188 



OTHER SPOROZOA 



pable of transmitting the disease, Hindle thinks is probably due to 

 insufficient numbers of Rickettsia in the lice. In the human host 

 the parasites, as shown by transmission experiments, occur in 

 the blood plasma and also in various body excretions. They are 

 ordinarily not filterable, but in some stages of their development 

 will pass through bacterial filters. 



Rocky Mountain Spotted Fever. For many years certain 

 limited districts in the Rocky Mountain region of northwestern 



United States (Fig. 57), partic- 

 ularly Idaho and Montana, 

 have been known to be affected 

 by this very serious disease. 

 Its yearly occurrence in well- 

 defined areas has given rise to 

 panic and hysterical fear of en- 

 tering the " haunted " places. 

 Houses were deserted, land de- 

 preciated in value, and some of 

 the richest valleys in the North- 

 west left unpopulated. In 1906 

 it was shown by Ricketts that 

 the disease was invariably pre- 

 ceded by the bite of a common 



FIG. 57. Map showing distribution of local WOOd-tick, DermacentOT 

 Rocky Mountain spotted fever. Com- , 001 i TT 



piled from U. S. Public Health Reports. veUUStuS (see p. 361, and Fig. 



156), which was experimentally 



shown to be the intermediate host of the parasite. Wolbach in 

 1919 demonstrated the regular appearance of minute organisms, 

 which he named Dermacentroxenus rickettsi, in the cells of blood- 

 vessels of infected mammals, and also in nearly all the tissues of 

 infected ticks. The organism shows several morphological 

 types, but most frequently appears as paired lanceolate bodies, 

 the length of a pair being about I/*. It is transmitted by ticks 

 to their young with the eggs. It requires from two to ten hours 

 feeding for a tick to become infective, but after an incubation 

 period of a few days it may remain infective for many months. 

 Although D. venustus appears to be the only transmitter in nature, 

 experimentally other ticks have been shown to act as suitable 

 hosts. 



In man the incubation period is usually from four to seven 



