McKiel et al. first described isolation of a novel rickettsia, R. canada (sic) strain 2678. The agent was isolated in embryonated chicken eggs from a pool of Haemaphysalis leporispalustris ticks collected from a domestic rabbit used as a sentinel animal near Ottawa, Ontario, Canada in 1962-63. A second isolate, strain H299, was recovered from ticks removed from a snowshoe hare in October 1964, but this strain was lost. Subsequent field work failed to detect this agent until 1980, when another isolate CA410 of R. canada was obtained from H. leporispalustris tick from a black-tailed jack rabbit, Lepus californicus from Mendocino County in California. Serologic evidence has been described that suggests R. canadensis may cause a febrile disease in man.

The biological and phylogenetic placement of R. canadensis has been problematic. While R. canadensis is antigenically more closely related to typhus group rickettsiae, like pathogenic spotted fever group rickettsiae, it causes a generalized infection in ticks, can be transmitted by ticks and is maintained transstadially and transovarially in them; it also invades the nuclei of cells⁶ and has both rOmpA and rOmpB proteins like spotted fever rickettsiae. On the other hand, like typhus group rickettsiae R. canadensis does not rapidly kill embryonated  chicken eggs where it grows to high titers, it hemolyzes red blood cells, it is susceptible to erythromycin, and it forms small plaques more slowly than spotted fever group rickettsiae; it also has a G+C % similar to that of typhus group rickettsiae. DNA-DNA hybridization and sequence comparisons of specific rickettsial genes like those of 16S rRNA and groEL place it at a similar distance from typhus and spotted fever group rickettsiae much like the mite and flea transmitted species, R. akari and R. felis. This data suggested that R. canadensis may be representative of a distinct lineage of rickettsiae which is poorly represented as yet among characterized isolates but which, like R. bellii, shares some ancestral characteristics with both classic typhus and spotted fever group rickettsiae; in that sense it may more closely resemble forms ancestral to this genus.

The above description has been condensed from a paper presented at the 2009 American Society for Rickettsiology meeting in Hilton Head, South Carolina: Eremeeva ME, Madan A, Shaw C, Tang K, Dasch GA. 2009. New Perspectives on Rickettsial Evolution from New Genome Sequences of Rickettsia, particularly R. canadensis, and Orientia tsutsugamushi.