INFECTIOUS DISEASES OF CATTLE. 487 



within the shell it ruj^tures this and gains its freedom. The time 

 required from the laying of the eggs to their hatching varies consid- 

 erably, according to the temperature. In the laboratory in the heat 

 of midsummer this was accomplished in about 13 days. In the late 

 fall, under the same conditions, it required from four to six wrecks. 

 The larva after emerging from the egg is very minute, six-legged, 

 and just visible to the naked eye. (PI. XLVIII, fig. 3.) If these 

 larvse be kept on a layer of moist sand or earth in a covered dish, 

 they may remain alive for months, but there is no appreciable in- 

 crease in size. As soon, however, as they are placed upon cattle 

 growth begins. 



On pastures these little creatures soon find their way onto cattle. 

 They attach themselves by preference to the tender skin on the 

 escutcheon, the inside of the thighs, and on the base of the udder. 

 Yet when they are very numerous they may be found in small num- 

 bers on various parts of the body, such as the neck, the chest, and 

 the ears. (Pi. XLIX.) 



The changes which they undergo during their parasitic existence 

 were first studied by Dr. Cooper Curtice in 1889. The young tick 

 within a week molts, and the seCond or nymphal stage of the para- 

 site's life is thus ushered in. After this change it has four pairs of 

 legs. Within another week another molt takes place by which the 

 tick passes from the nymphal to the sexual, or adult, stage. Im- 

 pregnation now takes place, and, with the development of the ova in 

 the body, the tick takes an increased quantity of blood, so that it 

 becomes very" much larger in a few days. That the rapid growth is 

 due to the blood taken in may be easily proved by crushing one. The 

 intestine is distended with a thick, tarry mass composed of partly 

 digested blood. When the female has reached a certain stage of 

 maturity she drops to the ground and begins to lay a large number 

 of eggs, which hatch in the time given above. 



The life of the cattle tick is thus spent largely on cattle, and 

 although the young, or larvae, may live for a long time on the 

 ground in the summer season, they can not mature excepting as para- 

 sites on cattle and horses. We have purposely omitted various details 

 of the life historj^, including that of the male, as they are not neces- 

 sary to an understanding of our present subject — Texas fever. How 

 this is transmitted we will proceed to consider. Before the enforce- 

 ment of the Federal quarantine southern cattle sent north during the 

 spring and summer months carried on their bodies large numbers of 

 the cattle ticks. These when matured would drop off and lay their 

 eggs on the northern pastures. After hatching, the young ticks 

 would soon get upon any northern cattle which happened to be on 

 the pasture. As soon as they liad attached themselves to the skin 

 they inoculated the cattle, and Texas fever would break out a week 



