Rickettsial treatment — DTM&H MCQ

Correct answer: A — Doxycycline

The correct answer is A, Doxycycline, because this presentation (fever, headache, thrombocytopenia and an eschar in a returning traveller from South East Asia) is classic for scrub typhus caused by Orientia tsutsugamushi, an intracellular pathogen transmitted by chigger mites. Doxycycline covers rickettsial organisms by inhibiting bacterial protein synthesis and penetrating intracellularly, which beta-lactams and other agents listed cannot achieve. With malaria and dengue excluded, an eschar is the single most specific clinical sign pointing to a rickettsial or scrub typhus infection, and UK clinical practice recommends starting doxycycline empirically before serology or PCR confirmation because delay increases risk of complications. Early empirical treatment is safe, cheap and shortens illness duration, which is why it is started on clinical suspicion alone in this scenario. Why the other options are wrong: E. Chloroquine: this is an antimalarial with no activity against Orientia tsutsugamushi or other rickettsiae, and malaria has already been excluded by negative testing. D. Cefalexin: a beta-lactam that only works on extracellular bacterial cell wall synthesis, so it has no useful activity against intracellular organisms like Orientia tsutsugamushi. B. Metronidazole: active against anaerobes and protozoa such as Giardia or Entamoeba, which is irrelevant to a febrile eschar illness with thrombocytopenia. C. Oseltamivir: an antiviral for influenza; there is no viral aetiology suggested by an eschar, and influenza does not classically cause eschar or this pattern of findings. Key point: an eschar plus fever and thrombocytopenia in a traveller from Asia should trigger empirical doxycycline for scrub typhus without waiting for confirmatory serology.

Reference: Nolder D et al. Imported rickettsial infections to the United Kingdom, 2015-2020. Journal of Infection, 2023. PubMed: https://pubmed.ncbi.nlm.nih.gov/36948252/ (see also BNF: Doxycycline, indications, https://bnf.nice.org.uk/drugs/doxycycline/)