Scrub typhus (Orientia tsutsugamushi) is endemic in India. The characteristic eschar at the bite site results from which pathological process, and what is the confirmatory serological test?

• A Eosinophilic infiltration at the bite site; confirmed by complement fixation test
• B Lymphocytic vasculitis at the mite bite site; confirmed by Weil-Felix test (OX-K titres ≥1:80) ✓
• C Necrotising neutrophilic infiltrate with secondary infection; confirmed by blood culture
• D Direct replication of tsutsugamushi in keratinocytes with apoptosis; confirmed by PCR of eschar biopsy

Explanation

Scrub typhus eschar (tache noire or inoculation eschar) forms at the Leptotrombidium mite bite site due to local endothelial invasion and lymphocytic vasculitis with thrombosis and necrosis. It is typically painless, has a central necrotic black crust with surrounding erythema, and is found in concealed areas (groin, axilla, behind ears). Serological confirmation uses the Weil-Felix agglutination test — O. tsutsugamushi reacts with Proteus mirabilis OX-K strain (titre ≥1:80 is significant). However, Weil-Felix is non-specific; IFA (indirect fluorescent antibody test using type-specific antigens) is the gold standard. Doxycycline is curative and should be started empirically in endemic areas without waiting for serology.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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