64 EXPEEIMENT STATION RECORD. 



the Bureau of Entomology of this Department, The history of the occurrence 

 of this disease, investigations relating to its transmission by ticks, and previous 

 knowledge of the life history and host relations of Dermacentor vennsius are 

 first discussed, together with an experiment which shows that unengorged 

 adult ticks collected in the district in which the disease obtains may convey the 

 infection. 



On the 717 small wild animals examined 4,595 ticks were found, of which a 

 large proportion were in immature stages and all but 1,446 belonged to the 

 genus Dermacentor. Six species of ticks were found to occur in the Bitter 

 Root Valley, which in the probable order of abundance as indicated by obser- 

 vations are D. venustus, D. albiinctus, Ixodes texanus, Hwmaphysulis leporis- 

 palustris, I. kingi, and /. angustus. " The adults of D. venustus are engorged 

 in the valley almost entirely on domestic animals, principally horses and cattle. 

 Sheep and dogs support them to a very much less extent. It is apparent that 

 on rare occasions such animals as the bear, coyote, and deer may drop en- 

 gorged adults in the valley, but it is belived that the numbers so deposited 

 have very little influence on tick abundance in the valley. ... In the 

 neighboring mountains 3 mountain goats (Oreamnos montanus) were shot 

 and examined on ' goat rocks ' above Rock Creek and Lake Como and some 

 200 adults of D. venustus were found on them, together with large numbers 

 of D. albipictus. Mountain goats do not leave the rocks and descend into the 

 valley and can not act as carriers of ticks into the valley. None of the im- 

 mature stages of D. venustus were taken from any of the domestic animals 

 but they were found in great abundance and apparently without much dis- 

 crimination on a wide variety of small mammals. . . . Considering the rela- 

 tive abundance of the various hosts of the early stages it can be stated beyond 

 a question that by far the greater part of the larvse and nymphs feed on the Co- 

 lumbian ground squirrel, the pine squirrel (Soiurus hudsonicus richardsoni) , 

 and the yellow-bellied chipmunk. They are common, however, on a large num- 

 ber of other small mammals, including the woodchuck, rock squirrel, wood rat, 

 and cottontail rabbit (SylvUagus nuttaUi).'" Nymphs and adults of D. albipictus 

 were found in abundance on horses during the fall and winter months but 

 neither adults nor nymphs of D. venustus were discovered. It is thought that 

 frequent reports of the occurrence of spotted-fever ticks on horses during the 

 fall and winter months are based on the presence of D. albipictus, the nymphs 

 and adults of which apparently feed only during the fall, winter, and spring 

 and which remain on the host for their nymphal molt. It is clearly indi- 

 cated that the presence of the spotted-fever tick in abundance is dependent 

 upon the joint occurrence of a sufiicient number of rodents and of domestic 

 animals, principally horses and cows. 



Observations made in the early spring indicate that ground squirrels, chip- 

 munks, and pine squirrels come out of their winter nests free of ticks and that 

 the nymphs with which they are later found Infested are picked up in the 

 regular way. Nymphs continued to be abundant until the latter part of June 

 when there was a gradual reduction in the number although scattering indi- 

 viduals were found up to September 8. " It now seems clear that 2 years 

 instead of 1 are required for the life cycle of this tick. It is believed that the 

 winter is passed as adults and as nymphs and that both become active and feed 

 in the spring and early summer, the former laying eggs that hatch into seeds 

 which feed and develop into nymphs by midsummer, and the latter maturing 

 adults by about the same time or later, neither showing a strong inclination to 

 feed during the first summer. It is not impossible that some of the newly- 

 developed nymphs and adults do feed during the first year." It is stated that 

 for the adult a longevity period of 353 days has been observed in the laboratory. 



