224 Arthropods as Essential Hosts of Pathogenic Protozoa 
this field contracted the disease. The fourth animal was not 
examined as to its blood but it showed no external symptoms of 
the disease. 
In these earlier 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 pastimes. “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 
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. 
