TRANSMITTERS OF SPOTTED FEVER 363 



again attack their rodent hosts. After dropping off these and 

 transforming into adults they no longer pay any attention to the 

 rodents but seek larger animals, especially preferring horses and 

 cattle, though they readily attack other large wild and domestic 

 animals and man. Their original wild hosts were probably the 

 mountain goats, elk and other wild game of the region, but with 

 the supplanting of these by domestic animals the latter have 

 become the main host animals of the ticks. Unlike most ticks, 

 this species may take two or even two and a half years to com- 

 plete its life cycle under unfavor- 

 able conditions. The winter is 

 passed in either the nymphal or 

 adult stages. 



Dermacentor venustus is found in a 

 limited area in northwestern United 

 States and British Columbia, east 

 to eastern Montana and eastern 

 Wyoming, west to the Cascade 

 Mountains and south into Nevada 

 and Colorado. This distribution 

 somewhat exceeds the present dis- 

 tribution of spotted fever (Fig. 58, 

 p. 191). 



Several different species of ticks FIG. 158. Spotted fever tick, 



have been found Capable of trans- Dermacentor venustus; engorged 



mitting spotted fever from rodent 



to rodent under experimental conditions. Several species of ticks 

 other than D. venustus are found in the spotted fever districts, 

 but none of these can have any hand in the transmission of the 

 disease to man since they do not attack him. A tick closely 

 related to D. venustus, the Pacific wood tick, D. occidentalis, oc- 

 curs west of the Cascades and Sierras in Oregon and California 

 and frequently attacks man. There is little doubt but that if 

 spotted fever once got a foothold in the territory occupied by this 

 tick, the latter would act as an efficient disseminator. In southern 

 and eastern states other ticks which attack man would probably 

 disseminate the disease were it once introduced. For this reason 

 it is of the utmost importance that the infection should not be 

 carried to parts of the country which are not now infected. 

 Measures for the prevention of this are discussed in Chapter X, 

 under " Spotted Fever." 



