Paper II — 2023 July (Supplementary) (2019 Scheme) — Q4
5 min read
Paper II
2023 July (Supplementary) (2019 Scheme) · 100 marks · 180 min
Question
Discuss the pathogenesis, antemortem lab diagnosis and post exposure prophylaxis of
Rabies.
Q48 marksShort Essays
Answer
Rabies
Pathogenesis: following inoculation via an infected animal’s bite/saliva, the rabies virus replicates locally in muscle tissue, then enters peripheral nerve endings and travels retrograde (centripetally) along axons to the central nervous system — the variable incubation period (weeks to months) corresponds to this neural transit, influenced by bite site/severity (shorter incubation for bites closer to the head/face). Once in the CNS, the virus replicates extensively (particularly brainstem and limbic structures), causing severe, characteristic encephalitis, and then spreads centrifugally (anterogradely) via peripheral/autonomic nerves to other tissues, notably the salivary glands, facilitating onward transmission.
Antemortem laboratory diagnosis:
Saliva — RT-PCR for viral RNA, or virus isolation.
Nuchal (neck) skin biopsy — direct fluorescent antibody test or PCR on cutaneous nerve twigs at the hair follicle base.
Corneal impression smear — direct fluorescent antibody detection of viral antigen.
CSF — antibody detection (in an unvaccinated patient) and RT-PCR.
Serum — detection of rabies-neutralizing antibody.
(Postmortem diagnosis — Negri bodies on brain histopathology and direct fluorescent antibody test on brain tissue — remains the definitive confirmatory method, but is not applicable antemortem.)
Post-exposure prophylaxis:
Immediate, thorough wound washing with soap and running water for at least 15 minutes — the single most effective, immediately available intervention.
Category-based management (WHO categorization):
Category I (touching/feeding animals, licks on intact skin) — no PEP required.
Category II (nibbling of uncovered skin, minor scratches without bleeding) — wound care plus vaccination.
Category III (single/multiple transdermal bites/scratches, licks on broken skin/mucous membrane, contact with bat) — wound care, vaccination, plus Rabies Immunoglobulin (RIG) infiltrated around the wound.
Active vaccination — modern cell-culture vaccines (e.g., PVRV, PCECV), given either via the intramuscular Essen regimen (days 0, 3, 7, 14, 28) or the dose-sparing intradermal (updated Thai Red Cross) regimen (two sites per visit on days 0, 3, 7, 28).
Rabies Immunoglobulin (RIG) — for Category III exposures, providing immediate passive protection while active immunity develops; human RIG or equine RIG (after appropriate sensitivity testing) infiltrated as much as possible around and into the wound.
