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Polio

Poliomyelitis, commonly known as polio, is an infectious disease caused by the poliovirus. It primarily affects children under five years of age, although it can also impact adults. The disease can range from asymptomatic to severe, including permanent paralysis and death in extreme cases.

Polio survivor
Polio survivor

Signs and Symptoms

Polio presents in various forms:

  • Asymptomatic: About 72% of cases show no symptoms.
  • Minor Illness: Around 24% of cases exhibit minor symptoms such as sore throat and fever, which resolve in one to two weeks.
  • Nonparalytic Aseptic Meningitis: Approximately 1–5% of cases progress to this form, showing symptoms like headache, neck stiffness, and paresthesia.
  • Paralytic Poliomyelitis: Occurs in 0.1–0.5% of cases, leading to muscle weakness and acute flaccid paralysis. Paralysis affects the legs most commonly but can also impact the head, neck, and diaphragm.
A TEM micrograph of poliovirus
A TEM micrograph of poliovirus

Cause and Transmission

Polio is caused by the poliovirus, a member of the genus Enterovirus. It spreads primarily via the faecal–oral route but can also transmit through oral–oral routes. The virus is highly contagious and can be excreted in faeces for up to six weeks post-infection. Poor hygiene significantly increases the risk of transmission.

Diagram of the spinal cord affected by poliovirus
A photomicrograph of the lumbar spinal cord depicting an infarct due to Polio Type III surrounding the anterior spinal artery.

Pathophysiology

Poliovirus enters through the mouth, infecting cells in the pharynx and intestinal mucosa before spreading to the tonsils, lymphoid tissue, and eventually the bloodstream. In about 1% of cases, it invades the central nervous system (CNS), causing inflammation and potentially leading to paralytic poliomyelitis. The virus targets motor neurons, resulting in muscle atrophy and paralysis.

Denervation of skeletal muscle tissue
Denervation of skeletal muscle tissue secondary to poliovirus infection can lead to paralysis.

Diagnosis

Diagnosis is primarily based on clinical signs of acute flaccid paralysis and confirmed through laboratory tests. The presence of poliovirus in stool samples or pharyngeal swabs, detected using reverse transcription polymerase chain reaction (RT-PCR) or genomic sequencing, is definitive.

Prevention

Vaccination

Two types of vaccines are used:

  • Inactivated Polio Vaccine (IPV): Developed by Jonas Salk, it is given by injection and induces immunity after three doses.
  • Oral Polio Vaccine (OPV): Developed by Albert Sabin, it contains live attenuated virus and is administered orally. Although effective, it carries a risk of vaccine-derived poliovirus (cVDPV).
A child receiving an oral polio vaccine
A child receiving an oral polio vaccine.

Treatment

There is no cure for polio; treatment focuses on supportive care. This includes pain relief, antibiotics to prevent secondary infections, and long-term rehabilitation such as physical therapy and orthopaedic surgery. Mechanical ventilation, historically provided by iron lungs, may be necessary for those with respiratory involvement.

Prognosis and Complications

Recovery varies: some patients recover completely, while others may have permanent paralysis. Complications include skeletal deformities, muscle atrophy, and joint contractures. Post-polio syndrome (PPS) can occur decades later, causing muscle weakness and fatigue.

A girl with genu recurvatum of her right leg due to polio
A girl with genu recurvatum of her right leg due to polio.

Epidemiology and Eradication Efforts

Polio was once a global epidemic, but vaccination campaigns have significantly reduced its prevalence. The Global Polio Eradication Initiative aims to eliminate the disease entirely. As of recent reports, wild poliovirus remains endemic in only a few countries, including Afghanistan and Pakistan.

The decade of the last recorded case of paralytic polio
The decade of the last recorded case of paralytic polio. Since the creation of this image, Nigeria has been certified free of wild polio as of August 2020 and one case was recorded in the US state of New York in July 2022.

Self-assessment MCQs (single best answer)

What is the primary age group affected by poliomyelitis?



What percentage of polio cases are asymptomatic?



Which form of polio leads to symptoms like headache, neck stiffness, and paresthesia?



What is the primary route of transmission for poliovirus?



Which vaccine for polio is administered orally?



In cases of paralytic poliomyelitis, which body part is most commonly affected by paralysis?



What is the definitive method for diagnosing polio?



Which of the following is a common complication that can occur decades after the initial polio infection?



Which countries remain endemic with wild poliovirus as of recent reports?



The Global Polio Eradication Initiative aims to:



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