Poliomyelitis, also known as polio or infantile paralysis, is a disease of the central nervous system (CNS) that preferentially infects and destroys the motor neurons, leading to muscle weakness and acute flaccid paralysis. Poliomyelitis is caused by infection with poliovirus (PV), a member of the genus Enterovirus. Although epidemics of the condition have been a more recent phenomena, polio is believed to be an ancient disease with steles from ancient Egypt and mummified remains of pharaohs demonstrating the tell-tale lower extremity withering resultant from polio-mediated paralysis. Outbreaks in Europe and the United States were first reported in the early and mid-19th century, respectively, and following industrialization, polio became one of the most feared childhood diseases of the 20th century, with infection peaking in the United States in the 1950s. Since then, worldwide prevalence has decreased significantly because of the development of the polio vaccine and as a result of aggressive immunization programs under the surveillance of the World Health Organization (WHO).

Polio was declared eradicated in the United States in 1979, and no cases of wild-type PV infection have been reported since then. From 1980-1999, an average of 8 cases per year were reported in the United States, 95% of which were due to vaccine-associated paralytic polio (VAPP) caused by OPV (with the remaining cases imported or indeterminant). Starting in 2000, the Advisory Committee on Immunization Practices recommended the exclusive use of IPV in routine immunizations in the United States to eliminate VAPP.

In July 2022, New York State reported a case of VAPP in an unvaccinated adult in Rockland County. Subsequently, poliovirus was detected in wastewater samples from Long Island, signaling community spread of the virus. While the risk of individuals contracting polio in the United States is low, this episode demonstrates the importance of vaccination in all children and adults.

Areas of endemic wild-type PV still exist in other areas of the world such as Pakistan and Afghanistan. Other countries are experiencing or at risk of outbreaks from imported wild or vaccine-derived poliovirus or circulation of vaccine-derived poliovirus. Refer to the Statement of the Thirty-second Polio International Health Regulations (IHR) Emergency Committee (June 24, 2022) for a list of countries.

Mortality observed in paralytic polio is due to respiratory failure, although this has not occurred in the United States since 1979 (a 2013 fatal case occurred in an infant who immigrated from India after receiving OPV in that country and who was severely immunodeficient due to an inherited condition). Incidence is equal in males and females. Poliomyelitis tends to be a disease mainly of children; however, it can affect individuals of any age, especially those who are immunocompromised.

Clinical course:
Primary viremia (abortive polio) is asymptomatic. In about 25% of patients, this is followed by a second major viremia manifesting as a flu-like illness with headache, sore throat, fever, nausea, vomiting, malaise, and fatigue. A fraction of patients (1%-5%) can have invasion of the CNS and inflammation of the meninges, causing neck stiffness, headache, fever, and vomiting and, in some patients, selective destruction of motor neurons; depending on the anatomical site, this causes severe back pain, neck / muscle pain, and the development of motor weakness. Paralysis occurs in 0.05%-0.5% of PV infections. Weakness may vary, from 1 muscle or group of muscles to quadriplegia and respiratory failure.

• Spinal polio – Tone is reduced, nearly always in an asymmetric manner, with lower motor neuron destruction. Proximal muscles usually are affected more than distal muscles, and legs more commonly than arms. Reflexes are decreased or absent. The sensory examination is normal. Weakness typically worsens over 2-3 days, although sometimes worsening can progress for up to a week. Muscle atrophy is generally observed several weeks after onset of symptoms. Recovery may be complete, partial, or absent.
• Bulbar polio – Bulbar involvement occurs in 5%-35% of patients with paralytic polio, producing dysphagia, dysarthria, and difficulty handling secretions. Approximately 19% of all paralytic polio cases have both bulbar and spinal symptoms.
• Respiratory polio (bulbospinal polio) – Bulbospinal polio affects the upper part of the cervical spinal cord (C3 through C5) and causes paralysis of the diaphragm along with difficulty in swallowing. There may be encephalitis, usually in infancy.
• Postpolio syndrome – Recurrences of weakness or fatigue may occur in some patients decades (20-40 years) after initial infection and disease. The same groups of muscles are usually involved, but weakness can extend to muscles that were not involved during the initial infection. The prognosis is generally good, although the actual cause of the syndrome is poorly understood. It is, at least, not secondary to persistent or reactivated PV infection. Patients may also experience chronic pain, contractures, depression, fatigue, and sleep problems.

It is worth noting that there are 2 types of paralytic diseases associated with OPV:

1. VAPP – Usually occurs after the first dose of OPV, and its estimated incidence is 1 person per 2.5 million doses. It is thought to be due to mutation of the vaccine virus to a neurovirulent type. The disease may occur in the vaccinated individual or a close contact of that person. Predisposing factors include B-cell immunodeficiencies and hypogammaglobulinemia.
2. Vaccine-derived poliomyelitis (VDPV) – Refers to outbreaks of poliomyelitis in regions with low immunization rates when OPV vaccination starts. The mechanism appears to be mutation of vaccine virus to naturally occurring wild-type PV.

Pathogenesis:
PV is transmitted feco-orally by the ingestion of contaminated water or, during epidemics, via pharyngeal spread. PV infects and causes disease only in humans. It is highly contagious. In temperate climates, infection is seasonal, with peak transmission occurring in summer and autumn. Three serotypes of PV have been identified: PV types 1 (PV1), 2 (PV2), and 3 (PV3). All are extremely virulent and produce the same symptoms. PV1 is the most common and is most frequently associated with paralysis.

After entering the mouth, the organism tends to infect the cells of the mouth, nose, and throat. PV infections are asymptomatic in 90%-95% of cases. There is an incubation period of 3-35 days (minimum range 6-20 days), during which the virus undergoes primary replication in the lymphatic tissues of the gastrointestinal tract.

An asymptomatic primary transient viremia is followed in 25% of patients by a flu-like illness. About 1%-5% have CNS involvement with meningitis as above with 0.05%-0.5% developing paralytic polio. The mechanism of spread of PV to the CNS is unclear. It may occur by direct passage of the virus from blood by crossing the blood-brain barrier, or it may occur by retrograde axonal transport from muscle to spinal cord and brain. After invading the CNS, the virus uncoats, viral replication occurs, and the motor neuron dies, leading to paralysis of muscle fibers supplied by this motor neuron. Spread of the virus to neighboring motor neurons may occur laterally, independent of axonal transport, or by transneuronal spread in a retrograde transport-dependent fashion. Other neurons can be infected as well, producing brain stem encephalitis and respiratory insufficiency.

The virus can cross the placenta during pregnancy but does not seem to affect the fetus, either by maternal infection or vaccination. Maternal antibodies also cross the placenta, providing passive immunity during the first few months of life.