Characterization of Rickettsia Tsutsugamushi Strains in Two Species of Naturally Infected, Laboratory-reared Chiggers

The strains of Rickettsia tsutsugamushi found in naturally infected, laboratory-reared Leptotrombidium (Leptotrombidium) arenicola and L. (L.) fletcheri chiggers were characterized by direct immunofluorescence (FA) and by mouse and monkey virulence tests. The strains existing in the L. (L.) arenicola chiggers consisted of different combinations of TA716, TA763, TA686, Karp, and Kato. In addition to these five strains, Gilliam was found in the L. (L.) fletcheri chiggers. Results indicate that individual chiggers can be simultaneously infected with several antigenic strains of R. tsutsugamushi. Although these antigens appear to remain stable within familial lines when several generations were viewed, the antigenic patterns observed in two succeeding generations did not always correlate. This variable expression of antigens was considered to be due to a quantitative fluctuation from one generation to the next in the strains of rickettsiae combined with a lack of sensitivity of the direct FA test in detecting small numbers of antigenically different rickettsiae. Phenotypic variation was considered to be a less probable explanation. Morbidity and mortality were minimal in ICR mice fed upon by individual chiggers of either species, but infection rates were 85-99%. Tissue suspensions prepared from mice infected by L. (L.) arenicola produced higher mortality and longer duration of illness in mice than those prepared from L. (L.) fletcheri-infected mice. Silvered leaf and cynomolgus monkeys were fed upon by the two species of chiggers or inoculated with the mouse tissue suspensions. In both cases, minimal clinical responses were observed.