Weil-Felix test
Summary
The Weil-Felix test is a historical serological assay for rickettsial infection that relies on cross-reaction between rickettsial antibodies and Proteus vulgaris O antigens. It is insensitive and non-specific and has been replaced by indirect immunofluorescence and PCR.
Detail
The test exploits shared carbohydrate epitopes between certain Rickettsia species and the OX-19, OX-2, and OX-K antigens of Proteus strains, so patient serum agglutinates Proteus. Classic patterns: Rocky Mountain spotted fever and epidemic typhus agglutinate OX-19 and OX-2; scrub typhus (Orientia tsutsugamushi) agglutinates OX-K; and Coxiella burnetii, the agent of Q fever, is negative for all, which is one of several features setting it apart from other rickettsiae along with its lack of a rash, its spore-like form, and its transmission by inhalation rather than arthropod bite. The test is a heterophile agglutination assay in the same conceptual family as the monospot test for EBV. Because sensitivity and specificity are poor and antibodies appear late, it has no place in modern practice; the key point for exams is that rickettsial disease is a clinical diagnosis treated empirically with doxycycline, including in children, since delaying treatment for serology increases mortality.
Sources
- Levinson Medical Microbiology and Immunology
- First Aid for the USMLE Step 1
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