The Weil-Felix reaction for diagnosis of rickettsial infections uses agglutination of Proteus vulgaris OX strains (OX-2, OX-19, OX-K). A patient with high fever, eschar, and regional lymphadenopathy shows OX-K agglutination (titer 1:160) with negative OX-2 and OX-19. This pattern is diagnostic of:
- A Epidemic typhus (Rickettsia prowazekii)
- B Rocky Mountain spotted fever (Rickettsia rickettsii)
- C Scrub typhus (Orientia tsutsugamushi) ✓
- D Murine typhus (Rickettsia typhi)
Correct answer: C. Scrub typhus (Orientia tsutsugamushi)
Explanation
Scrub typhus, caused by Orientia tsutsugamushi, characteristically causes agglutination of Proteus OX-K strain only (titer ≥1:80 significant), with the classic triad of fever, eschar at the mite bite site, and regional lymphadenopathy. OX-19 and OX-2 agglutination is positive in epidemic typhus (R. prowazekii) and spotted fever group rickettsioses (R. rickettsii), but not scrub typhus. The Weil-Felix test has poor sensitivity and specificity overall; IgM ELISA for Orientia is the preferred confirmatory test.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.