PIROPLASMOSES 483 



The blood of an ox containing the piroplasm is virulent 

 when injected into susceptible cattle (subcutaneous, intra- 

 vascular, intraperitoneally), but feeding such blood fails to 

 produce the disease. The parasite will remain virulent in a 

 recovered animal for years. 



How Texas Fever is Spread. — As noted, the cattle tick is 

 the carrier of the causal protozoon. As far as is known it is 

 the only carrier. These ticks are essentially parasitic; they 

 cannot attain full development unless they have access to 

 cattle. Their life history is important as the control and 

 eradication of the tick is naturally followed by a cessation 

 of the disease. The life history is simple: A pregnant female 

 falls to the ground from the skin of an ox and soon lays 2000 

 to 4000 eggs. Depending upon the temperature and moisture 

 present, in from two to six weeks a fully developed embryo 

 breaks' the egg shell and becomes free. When hatched the 

 little seed tick is a very active, six legged, spider-like insect 

 which crawls up the grass blades and lies in wait for an ox 

 to the skin of which it attaches itself. The larval tick can 

 grow only when on cattle. In moist soil, leaves, etc., it can 

 remain alive, however, for many months, even living through 

 the mild southern winters, and be capable of infecting cattle 

 the following spring. In the more rigorous northern winter, 

 however, it perishes. Once attached to the skin, the tick 

 moults, attains sexual maturity, copulates and fills itself 

 with the blood of its host. While withdrawing the blood of 

 the host the causal protozoon is introduced and the infection 

 brought about. 



Susceptible cattle placed in tick-infested pastures thus 

 become infected. This is commonly observed when sus- 

 ceptible cattle are turned on the usually tick-infested southern 

 pasture or are placed in northern pastures or enclosures in 

 which tick-laden southern cattle have been grazing or kept. 

 In permanently tick-infested districts the native cattle are 

 wholly or partially immune. If they contract the disease 

 at all, the attack is usually mild. One attack of Texas fever 

 confers a relative immunity, but the blood of an immune 

 animal remains virulent for several years. The disease can 

 therefore be spread by an immune or recovered animal 

 provided the purveying tick is present. 



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