A systematic study of 74 ticks, the infectivity or non-infectivity of which was determined by biting experiments, inoculation of emulsions, and specific immunity tests, showed the presence in some instances of several types of microorganisms morphologically resembling the inciting microorganism of spotted fever. The most frequently isolated was B. rickettsiformis, n. sp., those less commonly encountered were B. pseudoxerosis, n. sp., and B. equidistans, n. sp. These organisms are non-pathogenic for the guinea pig, rabbit, and Macacus rhesus. In morphological features they resemble the forms found in smears and sections of the ticks, yet their presence had no relation to infectivity. Immunologically they are not related to the spotted fever virus. All three are pleomorphic under cultural conditions, and the question arises whether or not the minute non-pathogenic Rickettsia forms and the somewhat coarser symbionts found in Dermacentor andersoni are morphological variations due to variations in nutrition, oxygen tension, tissue reactions, etc., in the different tissues and cells in which the organisms are embedded. At all events, the differentiation of the non-pathogenic Rickettsia-like organisms from Dermacentroxenus rickettsi is extremely difficult. In definitely infective ticks of the present study intranuclear forms were not constant.

A point of special interest is that these non-pathogenic micro-organisms from ticks grow best at room temperature, in this respect resembling culturally certain flagellates inhabiting the alimentary tract of insects. The difficulty of obtaining initial growth on artificial media and the gradual adaptation to less specialized media are other notable characteristics of these organisms. The possibility that B. rickettsiformis is a non-pathogenic phase of the spotted fever organism, comparable with the avirulent flagellate culture forms of Leishmanias, seems remote in view of the negative immunological findings.

Hereditary transmission of B. rickettsiformis is clearly indicated by its presence in large numbers in ovaries and egg cells, a characteristic also of the spotted fever organism and of other insect-borne Rickettsiæ.

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