U.S. Department of Health, Education, and Welfare, Public Health Service, National Institutes of Health, National Institute of Allergy and Infectious Diseases, Epidemiology Branch, Rocky Mountain Laboratories, Hamilton, Montana 59840
Adult Dermacentor andersoni ticks were collected by flagging vegetation in 18 canyons bordering the Bitterroot Valley, Montana, an area where nearly 400 cases of Rocky Mountain spotted fever (RMSF) have occurred since 1900. Three hundred and nine (8.3%) of the 3,705 ticks collected contained hemocyte-associated rickettsia-like organisms of three morphologic types, coccobacillary, fine bacillary, and coarse bacillary (long forms). Only the coccobacillary and fine bacillary organisms stained with fluorescein-conjugated antibody specific for the spotted fever group. One hundred and six isolates of spotted fever-group rickettsiae obtained by inoculation of Vero cells with suspensions of hemolymph test-positive ticks were serologically typed by microimmunofluorescence. Four sharply distinct serotypes were obtained, including Rickettsia rickettsii (10 strains), R. montana (8 strains), R. rhipicephali (47 strains), and a hitherto undescribed serotype referred to as 369-C (41 strains). All but two isolates were obtained from west-side canyons where virtually all cases of RMSF had been acquired. The four serotypes were widely distributed on the west side as evidenced by their presence in 5–11 of the 13 collecting sites. Each serotype induced distinctive plaques and cytopathogenicity in Vero cell culture.