Comparative studies were made of the microorganisms present in D. variabilis ticks, some of which served as a control series while the remainder were exposed to infection with D. rickettsi and thereafter maintained under various conditions.

All female ticks contained in their ovaries a coccoid intracellular microorganism.

About 50 per cent of all ticks after refeeding contained rickettsia-like microorganisms in variable numbers in nearly all organs.

The groups of ticks exposed to infection with the virus of spotted fever, in addition to the above mentioned microorganisms, usually harbored large numbers of D. rickettsi, distinguishable with certainty from the non-pathogenic organisms only by their localization in nuclei of tick cells.

No influence upon the size, number, or distribution of either the non-pathogenic rickettsiae or D. rickettsi in ticks was attributable to refeeding, variations of the temperature of incubation, or variations of the length of the period of incubation.

We conclude from the results of these studies that the non-pathogenic rickettsiae which occur in D. variabilis ticks have no well defined relationship to D. rickettsi since they differ from the latter organism not only in the absence of virulence and immunizing properties, but also in their distribution in tick tissues and inability to multiply in the nuclei of cells.

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