Injurious to Man. 40 



Dcrmaccntor venustus, Banks. 



The Eocky Mountain Spotted Fever Tick. 



(Plate III.) 



Dermacentor venustus is the carrier of the human tick-fever 

 occurring in Montana, Idaho, and several other States in the 

 Eocky Mountain region of the United States. The virulence of 

 this disease varies in different localities, the death-rate being from 

 5 to 7 per cent, in Idaho, whereas it is about 70 per cent, 

 in the Bitter Eoot Valley of Montana. This fever has been known 

 for many years in the localities in which it occurs, and it is 

 probable that it already existed there before the settlement of the 

 country by Europeans. It is not yet known with certainty 

 whether Eocky Mountain Spotted Fever is caused by a virus 

 secreted by the tick or by the transmission of some minute blood- 

 parasite by its bite. Dcrmaccntor venustus is known to occur 

 from British Columbia southward to northern New Mexico, and 

 from the foothills of the Eocky Mountains in Colorado to tbe base 

 of the Cascade Eange in Oregon and California. The immature 

 stages of the tick are generally found on various small mammals, 

 such as ground-squirrels, chipmunks, etc. The adults are usually 

 found on the larger domestic animals, especially on cattle and 

 horses, but also on hogs, sheep, and more rarely on dogs. Human 

 beings are bitten by the adult ticks and thus contract the 

 complaint. It is possible that the wild mountain goats may act 

 as a reservoir of infection. Gophers, rock-squirrels, woodchucks, 

 chipmunks, and mountain rats are also susceptible to Spotted 

 Fever, and may perhaps be sources of infection. 



Dr. C. Wardell Stiles has described the tick which causes the 

 disease under a new name (Dcrmaccntor andersoni, Stiles), and is 

 of opinion that D. venustus is a different tick. 



Dcrmaccntor occidentalism Neumann. 



This species, which has been confused with D. venustus, also 

 frequently attacks man, and often occurs in great numbers on 

 domestic live-stock. It is not known to spread disease, however. 

 Hooker, Bishopp, and Wood state that in Western California and 

 Western Oregon it is the most common tick which attacks man. 

 A number of cases were brought to their attention where the bite 

 of this species had caused considerable local inflammation, which 

 in some instances required a physician's attention. They say it is 

 quite common for the rostrum to be broken off when the ticks are 



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