DETECTION OF RICKETTSIAL ANTIBODIES USING WEIL-FELIX (OXK AND OX19) ANTIGENS AND THE INDIRECT IMMUNOPEROXIDASE ASSAY

Rickettsiae are grouped into 3 antigeni-cally defined groups: those causing scrub typhus, typhus and spotted fever. Although rickettsiae can be isolated from or detected in clinical specimens, serological tests still remain an indispensable tool in the diagnosis of rickett-sial diseases. The complement fixation test widely used in the past is being replaced by other tests which make differentiation of im-munoglobulin classes possible. These include microimmunofluorescence, latex agglutination, indirect hemagglutination, immunoperoxidase (IIP), and enzyme-linked immunosorbent as-say. The IIP assay has been developed for the diagnosis of Orientia tsutsugamushi (OT) (Yamamoto and Minamishima, 1982), Rickett-sia typhi (RT) (Kelly et al, 1988) and spotted fever group rickettsiae (SFGR) (Raoult et al, 1985). In this assay, specific antibody in patient serum reacts with rickettsial antigen dotted on slides and the level of IgG and IgM can be measured. The IIP titer of the test serum is the reciprocal of the highest dilution of serum that showed a positive reaction. With the IIP technique, specific IgM antibody can be de-tected on day 3 or 4 after the onset of illness and IgG antibody can be detected after day 7 (Kawamura et al, 1995). The well-known cross-reaction between certain Proteus strains and Rickettsia spp has been a presumptive diagnosis of rickettsial infection for many years (Weil and Felix, 1916). By the Weil-Felix test, agglutinating antibodies are detectable after 5 to 10 days following the onset of symptoms (Amano et al, 1992). Although poor sensitivity and specificity of the Weil-Felix test have been reported (Hechem