Question
Scrub typhus.
Answer
Scrub typhus is a rickettsial zoonosis caused by Orientia tsutsugamushi (formerly classified as Rickettsia tsutsugamushi), transmitted to humans by the bite of infected larval mites (chiggers), particularly of the genus Leptotrombidium, which maintain the organism transovarially in nature (mites act as both vector and reservoir).
Clinical features: an eschar (a painless, black, necrotic ulcer with a surrounding erythematous margin) at the site of the chigger bite is a highly characteristic clue, though often found in concealed body areas (axilla, groin) and easily missed; accompanied by fever, headache, myalgia, generalized lymphadenopathy, and a maculopapular rash; severe disease can progress to interstitial pneumonitis, myocarditis, meningoencephalitis, and multiorgan failure if untreated.
Laboratory diagnosis:
- Weil-Felix reaction — agglutination against Proteus OX-K antigen only (distinguishing it from typhus-group and spotted-fever-group rickettsioses, which react with OX-19/OX-2); a low-cost but only moderately sensitive/specific test.
- Indirect immunofluorescence assay (IFA) — the gold standard serological test, detecting specific IgM/IgG antibody.
- ELISA for IgM antibody — widely used, more practical than IFA in many settings.
- PCR on eschar swab/blood — allows early, specific diagnosis before antibody develops.
Treatment: doxycycline is the drug of choice (rapid clinical response is itself sometimes used as a diagnostic/therapeutic clue); azithromycin as an alternative, particularly in pregnancy and children.
Prevention: avoiding chigger-infested vegetation/scrub areas, protective clothing, insect repellents; no licensed vaccine is currently available.