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 
75 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. II. 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 
