Question
A four-year-old child was brought to casualty with altered behavior and difficulty to swallow liquids. Mother gave history of dog bite one month back and not taking vaccine as per schedule
- (a) What is the clinical diagnosis and the causative agent 2 mark(s)
- (b) Describe the pathogenesis of this condition 2 mark(s)
- (c) Describe the ante mortem laboratory diagnosis of this condition 2 mark(s)
- (d) Describe the post exposure prophylaxis recommended for this condition 4 mark(s)
Answer
a) Clinical diagnosis and causative agent: Rabies (furious/encephalitic form) — altered behaviour, hydrophobia (difficulty swallowing liquids), and a history of unvaccinated dog bite one month earlier is classical. Causative agent: rabies virus (a rhabdovirus, genus Lyssavirus).
b) Pathogenesis Rabies virus is introduced into a bite wound in the saliva of an infected animal; it first replicates locally in muscle tissue near the inoculation site, then enters peripheral nerve endings and travels retrograde along axons toward the CNS — incubation period depends on the distance from the bite to the CNS. Once in the CNS, the virus replicates extensively in neurons (producing Negri bodies, classically in hippocampal/Purkinje cells), causing severe encephalitis. From the CNS, the virus spreads centrifugally via peripheral nerves to the salivary glands (basis for further transmission) and other highly innervated tissues. The hydrophobia of furious rabies results from painful pharyngeal/laryngeal muscle spasm triggered by attempts to swallow, related to brainstem involvement.
c) Antemortem laboratory diagnosis
- Direct Fluorescent Antibody (DFA) test — on nuchal skin biopsy (hair-follicle-associated nerve tissue), corneal impression smear, or saliva.
- RT-PCR — on saliva, CSF, or nuchal skin biopsy — highly sensitive molecular detection.
- Serology — detection of rabies-neutralizing antibody in serum/CSF, useful in patients surviving long enough to mount a response.
- (Note: a single negative antemortem test cannot definitively exclude rabies, given variable/intermittent viral shedding; repeated/serial sampling improves sensitivity.)
d) Post-exposure prophylaxis
- Wound management: immediate, thorough washing with soap and water for at least 15 minutes, followed by a virucidal agent; suturing avoided/delayed.
- Category-based management: Category II (minor scratches) — vaccine alone; Category III (transdermal bites, deep scratches, mucous membrane exposure — applicable here) — vaccine PLUS Rabies Immunoglobulin (RIG), infiltrated around the wound.
- Active immunization: modern cell-culture-derived vaccines (HDCV, PCECV, PVRV) on a defined multi-dose schedule (e.g., days 0, 3, 7, 14, ± 28), given intramuscularly or intradermally.
- Passive immunization: RIG for Category III exposures, providing immediate neutralizing antibody while active immunity develops.
Note: since this child is already symptomatic, treatment at this stage would be palliative/supportive rather than curative — the described PEP measures are those that should have been instituted promptly after the bite.