poliomyelitis

Poliovirus is a non-enveloped, positive-sense single-stranded RNA virus of the Picornaviridae family, transmitted via the fecal-oral route. After replicating in the oropharynx and GI tract (Peyer's patches), the virus can enter the bloodstream (minor viremia) and, in a minority of cases, cross the blood-brain barrier to infect anterior horn cells of the spinal cord and motor neurons of the brainstem, causing lower motor neuron destruction. Clinical spectrum: ~95% asymptomatic; ~5% present with minor illness (fever, sore throat, GI upset); <1% develop nonparalytic aseptic meningitis; <1% develop paralytic poliomyelitis characterized by asymmetric flaccid paralysis, reduced/absent deep tendon reflexes, muscle atrophy, and fasciculations, with sensation intact (pure motor deficit) because sensory neurons are spared. Bulbar involvement can cause respiratory failure and death. CSF shows lymphocytic pleocytosis with normal glucose. Diagnosis is via viral culture/PCR from stool, throat, or CSF. There is no specific antiviral treatment; management is supportive (including mechanical ventilation for respiratory failure, as historically with the 'iron lung'). Prevention is via vaccination: the inactivated poliovirus vaccine (IPV, Salk) confers humoral immunity without risk of vaccine-associated paralytic polio, while the live attenuated oral poliovirus vaccine (OPV, Sabin) induces mucosal (IgA) immunity and is used in global eradication campaigns but carries a rare risk of vaccine-associated paralytic poliomyelitis (VAPP) due to reversion to virulence. Post-polio syndrome is a delayed complication occurring decades after initial infection, presenting with progressive muscle weakness, fatigue, and pain due to degeneration of previously affected neurons. Poliomyelitis is a classic board topic for distinguishing lower motor neuron lesions and for understanding live vs. inactivated vaccine immunology.