A patient from Andhra Pradesh presents with acute febrile illness, eschar (tache noire) at the site of tick bite, and rash. IgM serology for Rickettsia is positive. Weil-Felix reaction shows agglutination of OX-2 proteus strain but not OX-19 or OX-K. The most likely organism is:
- A Rickettsia rickettsii (Rocky Mountain spotted fever)
- B Rickettsia conorii (Indian tick typhus / Mediterranean spotted fever) ✓
- C Orientia tsutsugamushi (scrub typhus)
- D Rickettsia prowazekii (epidemic louse-borne typhus)
Explanation
Rickettsia conorii causes Indian tick typhus (Mediterranean spotted fever), transmitted by Rhipicephalus sanguineus (brown dog tick). Characteristic features include eschar (tache noire) at the bite site, fever, and maculopapular rash. Weil-Felix reaction pattern: R. conorii agglutinates OX-19 and OX-2 (both), but OX-2 predominates — however if only OX-2 is positive, it still points to spotted fever group rickettsia, with conorii being the dominant species in India. Scrub typhus (O. tsutsugamushi) is OX-K positive, mite-borne. R. prowazekii is OX-19 dominant (epidemic typhus). Treatment is doxycycline.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.