226 Arthropods as Essential Hosts of Pathogenic Protozoa 



to infest vertebrates and in all the cases where the method has been 

 worked out it has been found that the conveyal was by ticks. We 

 shall not consider the cases more fully here, as we are concerned 

 especially with the method of transfer of human diseases . 



Ticks and Rocky Mountain Spotted Fever of Man Ever since 

 1873 there has been known in Montana and Idaho a peculiar febrile 

 disease of man, which has gained the name of ''Rocky Mountain 

 spotted fever." Its onset is marked by chills and fever which rapidly 

 become acute. In about four to seven days there appears a charac- 

 teristic eruption on the wrists, ankles or back, which quickly covers 

 the body. 



McClintic (1912) states that the disease has now been reported 

 from practically all of the Rocky Mountain States, including Arizona, 

 California, Colorado, Idaho, Montana, Nevada, Oregon, Utah, 

 Washington, and Wyoming. "Although the disease is far more 

 prevalent in Montana and Idaho than in any of the other States, 

 its spread has assumed such proportions in the last decade as to call 

 for the gravest consideration on the part of both the state and national 

 health authorities. In fact, the disease has so spread from state 

 to state that it has undoubtedly become a very serious interstate 

 problem demanding the institution of measures for its control and 

 suppression." 



A peculiar feature of the Rocky Mountain spotted fever is a 

 marked variation in its severity in different localities. In Montana, 

 and especially in the famous Bitter Root Valley, from 33 per cent to 

 7 5 per cent of the cases result fatally. On the other hand, the fatality 

 does not exceed four per cent in Idaho. 



In 1902, Wilson and Chowning reported the causative organism 

 of spotted fever to be a blood parasite akin to the Babesia of Texas 

 fever, and made the suggestion that the disease was tick-borne. 

 The careful studies of Stiles (1905) failed to confirm the supposed 

 discovery of the organism, and the disease is now generally classed 

 as due to an invisible virus. On the other hand, the accumulated 

 evidence has fully substantiated the hypothesis that it is tick-borne. 



According to Ricketts (1907) the experimental evidence in sup- 

 port of this hypothesis was first afforded by Dr. L. P. McCalla and 

 Dr. H. A. Brereton, in 1905. These investigators transmitted the 

 disease from man to man in two experiments. "The tick was 

 obtained 'from the chest of a man very ill with spotted fever' and 



