Sensitive Enzyme-Linked Immunosorbent Assay for Detection of Antibodies Against Typhus Rickettsiae, Rickettsia prowazekii and Rickettsia typhi

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Abstract
An enzyme-linked immunosorbent assay (ELISA) was developed for titration of rickettsial antibodies in human and animal sera. Two preparations of soluble typhus-group antigens were obtained from R. typhi and R. prowazekii by ether extraction: a standard antigen from infected yolk sacs (YS antigen) and 1 free of yolk sac contaminants from Renografin-purified rickettsiae (PR antigen). Rabbit, mouse and guinea pig sera were obtained by immunization with viable purified R. typhi or R. prowazekii. Human sera were obtained from individuals who recovered from laboratory infections with either typhus rickettsia months or years previously. Goat-derived anti-immunoglobulins were conjugated to alkaline phosphatase with glutaraldehyde. Although the PR and YS antigens gave equivalent antibody titers in the complement fixation test, the PR antigen was clearly superior in the ELISA. With this antigen, the titration curves of all antisera were linear over a wider range of serum concentrations and the titers were higher than with the YS antigen. With YS and PR antigens, ELISA titers were higher than those obtained by complement fixation by 1 and 2 orders of magnitude, respectively. In human sera, immunoglobulin G and immunoglobulin M antibodies were demonstrated by their respective anti-immunoglobulins and by differential susceptibility to ethanethiol. ELISA titers showed some type specificity, whereas none was observed in complement fixation tests. The ELISA is highly sensitive, reproducible and easily adaptable to various laboratories.