Topic: Herpes, Dengue, Polio
Question: Briefly describe the pathogenesis and laboratory diagnosis of polio.
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Pathogenesis:
– Causative agent is poliovirus, a positive-sense, single-stranded RNA virus in the Picornaviridae family. Three serotypes – PV1, PV2, PV3.
– Primary site of infection is the gastrointestinal tract. Virus is ingested and initially replicates in the oropharynx and Peyer’s patches of the intestine.
– Then spreads to draining lymph nodes via bloodstream. Viremia leads to invasion of reticuloendothelial system.
– During viremia, poliovirus crosses blood-brain barrier and infects motor neurons in the anterior horn cells of the spinal cord and brainstem.
– Virus has tropism for motor neurons due to binding of viral capsid to poliovirus receptor CD155 on neuron cell surface.
– Viral replication in neurons leads to cell destruction and nerve cell loss, resulting in flaccid paralysis.
– In minor infections, immune system clears virus before paralysis occurs. In small %, virus spreads to CNS leading to major damage.
Lab Diagnosis:
– Virus isolation: Poliovirus can be isolated from stool, CSF or pharyngeal secretions. Optimally collected within 14 days of paralysis onset.
– Samples inoculated into cell culture (RD, L20B cells) and observed for cytopathic effect.
– RT-PCR: Viral RNA detected from CSF, stool, throat swabs using reverse transcriptase PCR. High sensitivity.
– Intrathecal antibody production: Poliovirus IgM in CSF indicates CNS infection. Paired serum samples show rising titers.
– Neutralizing antibodies: Measured by seroneutralization. Fourfold rise indicates recent infection.
– Single high IgM titer is diagnostic in endemic regions. In disease-free areas, preference is isolation from two different samples.
In summary, poliovirus infects motor neurons leading to cell destruction and flaccid paralysis. Definitive laboratory diagnosis involves virus isolation from multiple samples and/or intrathecal IgM detection.
