Scrub typhus is caused by Orientia tsutsugamushi, transmitted by the bite of infected larval mites (chiggers) of the genus Leptotrombidium. Which clinical feature, when present, is highly specific for the diagnosis of scrub typhus?
- A Maculopapular rash on palms and soles
- B Eschar (painless blackened necrotic ulcer) at the bite site
- C Weil-Felix reaction positivity with Proteus OXK
- D Both B and C are specific for scrub typhus ✓
Explanation
Eschar (a painless blackened necrotic ulcer at the chigger bite site, usually in skin folds) is pathognomonic for scrub typhus when present, but it occurs in only ~50% of cases. Weil-Felix reaction using Proteus OXK agglutination is positive in scrub typhus (Orientia tsutsugamushi cross-reacts with OXK antigen) — this is distinct from epidemic typhus (Rickettsia prowazekii, positive with OX-2 and OX-19) and murine typhus (R. typhi, positive with OX-2 and OX-19). Both eschar (when present) and positive OXK together are highly specific. Maculopapular rash on palms and soles is characteristic of syphilis (secondary) and Rocky Mountain spotted fever, not scrub typhus.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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