Serological cross-reactions between Rickettsia typhi, Proteus vulgaris OX19, and Legionella bozemanii in a series of febrile patients.

The sera of 19 patients with a febrile disease of undetermined etiology were positive in the indirect immunofluorescence assay (IFA) to Legionella bozemanii serogroup 1 (Lb) and Rickettsia typhi (Rt). To both antigens, high titers of IgG-class and IgM-class antibodies were demonstrated. Several of the patients also had positive IFA and Weil-Felix reactions to Proteus vulgaris OX19 (PX 19). A sharp reduction of the serotiters to all three antigens was achieved by absorption of the sera with any one of the organisms. We demonstrated, by crossed immunoelectrophoresis with an Lb extract and a rabbit reference anti-Lb serum, that a heat-stable and trypsin-resistant antigen (antigen no. 1) reacted consistently with patients' sera that had been incorporated into an intermediate gel. Sera from five patients with high-titer IFA reactions to Rt, but no reaction to Lb, showed no interaction with antigen no. 1.