Polio

Basics

Description

  • Vaccine-preventable disease caused by highly infectious poliovirus
  • Infected individuals are usually asymptomatic, but can present with symptoms ranging from flu-like illness to acute flaccid paralysis or meningitis
  • Incubation period 3–6 d for nonparalytic infection and 7–21 d for the onset of paralytic polio
  • Clinical manifestations:
    • Subclinical (ie, not apparent) 90–95%
    • Abortive poliomyelitis 4–8%:
      • Clinically indistinct from many other viral infections (fever, myalgias, malaise)
      • Only suspected to be polio during an epidemic
    • Nonparalytic poliomyelitis 1–2%:
      • Differs from abortive poliomyelitis by the presence of meningeal irritation
      • Course similar to any aseptic meningitis
    • Paralytic poliomyelitis 0.1%, which is further subdivided:
      • Spinal paralytic poliomyelitis (frank polio)
      • Bulbar paralytic poliomyelitis (10% of paralytic polio): Paralysis of muscle groups innervated by cranial nerves; involves the circulatory and respiratory centers of the medulla with high mortality
      • Mixed bulbospinal poliomyelitis
    • Postpoliomyelitis syndrome:
      • New onset of increased muscle weakness, pain, and focal or generalized atrophy
      • Usually occurs over 15 yr after active illness, often in the previously affected limb
      • Risk factors include age at time of infection, extent of recovery and female sex (increased risk with better recovery)
      • Gradual progression

Etiology

  • Polioviruses:
    • Small, nonenveloped RNA viruses of the enterovirus genera
    • 3 strains of wild poliovirus: 1, 2, 3
      • Types 2 and 3 are eradicated; type 1 endemic only to Pakistan and Afghanistan
  • Humans are the only natural host and reservoir
  • Fecal–oral or oral–oral transmission:
    • Oral polio vaccine:
      • Contains a weakened version of virus excreted by vaccinated person for weeks
      • Contacts can become inoculated and gain immunity through secondary exposure
      • In very rare instances, however, the vaccinated person or an unvaccinated person around them can become infected from the neurovirulent conversion of vaccine virus
      • To avoid vaccine-associated paralytic poliomyelitis (VAP) the US has only used intramuscular inactivated polio virus (IPV) since 2000
      • Global decrease in VAP incidence since the worldwide introduction of IPV in 2016
      • IPV is costly, however, so OPV is still used during outbreaks and in endemic areas

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