Weil Felix Test

Excerpt

First described in 1916, the Weil-Felix reaction is a test used to diagnose rickettsial infections. While it has largely been replaced with new serological techniques, the Weil-Felix test continues to hold great importance in resource-limited areas where more advanced methods are unavailable. The known pathogenic rickettsia species are gram-negative, obligate intracellular bacteria that include an increasing number of identified organisms belonging to seven genera (Rickettsia, Orientia, Ehrlichia, Anaplasma, Neorickettsia, Candidatus, Neoehrlichia, and Coxiella). They are closely related and are traditionally separated into three groups: the epidemic and endemic typhus group, the scrub typhus group, and the spotted fever group.

The test was developed upon the observation that certain serotypes of Proteus bacteria display antigenic cross-reactivity with Rickettsia species. Through the isolation of these Proteus antigens, a heterophile agglutination reaction was developed to identify antibodies against the Rickettsia disease groups. P. vulgaris OX19 antigen reacts with antibodies to the typhus group (TG), P. mirabilis OXK antigen reacts with antibodies to the scrub typhus group (STG), and both P. vulgaris OX2 and OX19 antigens react with antibodies to the spotted fever group (SFG).

Due to its low sensitivity and specificity, the Weil-Felix test has fallen out of favor in most clinical settings, and its use is no longer recommended in routine practice. The current gold standard in diagnosing rickettsial infections is indirect immunofluorescence, which is available through most state health departments in areas where infections are common.