Question
Describe the pathogenesis, laboratory diagnosis and prophylaxis of Polio myelitis. (PTO)
Answer
Poliomyelitis
Pathogenesis: poliovirus (an enterovirus) is transmitted via the faeco-oral route; after ingestion, it replicates initially in the oropharyngeal/intestinal mucosa and associated lymphoid tissue, producing a primary (minor) viraemia. In most infected individuals, this remains subclinical or produces only mild non-specific illness (“abortive poliomyelitis”), with the infection cleared at this stage. 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), performed 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), essential for global eradication surveillance.
- Serology — demonstration of significant rise in neutralizing antibody titre between paired acute and convalescent sera; limited utility for acute individual diagnosis given widespread vaccine-induced background seropositivity.
- CSF examination — mild lymphocytic pleocytosis with normal/mildly elevated protein, consistent with aseptic (viral) meningitis, a supportive, non-specific finding.
Prophylaxis:
- Oral Polio Vaccine (OPV) — live attenuated (Sabin) vaccine, given orally; induces both humoral and strong mucosal (intestinal) immunity, interrupting faeco-oral transmission, and is the mainstay of eradication campaigns given ease of administration and cost. Carries a very small risk of Vaccine-Associated Paralytic Poliomyelitis (VAPP) and can give rise to circulating Vaccine-Derived Poliovirus (cVDPV) in areas of low immunization coverage.
- Inactivated Polio Vaccine (IPV) — killed (Salk) vaccine, given by injection; induces strong humoral immunity without the (rare) risk of VAPP, but induces weaker mucosal immunity than OPV.
- Current global strategy (as part of polio eradication efforts) uses a combination of OPV and IPV in national immunization schedules, alongside intensive AFP (Acute Flaccid Paralysis) surveillance to detect and respond to any residual wild poliovirus or VDPV transmission.