Scrub typhus — DTM&H MCQ

Correct answer: B — Doxycycline

The correct answer is B, Doxycycline. This vignette describes scrub typhus (Orientia tsutsugamushi infection), a rickettsial disease transmitted by chigger mites in scrub vegetation across rural Southeast Asia, including northern Thailand. The classic triad of fever, headache and an eschar at the bite site (often in a warm, moist area such as the groin or axilla), accompanied by thrombocytopenia and mild transaminitis, is highly specific for this diagnosis. Doxycycline is the first line agent because it is rapidly bactericidal against intracellular Orientia and other rickettsiae, and empirical treatment should be started on clinical suspicion without waiting for confirmatory serology, since delay increases the risk of complications such as meningoencephalitis, pneumonitis and multi organ failure. Why the other options are wrong: E. Chloroquine: this is an antimalarial acting on the erythrocytic stage of Plasmodium species; it has no activity against Orientia tsutsugamushi and would leave the underlying rickettsial infection untreated. D. Praziquantel: this is used for trematode and cestode infections such as schistosomiasis and is irrelevant to an intracellular bacterial pathogen with no helminthic features in this stem. C. Metronidazole: this covers anaerobic bacteria and protozoa such as Giardia and Entamoeba, none of which explain an eschar, fever and thrombocytopenia in this exposure history. A. Cefalexin: beta lactams, including cephalosporins, are ineffective against Orientia tsutsugamushi because it lacks a peptidoglycan cell wall target and survives intracellularly, so cell wall active agents fail clinically. Key point: an eschar plus fever after scrub exposure in Southeast Asia should trigger empirical doxycycline for scrub typhus without waiting for serology.

Reference: BNF (NICE), Tetracyclines: Doxycycline, indications and use in rickettsial infections, https://bnf.nice.org.uk/drugs/doxycycline/