348 PARASITES OF THE DOMESTIC ANIMALS 



and here undergoes its complete development. (Ref. Margaropus annul- 

 atus, Life History, p. 148.) For a number of days following her fer- 

 tilization the female tick engorges with the blood of her host and then 

 drops to the ground where a few days later she deposits her eggs and, 

 having completed her cycle, soon dies. The parasites contained in the 



blood upon which the tick has fed reach the eggs and 



are present in the larval ticks when these are hatched. 



Thus the larvse have the power to infect any susceptible 



animal to which they attach. 

 In the first stage of development after gaining the 



circulation the piroplasma is within the red corpuscle 



as a single body near the corpuscle's margin. Later it 

 Piroplasma,^ "big- ^^'^^^6^ ^^^^ ^^^ bodies which remain slightly connected 

 eminum (after by a Small filament. A single corpuscle may contain as 

 Crawley, from many as four or even six parasites. The doubled bodies 

 194, "bu. An. Ind! enlarge, assuming a spindle-shaped and later a pear- 

 U. s. Dept. Agr.) shaped appearance. Finally, as a result of this invasion, 



the corpuscles break down, and the parasites become 

 free bodies in the plasma. That a multiplicative stage occurs within 

 the bovine host is evidenced from the fact that inoculation of sus- 

 ceptible cattle with a small quantity of virulent blood will produce the 

 disease with the development of myriads of the parasites in the blood of 

 the inoculated animals. 



Occurrence. — Numerous attempts have been made to produce Texas 

 fever in other species of animals by inoculating them with infected blood 

 from cattle. That all of these experiments have proved negative in- 

 dicates that the disease is one purely bovine. All bovine animals that 

 have never been exposed are susceptible, and in all cases natural in- 

 fection with the protozoan causing the disease is due to puncture by 

 the cattle tick. 



The disease exists in European and Asiatic countries, Africa, Australia, 

 and the Philippine Islands. It was probably introduced into the United 

 States by cattle brought over by the early Spanish settlers. The terms 

 " Texas fever " and " southern cattle fever " are misleading to some in 

 giving the impression that the disease is confined to the Southern States. 

 Southern cattle carrying the infecting organism in their blood, though 

 themselves possessing degrees of immunity to Texas fever, disseminate 

 it through ticks from their bodies among cattle in the. North or among 

 those of the South which are susceptible to the disease in a virulent 

 form. 



Exposure and Development. — The period from exposure to tick in- 

 fested pastures or pens to the appearance of the disease depends upon 

 the time which elapses from the dropping of the female ticks from the 

 southern cattle to the hatching of the larvae from their eggs, and this 



