Weil-Felix Reaction

WEIL-FELIX REACTION (Weil-Felix), a serodiagnostic test for the recognition of typhus fever by means of agglutination (see) of the test serum with Bacillus proteus X₁₉. The reaction is named after the authors Weil and Felix, who in 1915 isolated from typhus patients a microbe identical in its properties, as discovered in cultures, to the common proteus (Bacillus proteus vulgaris of Hauser), but which was highly agglutinated by the sera of typhus patients. This microbe was named by them Bacillus proteus X₁₉. The majority of authors (Dietrich, Etinger, Soucek) found this reaction in 100% of cases during repeated examinations throughout the course of the disease in typhus patients, with a positive reaction often noted as early as the 3rd or 4th day of the illness. The degree of serum dilution that ensures a diagnosis of typhus fever is determined by some authors as 1:100, and by others as 1:200, with some (Wolff) pointing out the necessity of noting the final results of the reaction no later than two to three hours after the test tubes have been kept at 37°. The reaction is performed in the same way as other agglutination reactions. Instead of the commonly used live 24-hour cultures, one can also use cultures killed by heating to 60° (Sadov), 50% alcohol (Bich, Sontag), or ether (Shapshev) when performing the Weil-Felix reaction. The specificity of the reaction is very high. Among many thousands of observations, only isolated cases of a positive Weil-Felix reaction with the sera of non-typhus patients have been described (and even then, only in very low dilutions of serum, not exceeding 1:200). As for the theoretical interpretation of the Weil-Felix reaction, despite more than 10 years of study, there is no consensus. Of the numerous theories, only two are currently defended: one, which considers X₁₉ to be the causative agent or a mutant of the causative agent of typhus and the Weil-Felix reaction to be a standard immunity reaction (Friedberger, Weigl, Breinl, Fejgin), and the other, which considers the Weil-Felix reaction to be a para-agglutination and X₁₉ to be a para-microbe (Otto, Papamarku, Etinger, Grütz, Sadov, Zilber). The most important argument for this latter theory is the fact that Bacillus proteus vulgaris acquires the properties of X₁₉ upon contact with the typhus virus. The question of the nature of the Weil-Felix reaction has great theoretical significance. The reaction can also be obtained with the serum of a corpse, which has important diagnostic significance (Pashin).