Question
Pathogenesis and laboratory diagnosis of polio.
Answer
Pathogenesis of poliomyelitis
Poliovirus (an enterovirus) is transmitted via the faeco-oral route. After ingestion, the virus initially replicates in the oropharyngeal and intestinal mucosa/lymphoid tissue (tonsils, Peyer’s patches), producing a primary (minor) viraemia. In most individuals, the infection remains subclinical or produces only a mild, non-specific febrile illness (“abortive poliomyelitis”), and is cleared at this stage. In a minority of cases, the virus disseminates further, producing a secondary (major) viraemia, which allows the virus to reach and invade the central nervous system, crossing the blood-brain barrier and/or spreading via peripheral nerves. Within the CNS, poliovirus shows a specific tropism for and destroys the motor neurons of the anterior horn of the spinal cord (and, in bulbar poliomyelitis, brainstem motor nuclei), causing irreversible, asymmetric flaccid paralysis of the muscles innervated by the affected neurons — the hallmark clinical manifestation of paralytic poliomyelitis.
Laboratory diagnosis:
- Virus isolation — from stool samples (the specimen of choice, given prolonged viral shedding — collected as two samples 24–48 hours apart within 14 days of paralysis onset, per WHO AFP surveillance protocol), and also from throat swabs (early in illness) or, less commonly, CSF; isolation performed in cell culture (e.g., using susceptible cell lines showing characteristic cytopathic effect).
- Intratypic differentiation and genomic sequencing (RT-PCR/sequencing) — performed on isolates to distinguish wild poliovirus from vaccine (Sabin) strains, and to identify Vaccine-Derived Poliovirus (VDPV) — an essential step in polio surveillance/eradication programmes.
- Serology — demonstration of a significant rise in neutralizing antibody titre between acute and convalescent sera; of limited use for acute individual diagnosis given widespread background seropositivity from vaccination, but used in specific epidemiological/immunity assessment contexts.
- CSF examination — in paralytic disease, typically shows a mild lymphocytic pleocytosis with normal/mildly elevated protein and normal glucose, consistent with aseptic (viral) meningitis — supportive, non-specific finding.
- Surveillance for poliomyelitis relies heavily on Acute Flaccid Paralysis (AFP) surveillance, with virological confirmation via stool sample testing being central to the WHO Global Polio Eradication Initiative.