Question
A four-year-old child was brought to paediatric casualty with fever, altered behaviour and difficulty to swallow liquids. Mother gives a history of dog bite two months back and not taking vaccine as per schedule. (
- (a) What is your clinical diagnosis. ( 1 mark(s)
- (b) Name the causative agent. ( 1 mark(s)
- (c) Describe the pathogenesis of this condition. ( 2 mark(s)
- (d) List the samples collected and the methods of antemortem diagnosis of this condition. (e) Name cell culture vaccines for this disease. (f) What is IDRV. Discuss the post exposure vaccination schedule. 4 mark(s)
Answer
(a) Clinical diagnosis: Rabies (furious/encephalitic type) — fever, altered behaviour, and hydrophobia (difficulty swallowing liquids) following a dog bite two months earlier in an unvaccinated child is the classical presentation.
(b) Causative agent: Rabies virus (genus Lyssavirus, family Rhabdoviridae).
(c) Pathogenesis: following inoculation via an infected animal’s bite/saliva, the virus initially replicates locally in muscle tissue, then enters peripheral nerve endings and travels retrograde (centripetally) along axons to the central nervous system — the (typically prolonged, variable, weeks-to-months) incubation period corresponds to this neural transit time and is influenced by the site/severity of the bite (shorter incubation with bites closer to the head/face, richer nerve supply). Once in the CNS, the virus replicates extensively, particularly in the brainstem and limbic system, and then spreads centrifugally (anterogradely) via peripheral and autonomic nerves to other tissues, including the salivary glands (facilitating transmission), causing the characteristic fatal encephalitis with autonomic hyperactivity, hydrophobia, and aerophobia.
(d) Samples and methods of antemortem diagnosis:
- Saliva — for viral RNA detection by RT-PCR, or virus isolation.
- Nuchal (neck) skin biopsy — for detection of viral antigen (direct fluorescent antibody test) or RNA (RT-PCR) in cutaneous nerve twigs at the hair follicle base.
- Corneal impression smear — for direct fluorescent antibody detection of viral antigen.
- CSF — for antibody detection (in unvaccinated patients) and RT-PCR.
- Serum — for detection of rabies-neutralizing antibody (in an unvaccinated patient, positive antibody supports diagnosis).
(e) Cell culture vaccines for rabies: Human Diploid Cell Vaccine (HDCV), Purified Chick Embryo Cell Vaccine (PCECV), and Purified Vero Cell Rabies Vaccine (PVRV) — all modern, cell-culture-derived inactivated vaccines that have largely replaced older nerve-tissue vaccines.
(f) IDRV and post-exposure vaccination schedule: IDRV (Intradermal Rabies Vaccine): a dose-sparing method of administering cell-culture rabies vaccine via the intradermal route rather than intramuscularly, using a fraction of the standard IM dose at each site while achieving equivalent immunogenicity — cost-effective and increasingly the WHO-recommended standard for post-exposure prophylaxis, especially in resource-limited settings.
Post-exposure vaccination schedule: as per the WHO-recommended updated Thai Red Cross (2-site intradermal) regimen, vaccine is given intradermally at two sites per visit on days 0, 3, 7, and 28 (a total of 4 visits/8 intradermal doses); alternatively, the intramuscular Essen regimen (one dose IM on days 0, 3, 7, 14, and 28) may be used. Post-exposure management additionally requires thorough wound washing with soap and water, and, for Category III exposures (deep bites/wounds, or exposure to mucous membranes, or bites from a confirmed/suspected rabid animal), Rabies Immunoglobulin (RIG) should be infiltrated around the wound to provide immediate passive protection alongside active vaccination.