2 24 Arthropods as Essential Hosts of Pathogenic Protozoa 



this field contracted tlie disease. The fourth animal was not 

 examined as to its blood but it showed no external symptoms of 

 the disease. 



In these earher experiments it was believed that the cattle tick 

 acted as a carrier of the disease between the Southern cattle and the 

 soil of the Northern pastures. "It was believed that the tick ob- 

 tained the parasite from the blood of its host and in its dissolution 

 on the pasture a certain resistant spore form was set free which 

 produced the disease when taken in with the food." The feeding of 

 one animal for some time with grass from the most abundantly 



/}da.rcd 

 ■shield 



Chitmiz&ci points 



140. Hyalomma aegypticum. After Nuttall and Warburton. 



infected field, without any appearance of the disease, made this 

 hypothesis untenable. 



In the experimental work in 1890 the astonishing fact was brought 

 out that the disease was conveyed neither by infected ticks dis- 

 integrating nor by their directly transferring the parasite, but that 

 it was conveyed by the young hatched from eggs of infected ticks. 

 In other words, the disease was hereditarily transferred to ticks of 

 the second generation and they alone were capable of conveying it. 



Thus was explained the fact that Texas fever did not appear 

 immediately along the route of Southern cattle being driven to 

 Northern markets but that after a certain definite period it mani- 

 fested itself. It was conveyed by the progeny of ticks which had 

 dropped from the Southern cattle and deposited their eggs on the 

 ground. 



