Overview

Our health is one of the most important aspects of our lives. In day-to-day life, infections are inevitable. Some of these infections are viral infections. Viruses can vary in severity, like the flu, and others can be more severe, like polio.¹ In this article, we will focus on polio and how it relates to other viruses.

Polio is an RNA virus that is responsible for acute poliomyelitis and post-polio syndrome.¹ It can lead to paralysis (mostly in the legs) in 1 in 200 cases, and around 5 to 10% of these individuals suffering from paralysis end up dying due to inability to breathe due to muscle paralysis.² Polio is highly contagious and affects children below the age of 5, however any unvaccinated person can get this infection regardless of their age.²

Disability resulting from polio in the early 20^th century was a major health problem, however with the introduction of vaccines, the cases of polio infections decreased by 99% since 1988.² Although it is largely controlled and mostly eradicated, polio remains present in the world – it went from 350,000 cases in more than 125 countries to just 2.² Hence, treatments were effective, but the disease must still be monitored closely for any breakouts.

This article will cover the basics of polio, such as its classification, transmission and vaccination, to shed more light on this topic.

Basics of poliovirus

Classification

Polio or poliovirus (PV) is classified as a virus in the Enterovirus genus, which belongs to the Picornaviridae family.¹ It has 3 wild serotypes known as WPV1, 2 and 3.¹ However, after interventions and control efforts that were placed in 1988, types 2 and 3 were eradicated.¹

Structure and genome

PV is a positive single-stranded RNA virus that is non-enveloped with an icosahedral capsid.¹ Its genome size is around 7,400 kilobases.¹ Once infected, PV enters the host cell and gets inside the cytoplasm, where it can later utilise the host cell’s machinery to translate the viral RNA into proteins.^1,3

Transmission

PV can be transmitted from one person to another by the faecal-oral route from contaminated food and water.¹ Polio has a specific range of hosts, with humans being the only natural reservoir.¹ Once infected, people can start shedding this virus and infect others even when symptoms disappear – the virus continues to shed in stool for several weeks after being asymptomatic.¹

Pathogenesis

Once polio enters the body through contaminated food or water, it will bind to the common cell receptor known as CD155.¹ This binding causes structural changes to occur in poliovirus, which leads to a process known as decapsidation to occur, which is when the viral capsid changes its structure and allows the RNA to enter the host’s cell cytoplasm.¹

PV first replicates in the throat and small intestines, and in some cases, it can migrate to the central nervous system through the blood-brain barrier and cause nerve damage, which then can result in paralysis without permanent sensory loss – it only attacks the motor neurons in the spine.¹

Polio vs other viral infections

Structural/genomic similarities and differences

RNA viruses:

Poliovirus is an RNA virus and has similarities with RNA viruses such as:

1. Coxsackievirus and echovirus: these viruses are labelled as non-polio enteroviruses, and commonly occur in children globally.⁴ They cause mild illnesses like hand-foot-mouth disease⁴
2. Rhinovirus: is the cause of common cold infections and differs from enteroviruses (like polio) by infecting the respiratory tract, while polio affects the intestines and nervous system^1,5
3. Influenza virus: these viruses belong to the Orthomyxoviridae family and are enveloped viruses, unlike polio, and have segmented RNA, which can cause easier mutations to occur, known as antigenic drift⁶

DNA viruses:

Poliovirus can also be compared to DNA viruses such as herpes simplex virus (HSV).⁷ HSV has a linear double-stranded DNA structure, which is enveloped and shows latency in nerve cells.⁷ This is the opposite of polio, which has no envelope and does not become latent.

Transmission

Polioviruses are transmitted through the faecal-oral route, where there is poor sanitation. Meanwhile, other viruses, such as influenza, can be transmitted through respiratory droplets.⁸ Another example would be the Rhinoviruses, which are transmitted through the air or direct contact with sick individuals.⁹

Other forms of transmission include bloodborne and vector-borne transmissions – for example, HIV can be transmitted sexually or through blood or from a mother to a child, which is a bloodborne transmission, while malaria spreads through mosquito bites, which is a vector-borne type.^10,11 These modes of transmission differ from poliovirus transmission.

Immune response and pathogenesis

Polio can affect the nervous system and lead to paralysis in severe cases; however, sometimes there are no symptoms except generalised fatigue and fever.^12,13 Additionally, poliovirus initially infects the gut and replicates there before spreading to other parts of the body, hence evading immune response.¹

We can further identify differences between polio and other viral infections based on location and time – meaning localised vs systemic infections and chronic vs acute infections. Localised infections can be like rhinoviruses that affect the upper respiratory tract, while systemic infections can be like influenza, where multiple organs are affected and can lead to organ failure.¹⁴ Polio is a systemic infection as it can infect the throat, gut, and even the nervous system.¹

When looking at chronic and acute infections, Polio is an acute infection with lifelong immunity.^1,2 While infections like HIV are chronic, lifelong infections that slowly weaken the immune system over time.¹⁵ 

Prevention

Currently, polio has no cure and is approached by prevention.² This can be done through vaccines:

• Poliovirus vaccine: this can be taken in either oral form (OPV) or inactivated form (IPV).¹⁶ OPV is a live-attenuated virus and is administered as a liquid in the mouth.¹⁶ This form can provide immunity for the community, where this virus gets shed in the stool or spit of individuals and is passed on to others to strengthen their immune system.¹⁶ Meanwhile, IPV is a dead poliovirus which does not cause polio and is given in shots.¹⁶ This form of vaccine does not provide community immunity and is given in countries that have a low risk of polio infection, such as Canada or Europe.¹⁶

FAQs

Can you recover from polio?

Individuals experiencing mild polio symptoms should make a full recovery in 1 - 2 weeks, but those with more severe symptoms.¹⁷ Some patients in this category end up developing post-polio syndrome for 30 to 40 years after the first infection.¹⁷ Hence, it is important to have the appropriate measures taken to be protected from polio.

What is polio’s survival rate?

If the virus reaches a paralysis stage, then the mortality rate can be between 5 to 15%.¹⁸

Why is polio coming back?

While polio was seen to be eradicated after the control measures placed in the 20^th century, it is showing signs of resurfacing. In countries that have consistently low immunity coverage, the weakened vaccine can circulate for a longer time and start gaining paralytic ability and causing outbreaks across different countries.¹⁹

Summary

Polio can be a severe and debilitating infection. It usually affects children below the age of 5 and can lead to paralysis and even death. However, sometimes polio appears as a mild flu and can not be caught as easily, especially when compared to other viral infections like the common cold and others. It is transmitted through the faecal-oral route and can evade the immune system initially before reaching the central nervous system. Since polio can lead to such severe situations, the best method to approach it is prevention – there is currently no cure for polio. Hence, having the right vaccinations required and maintaining a good hygiene routine can lower the risk of polio infections and their severity, as vaccines provide lifelong immunity against this virus.