The views of Ricketts, Wolbach and Frick as to etiology are discussed under that heading.
Geographical Distribution.—The two best known regions of prevalence of the disease are the Bitter Root Valley of Montana and the Snake River Valley of Idaho. It is also reported from limited sections of Washington, Oregon and California, as also from Nevada and Utah.
In Wyoming it is rather widely distributed.
Etiology and Epidemiology
Etiology.—Wolbach states that he has noted certain bacterial forms in the endothelial cells of the blood vessels of guinea pigs infected with the virus, as well as a very general distribution in infected ticks.
There are two morphological types—one, a chromatic-staining lanceolate diplococcoid organism, found in the circulating blood as well as in the endothelial cells, the other type—a blue-staining rod-shaped form.
Ricketts noted certain chromatin-staining bacteria, in man and in eggs of infected ticks, which were about 1 micron long by ⅓ micron broad, showed chromatin staining, were about the size of B. influenzae, and appeared as two lanceolate-shaped bodies. These bodies are now considered as belonging to the Rickettsia group of organisms. Wolbach has named the organism of spotted fever of the Rocky Mountains Dermacentroxenus rickettsi. In infected guinea pigs Wolbach found these bodies particularly abundant in the endothelial cells.
Frick has also found bodies within the red cells of human cases and infected guinea pigs, as well as extracellularly, which showed chromatin-staining characteristics, there often being an elongated reddish body joined on to a larger blue-staining protoplasm.
In 1902 Wilson and Chowning reported the finding of piroplasm-like bodies in the blood of human cases of Rocky Mountain Spotted Fever. Ricketts proved that the virus was not filterable. A tick, Dermacentor andersoni transmits the disease.
Epidemiology.—The transmitting tick, D. andersoni (D. venustus) lives on the domesticated animals of the region of geographical distribution of the disease. Ricketts showed that the reservoir of the virus was to be found in ground squirrels, chipmunks, mountain rats, etc., and that ticks feeding on these rodents become infected and transmit the disease to man. The guinea pig, white rat and monkey are also susceptible.