Poliomyelitis

Poliomyelitis, also known as polio, is an acute communicable disease that primarily affects children younger than 5 years of age.¹ There are three serotypes – how groups of pathogens are identified based on their unique surface proteins or antigens  – of poliovirus: type 1, type 2, and type 3. Acquiring immunity to one serotype does not offer significant protection against the other serotypes.²

Pathogenesis

Polio virus spreads through the oral-faecal route, where the virus is shed in faeces. It enters an individual’s mouth through poor handwashing, or coming into contact with contaminated food, water or surfaces.¹ After entering the body through the mouth, the poliovirus replicates in the oropharynx and digestive tract. It stays for 1-2 weeks in nasopharyngeal secretions. Even after infection, people can shed the virus in their stools for several weeks, including those who are asymptomatic or have relatively minor symptoms. The virus spreads to local lymphoid tissues and the bloodstream, and may eventually infect cells in the central nervous system. The hallmark signs of poliomyelitis are shown when the virus replicates in the motor neurons of the anterior horn of the spinal column and brain stem, causing cell death.²

It spreads more rapidly in areas with poor sanitation, especially among the non-immune population.³ Although extremely rare in regions outside endemic areas (Afghanistan and Pakistan), polio still poses a risk to populations with low vaccination rates, even in industrialised countries.⁴

Image: Diagram showing how polio affects the body. Author: Nikita Cranston, using Canva.

Clinical presentation

The incubation period for nonparalytic poliomyelitis ranges from 3 to 6 days. For paralytic poliomyelitis, paralysis usually occurs between 7 and 21 days following the initial infection. The likelihood of experiencing severe illness or death after a primary infection with poliovirus increases with age.² Its presentation is highly variable and depends on the severity of the symptoms.

Asymptomatic poliomyelitis

Most individuals who are exposed to the virus (approximately 95%) do not show any symptoms.¹ The virus during this period sheds through the stool and can also be isolated from the throat swabs. The ratio of asymptomatic to paralytic cases ranges from 1 in 200 to 1 in 2000.⁶ 

Abortive poliomyelitis

This is the mildest form of the disease, representing about 4% to 8% of cases. It may present as gastroenteritis, an influenza-like illness, or mild respiratory tract infections that usually last 1 week.³

Nonparalytic poliomyelitis

Nonparalytic poliomyelitis, or aseptic meningitis, is a moderate form of the infection. It’s considered more severe than abortive polio, since the virus affects the meninges (protective layers of the brain), but it is less severe than paralytic poliomyelitis. Nonparalytic poliomyelitis is reported in approximately 1% of cases. Symptoms include: severe muscle spasms in the neck, back, and lower limbs, often following a brief early phase similar to that seen in abortive poliomyelitis. Full recovery typically occurs within 10 days.^3,13

Paralytic poliomyelitis

Paralytic poliomyelitis presents in less than 1% of patients and is the most severe form. It is characterised by severe pain in the back and lower limbs followed by muscle weakness, loss of reflexes and paralysis.⁸ In children, the disease can appear in two phases: an initial prodromal (symptom-presenting) period followed by a short symptom-free interval lasting 7 to 10 days, after which asymmetrical limb paralysis develops. The characteristic feature is flaccid paralysis, accompanied by the eventual loss of deep tendon reflexes.⁷ Some patients may experience full recovery, but if motor function loss continues for more than 12 months, it typically leads to permanent disability.⁷

How does the polio virus cause asymptomatic infections? 

The poliovirus can cause asymptomatic infections due to its interaction with the host's immune system and the viral infection's nature.

Mechanism of asymptomatic infections

• Initial infection: The poliovirus usually enters the body orally, where it infects intestinal epithelial cells and the oropharynx.⁶ The virus replicates in these tissues, and the immune system often clears the infection before it spreads
• Immune response: Many people don't have any symptoms because their immune system effectively controls the illness or because they have already received the OPV vaccine. Asymptomatic individuals may still have viral particles in their gastrointestinal tract, allowing them to shed the virus without exhibiting clinical signs of the disease⁹
• Viral Shedding: Asymptomatic carriers can shed poliovirus in their faeces for several weeks or even months post-infection, facilitating environmental contamination and the potential for transmission to others⁶

Implications for transmission

Continued transmission

Asymptomatic carriers are significant for the transmission of poliovirus, especially in areas with inadequate sanitation. They can unknowingly spread the virus through faecal-oral routes.³

This is particularly concerning in endemic regions where vaccination coverage is low, as asymptomatic carriers can help maintain the circulation of the virus within communities.¹⁰

Challenges in eradication

The presence of asymptomatic infections complicates efforts to eradicate polio. Surveillance systems may miss cases, leading to underreporting of the disease and hindering vaccination campaigns.¹¹

Understanding the role of asymptomatic carriers is crucial for effective public health strategies, including vaccination campaigns and sanitation improvements.

Vaccination strategies

Immunisation not only protects individuals from developing polio but also helps reduce the viral load in the community, decreasing the likelihood of asymptomatic carriers spreading the virus.¹² Oral poliovirus vaccines (OPV) can contribute to herd immunity, reducing transmission even among unvaccinated individuals.

Conclusion

Asymptomatic infections significantly impact the epidemiology of the poliovirus by enabling transmission and complicating eradication efforts. These carriers can sustain the virus within vulnerable populations, posing challenges for effective surveillance, as they often remain undetected. Continued public health measures, including vaccination and improved sanitation, are crucial even in areas with low incidence rates to reduce faecal-oral transmission. Future research should focus on developing vaccines that limit viral shedding and innovative surveillance methods to identify asymptomatic carriers. Addressing these issues is crucial for reducing polio incidence and advancing toward global eradication.