Polio

Basics

Description

  • Caused by poliovirus infection
  • Incubation period 7–14 d
  • Duration <1 wk
  • Clinical manifestations are defined as follows:
    • Subclinical (i.e., 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:
    • Picornaviruses
    • Small, nonenveloped RNA viruses of the enterovirus genera
    • 3 subtypes: 1, 2, 3
  • Fecal–oral route transmission:
    • Enters through oral cavity
    • Replicates in pharynx, GI tract, and lymphatics
  • Humans are the only natural host and reservoir
  • Poliovirus selectively destroys motor and autonomic neurons
  • Natural (wild) virus has been completely eliminated in the U.S. since 1979
  • Oral poliovirus vaccine (OPV):
    • Accounts for only poliomyelitis seen in the U.S. (last case reported in the U.S. was in 2013):
      • Vaccine-associated paralytic poliomyelitis (VAP): Neurovirulent conversion of vaccine virus; decreased since widespread use of inactivated poliovirus vaccine (IPV)
      • VAP occurs in poorly immunized regions by acquiring properties of wild-type virus
      • There has been a global decrease in VAP incident since the worldwide introduction of IPV in 2016, as well as, the cessation of trivalent OPV use

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