Hypersensitivity Reaction to Equine Rabies Immunoglobulin with Confirmed Positive Rechallenge in Category III Dog Bite Exposure: A Comprehensive Case Report

Background: Equine rabies immunoglobulin (ERIG) constitutes the cornerstone of category III rabies post-exposure prophylaxis (PEP) in resource-limited settings, yet acute hypersensitivity reactions complicate 5-46% of administrations. Deliberate positive re challenge re-administration post-reaction to confirm causality remains exceptionally rare in published literature, precluding definitive risk-benefit analyses. Case Presentation: A healthy 17-year-old male (55 kg) manifested severe multisystem hypersensitivity (nausea, emesis, Grade II local reaction, vertigo) 30 minutes following 40 IU ERIG for right forefinger dog bite. Temporary de challenge yielded partial resolution, while deliberate re challenge produced attenuated but confirmatory recurrence (Naranjo score: 8, "Probable"; WHO-UMC: "Probable/Likely"). Complete recovery ensued upon permanent discontinuation, permitting unhindered ARV completion. Conclusions: This meticulously documented re challenge case delineates ERIG's acute Type I hypersensitivity profile, informing premedication strategies, HRIG substitution criteria, and pharmacovigilance imperatives in India's 20 million annual Category II/III exposures.