590 PIROPLASMA BOVIS 



infected territory. This is due to the fact that very young animals 

 are not very susceptible to the disease; they acquire it in a mild 

 form and a certain degree of immunity becomes established by 

 repeated annual infections. The immunity so acquired, however, is 

 no real immunity, but simply consists in the presence of a small 

 number of piroplasmata and a tolerance against them. If such 

 animals are exposed to fatigue (in transit) or to other diseases they 

 may develop a malignant outbreak of the disease. 



Such partially immune animals if brought to a non-infected 

 district may become the source of violent outbreaks among the 

 animals of the hitherto free territory. Transmission in these cases 

 may be brought about by either one of two ways: (1) The animals 

 from the infected district may carry with them infected ticks which 

 will directly spread the disease in a hitherto free territory, or (2) they 

 may not bring ticks along, but find in their new surroundings ticks 

 which can and will spread the disease. 



Immunization of Cattle. It was noticed by Smith and Kilbourne 

 that animals after recovery from an attack of Texas fever in one 

 year were comparatively immune against new attacks in subsequent 

 years in spite of being much exposed to infected ticks. Schroeder 

 was one of the first in this country experimentally to inoculate young 

 northern cattle with blood from infected Southern animals, producing 

 by this method a mild attack of Texas fever. Subsequently he 

 exposed the inoculated animals together with non-protected control 

 animals in the South to the natural tick infection. The results were 

 very favorable and promising : most of the protected animals lived, 

 and all the controls died. 



Dalrymple 1 has published a report on the results and experiences 

 of protective inoculation of cattle against Texas fever. He states 

 (1) that sterile blood serum of infected animals obtained by centri- 

 fuging the blood and separating the corpuscles from the serum, 

 has no value whatever in immunizing animals; (2) that susceptible 

 cattle may be immunized by infecting them with piroplasma through 

 the medium of infected seed ticks, but on account of certain trouble- 

 some conditions the method is not as practical as could be desired. 

 The results of experiments to utilize the blood from ticks in immuni- 

 zing inoculations is summarized as follows by Dalrymple: 



"The blood with which the adult ticks are filled, after maturing 

 on Southern cattle, carries with it the power to produce Texas fever 

 when injected under the skin of a susceptible animal. 



"Experiments indicate that we may be able to take ticks from 

 recently immunized animals, ship them considerable distances, and 

 utilize them as a substitute for the blood drawn from the vein, where 

 recently immunized animals are not available. 



"Experiments further indicate that this will give a milder form 



1 Louisiana State University and A. and N. College Bulletin, No. 84, October, 1905. 



