Principle:

• Weil-Felix reaction is a serological test for detecting Rickettsial antibody in serum of patient using heterophile antigen. This is a non specific agglutination reaction.
• Antibodies produced against Rickettsial antigen cross reacts with OX19 and OX2 strain of Proteus vulgaris and OXK strains of Proteus mirabilis. This cross reactivity between some strains of Rickettsiae and Proteus is observed for the first time by Weil and Felix in 1916 and developed a serological test for detecting Rickettsial disease.
• Patients infected with R. rickettsiae develops antibody which is reactive to Proteus vulgairis OX19 and OX2 while patients infected to R. prowazekii develops antibody which is reactive to Proteus vulgaris OX19.
• Anti-Rickettsial antibodies  develops within 5-7 days of infection and titer reaches peak within 2 weeks. Serum of typhus patient agglutinate in presence of non motile Proteus vulgaris  OX strain.
• Somatic antigen (O) of Proteus vulgaris reacts with anti-Rickettsial antibody giving agglutination reaction.

Procedure:

Weil-Felix test can be performed in slide or in test tube.

1. Weil-Felix test by Slide method:

• Place 50-100µl of patient serum on the slide
• Add a drop of desired antigen ( Proteus OX19 or OX2 or OXK)
• Mix the suspension by rotating the slide for 1 minutes
• Visible agglutination indicates the positive test

2. Weil-Felix test by Tube method

• Dilute the patient serum (two fold) using 0.25% phenol saline and make final volume of 1 ml.
• Add 1 drop of desired antigen ( Proteus OX19 or OX2 or OXK) in each test tube
• Incubate the test tubes at 50-55 C for 4-6 hours
• Visible flocculation or granulation on slight agitation indicates the positive test

Application:

• Very useful for diagnosis in case of epidemic typhus

Limitations:

• Weil-Felix test is non specific test and cannot be relied for diagnosis of rickettsial infection. Orientia tsutsugamushi gives false negative test while Proteus spp give false positive test.
• Sensitivity= less than 33%
• Specificity= 46%

Weil-Felix test: Principle, procedure and application

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