INFECTIOUS DISEASES OP CATTLE. 483 



and but one field under cultivation, the above plan can be made appli- 

 cable by fencing off three inclosures in the latter and by rotating the 

 cattle in them every twenty days in the manner just described. The 

 same precautions should be observed in changing the cattle from one 

 lot to another and in preventing ticks from getting into the cultivated 

 field as are mentioned above. 



IMMUNIZATION OF SUSCEPTIBLE CATTLE. 



By blood inoculation. — It is often desirable to ship well-bred cattle 

 into infested districts, that they may be used to improve the quality 

 of the native cattle already there. Previous to the discovery of the 

 cause of Texas fever it was found to be well-nigh impossible to 

 introduce pure bred cattle from the North into any of the infected 

 regions without siiffering gi'eat loss — sometimes as high as 90 per 

 cent — within a few months of their arrival at their southern destina- 

 tion. At first it was thought that the fatalities were due to climatic 

 changes, but later the discovery was made that Texas fever was 

 causing these numerous deaths. 



It has now been found practicable to immunize this class of cattle 

 so perfectly that the losses which follow their transportation to a 

 tick-infested region are reduced to a minimum. Young animals six 

 to fifteen months old should, so far as possible, be selected for this 

 purpose, as they are more readily immunized than adults, are more 

 easily handled, and the dangers which may arise from pregnancy 

 while undergoing the immunizing treatment are thus avoided. 



Immunity in these cattle is obtained by introducing the micro- 

 parasite of the blood into their systems. It may be done by direct 

 artificial inoculation or by placing virulent young ticks upon the 

 animals and allowing them to perform the inoculation in the natural 

 manner. The subcutaneous injection of a small amount of defibri- 

 nated virulent blood has been found, by means of prolonged experi- 

 ment, the preferable method, as the number of micro-organisms intro- 

 duced can be more accurately gauged from the syringe than by 

 allowing the infection to be produced by bites of ticks. Two or three 

 inoculations, if repeated at proper intervals, are accomplished with 

 greater safety to the animal than would be possible by means of a 

 single inoculation. The amount first injected should be small and 

 then gradually increased in the succeeding treatments. 



The inoculation always results in a more or less serious attack of 

 Texas fever. Besides having a fever, there is great diminution of 

 red blood corpuscles, and in about 3 per cent of the cases a fatal ter- 

 mination; but the proportion of deaths resulting from the inoculation 

 is small when compared with the fatalities among untreated animals 

 taken into infested districts. To this number should be added those 

 animals (less than 7 per cent) that do not receive sufficient immunity 

 by this method and which succumb when exposed to infested pastures. 

 Combining these failures it will be seen that by this method of 



