Question
Describe the pathogenesis, laboratory diagnosis and prophylaxis of poliomyelitis
Answer
Poliomyelitis
Pathogenesis: poliovirus (an enterovirus) is transmitted via the faeco-oral route; after ingestion, it replicates in the oropharyngeal/intestinal mucosa and associated lymphoid tissue, producing a primary (minor) viraemia. In most individuals, this remains subclinical or produces only mild non-specific illness (“abortive poliomyelitis”), and is cleared. In a minority, the virus disseminates further via a secondary (major) viraemia, reaching the central nervous system, where it shows specific tropism for and destroys anterior horn motor neurons of the spinal cord (and, in bulbar disease, brainstem motor nuclei), producing irreversible, asymmetric flaccid paralysis.
Laboratory diagnosis:
- Virus isolation — from stool (the specimen of choice, given prolonged shedding — two samples collected 24–48 hours apart within 14 days of paralysis onset, per WHO AFP surveillance protocol), by inoculation into susceptible cell culture, observing characteristic cytopathic effect.
- Intratypic differentiation/genomic sequencing (RT-PCR/sequencing) — to distinguish wild poliovirus from vaccine (Sabin) strains, and to identify Vaccine-Derived Poliovirus (VDPV).
- Serology — significant rise in neutralizing antibody titre between paired acute/convalescent sera; limited utility given widespread vaccine-induced seropositivity.
- CSF examination — mild lymphocytic pleocytosis with normal/mildly elevated protein, consistent with aseptic (viral) meningitis, a supportive finding.
Prophylaxis:
- Oral Polio Vaccine (OPV) — live attenuated (Sabin) vaccine; induces strong mucosal (intestinal) immunity, interrupting faeco-oral transmission, central to global eradication campaigns; small risk of Vaccine-Associated Paralytic Poliomyelitis (VAPP) and can give rise to circulating VDPV in areas of low coverage.
- Inactivated Polio Vaccine (IPV) — killed (Salk) vaccine, given by injection; strong humoral immunity without VAPP risk, but weaker mucosal immunity than OPV.
- Current strategy combines both vaccines in national immunization schedules, alongside intensive Acute Flaccid Paralysis (AFP) surveillance as part of the WHO Global Polio Eradication Initiative.