Weil-Felix test is a heterophile agglutination test used in rickettsial disease diagnosis. The test relies on cross-reactive antigens between Rickettsia species and certain Proteus OX strains. However, the test is NEGATIVE in which rickettsial disease?

• A Epidemic typhus (R. prowazekii)
• B Rocky Mountain spotted fever (R. rickettsii)
• C Scrub typhus (Orientia tsutsugamushi) ✓
• D Murine typhus (R. typhi)

Explanation

The Weil-Felix test detects antibodies cross-reactive with Proteus OX-2, OX-19, and OX-K antigens. Scrub typhus (Orientia tsutsugamushi) characteristically agglutinates only OX-K (Proteus mirabilis OX-K), while R. prowazekii, R. typhi, and R. rickettsii show OX-2 and/or OX-19 reactivity. However, the question addresses when the Weil-Felix is completely non-reactive or unreliable — Q fever (Coxiella burnetii) gives a uniformly negative Weil-Felix test because it lacks the relevant cross-reactive polysaccharide shared between Rickettsia spp. and Proteus strains.

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

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