-term issues, it is likely that they will require ongoing therapy and support. This includes occupational therapy, which will provide day-to-day support to those with physical problems from the disease. Physiotherapy may be required for similar reasons, which include the use of splints or braces to support weak limbs. Surgery may also be probable to correct any deformities. These intend to increase the independence and quality of life of the patients.⁷

Polio vaccine

Although there is no cure for polio once infected with the virus, there are means of preventing the disease through vaccination. Vaccines contain a safe and weakened or inactive form of the virus and are usually injected into a person or child. Upon administration of the vaccine, our body’s immune system starts to produce molecules called antibodies that help fight against the disease. The body will then have these antibodies for life that ‘remember’ the specific virus if you were to be infected with poliovirus in the future. This will allow the body to fight poliovirus at a faster rate, achieving immunity in the process and preventing you from getting unwell. For the best outcomes of eradicating polio from the community, everyone must get vaccinated as it prevents the spread of the virus.⁸

Before the polio vaccine was discovered in the 1950s, polio was one of the worst childhood diseases. In the 1980s, a substantial amount of children were affected until 1988, when the World Health Organization (WHO) launched the Global Polio Eradication Initiative. This aimed to vaccinate all children in the world and detect the circulation of the virus.

There are two types of polio vaccines: injectable polio vaccine (IPV) and oral polio vaccine (OPV).⁹

• IPV was developed by Jonas Salk in the 1950s, contains inactivated versions of all three types of poliovirus and provides strong protection by building good overall immunity. Since this vaccine requires multiple injections and regular boosters, it needs to be administered in sterile conditions. While it is completely safe, the cost of production and the logistics involved in using it limited its widespread distribution, especially in developing countries. However, its use has been steadily increasing over time⁹
• OPV was developed by Albert Sabin and was also released in 1950. It contains live but weakened forms of the specific type of poliovirus. The strains in the vaccine, one for each type of poliovirus, can infect vaccinated people but are much less likely to reach the central nervous system compared to the natural virus. OPV is easy to administer as it does not require an injection but rather is administered orally. It is a cheaper option than IPV and provides strong immunity, which helps stop the spread of wild poliovirus more effectively. However, since it’s a live vaccine, there’s a small risk that OPV can cause polio, which can spread in communities with low vaccination rates⁹

Summary

Poliovirus are small viruses belonging to the Picornaviridae family that cause polio disease. Polioviruses are spread through contaminated water or food, and once in the body, they use our cell’s machinery to replicate in the regions of the throat and intestines. Poliovirus infections are usually fought off by our body's defence systems (immune system) however, in some cases, the infection develops, which leads to polio. Polio attacks the nervous systems and can potentially leave lasting paralysis and disability. It has been prevalent throughout history up until the 1950s when vaccines were produced to provide immunity. Although polio has been mostly eradicated, especially in Europe, new cases are found globally in regions where vaccines are uncommon. By understanding the way polioviruses work, we can work towards completely preventing and eradicating polio.