A laboratory reports a Weil-Felix reaction as follows: Proteus OX19 positive at 1:320, Proteus OX2 positive at 1:80, Proteus OXK negative. This pattern is most consistent with which infection?

• A Scrub typhus caused by Orientia tsutsugamushi
• B Murine typhus caused by Rickettsia typhi
• C Epidemic typhus caused by Rickettsia prowazekii ✓
• D Rickettsialpox caused by Rickettsia akari

Explanation

The Weil-Felix test exploits cross-reactivity between rickettsial antigens and Proteus vulgaris strains. Strong OX19 reactivity indicates the typhus group (Rickettsia prowazekii, Rickettsia typhi), while isolated OXK positivity indicates scrub typhus (Orientia tsutsugamushi). A high OX19 titre with low OX2 favours epidemic typhus; murine typhus also gives OX19 but the question pattern with dominant OX19 is classically taught as epidemic typhus. Rickettsialpox does not react with any Proteus antigen.

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

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