Rickettsia typhi is the agent of endemic or murine typhus. It is maintained in an enzootic cycle (rats, opossums and other small mammals) and the associated ectoparasites (fleas, mites and lice). Infection of humans occurs with the attendant scratching of the insect bites, which introduces the flea feces into abraded skin where infection of endothelial tissues may commence.
Antibody detection is dependent on 2 immunodominant antigens, both found in the s-layer surrounding the bacterium. The LPS (lipopolysaccharide) antigen is most commonly detected, both in acute samples and seroprevalence studies of healthy blood donor sera. The OmpB protein is also part of the s-layer and reactivity to this antigen is usually found along with LPS reactivity. The optimized mixture of these antigens (RTG-96) provides the most sensitive and specific ELISA for IgG class antibody assay. Testing for IgM class antibody will continue to use only pure OmpB as antigen.
Data for individual antigens is presented in the Flea-Borne Typhus poster. To summarize, IgG antibody was detected by OmpB antigen alone in 39% of positives and by LPS antigen alone in 62% of positives. Both antigens were detected by only 50% of the positive sera. The optimized mix of these antigens detected 100% of positives.
The ELISA antigen currently used is a full-length OmpB passenger domain, but we are transitioning to the use of a recombinant OmpB passenger domain. This antigen has been provided with native methylation enzymatically in order to be a sensitive and specific as the antigen that previously was purified from live R. typhi. This antigen will also be used in a reciprocal cross-absorption protocol to differentiate endemic (typhi) from epidemic typhus (prowazekii). This should be available by June-July 2015.