Epidemic typhus — DTM&H MCQ

Correct answer: A — Epidemic typhus

Epidemic typhus (A) is correct because the combination of louse exposure in a crowded, unsanitary field setting, high fever, severe headache, and a centrifugally spreading macular rash on the trunk and limbs, plus a positive Weil-Felix OX-19 agglutination, is classic for Rickettsia prowazekii infection. R. prowazekii is transmitted by the human body louse (Pediculus humanus corporis), which thrives in refugee camps, conflict zones, and other settings with poor hygiene, exactly the exposure described. The OX-19 antigen cross-reacts with typhus group rickettsiae (epidemic and murine typhus), and when this is paired with a definite louse vector rather than a flea or mite, epidemic typhus is the diagnosis to select. Prompt recognition matters because untreated epidemic typhus has significant mortality, and doxycycline is highly effective. Why the other options are wrong: E. Scrub typhus: caused by Orientia tsutsugamushi, transmitted by trombiculid mite (chigger) bites, classically gives OX-K agglutination on Weil-Felix, not OX-19, and often has an eschar. C. Relapsing fever: caused by Borrelia species also transmitted by body lice or ticks, but presents with recurrent febrile episodes and spirochaetes seen on blood film, not a Weil-Felix reactive illness with this rash pattern. B. Murine typhus: caused by Rickettsia typhi, transmitted by the rat flea, not lice; the vector history in the stem excludes this despite similar OX-19 reactivity. D. Q fever: caused by Coxiella burnetii, acquired by inhalation of aerosols from infected livestock (not lice), does not produce a Weil-Felix reaction, and rash is uncommon. Key point: Louse-borne exposure plus OX-19 positive Weil-Felix in a crowded, unsanitary setting points to epidemic typhus (Rickettsia prowazekii), whereas OX-K points to scrub typhus and a flea vector points to murine typhus.

Reference: UK Health Security Agency, Imported rickettsial infections to the United Kingdom 2015-2020, Journal of Infection (UKHSA Rare and Imported Pathogens Laboratory guidance on rickettsial diagnostics and doxycycline treatment), https://researchportal.ukhsa.gov.uk/en/publications/imported-rickettsial-infections-to-the-united-kingdom-20152020/