Circular No. 1 36. issued March 31, 1911. 



United States Department of Agriculture, 



BUREAU OF ENTOMOLOGY. 

 L. O. HOWARD, Entomologist and Chief of Bureau. 



THE DISTRIBUTION OF THE ROCKY MOUNTAIN 

 SPOTTED-FEYER TICK. 



{Dennacentor veaustus Banks.) 

 By F. C. BisHOPP, 

 Agent and Expert. 



The demonstration of the fact that Rocky Mountain spotted fever, 

 an important disease of man, is conveyed by the common wood tick 

 of the Rockj^ Mountains, has naturally attracted considerable atten- 

 tion. In the year 1902 Drs. Wilson and Chowning advanced the 

 theory that the " wood tick " acts as a carrier of the disease. In 

 1906 the late Dr. H. T. Ricketts began a series of admirably planned 

 and executed experiments which showed that the tick Dermacentoi' 

 venustiis Banks is principally, if not entirely, responsible for the 

 transmission of this disease to man. 



While the disease appears in its most virulent form in the Bitter 

 Root Valley in western Montana, it is known definitely to occur in a 

 less severe form in parts of Idaho, Wyoming, Utah, and Nevada. 

 Although no authentic records to that effect are to be found, there is 

 little doubt that it occurs occasionally in certain sections of Colorado 

 and Oregon. In the Bitter Root Valley from 70 to 80 per cent of 

 the cases terminate fatallj'^, while in southern Idaho there is a mor- 

 tality of about 5 per cent. The cases of the disease which have ap- 

 peared in the other States mentioned seem to be of this less virulent 

 type. 



Since it has been determined that a tick is the transmitter of this 

 disease, the importance of ascertaining the distribution of the sjpecies 

 concerned is at once apparent. In 1909 Prof. R. A. Cooley, as Ento- 

 mologist of the Montana Agi-icultural Experiment Station, under- 

 took the determination of the distribution of this tick in the State of 

 Montana. He accumulated 172 lots of ticks, including 142 lots of 

 Dermacentor venustus, from 49 localities. During the same season 



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