INFECTION 



357 



cycle of the tick will, therefore, explain the variation in the 

 time elapsing between the exposure of northern to southern 

 cattle and the appearance of the disease. Starting with tick 

 infested animals placed with native cattle in a northern pasture 

 the adult female ticks drop to the ground almost daily, so that 

 the following life cycle may be assumed to begin at once. 



1. Adult ticks drop to the ground in from i to 3 days 

 after the infested cattle are placed in the field. 



2. Adult ticks lay their eggs in about 7 days after drop- 

 ping to the ground. 



3. Eggs are hatched in about 20 days after they are laid.* 



4. Young ticks 

 crawl upon cattle from 

 I to several days after 

 they are hatched. 



5. In about ten 

 days from the time the 

 young ticks crawl upon 

 the susceptible cattle the 

 rise of temperature ap- 

 pears. 



The length of time 

 that must elapse (period 

 of incubation) from the 

 exposure of susceptible cattle to the development of the 

 disease depends on whether or not the whole life cycle of the 

 tick must be passed or part of it has already gone by. If 

 susceptible animals are placed in a pasture where the young 

 ticks are just ready to crawl upon them the infection of the 

 cattle is accomplished at once and the high fever appears in 

 about ten days, practically the minimum time. It has been 

 experimentally demonstrated that the young ticks are able to 

 travel for a considerable distance in a pasture. In pastures 

 where tick infested cattle are grazing, young ticks are very 



*Moliler states that the time required for the hatching varies from 

 13 days to six weeks, depending upon temperature, moisture, soil, etc. 

 He states further that the eggs may remain dormant for several months. 



Fig. 96. Sexually mature female after 

 the last moult, dorsal view {Smith). 



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