162 HOG CHOLERA 



words, give serum alone to the sick animals, fol- 

 low-up treatment to those that are apparently 

 well. 



Regardless of the method selected, we must 

 proceed promptly with the one that becomes our 

 final choice, and we must take immediate precau- 

 tions to prevent spread of the disease to other 

 herds. 



Method number one may prove highly satisfac- 

 tory in some cases of this kind, but it is open to 

 the serious objection that it may not produce per- 

 manent immunity in all the animals. In the indi- 

 vidual, serum alone plus hog cholera infection 

 produces permanent immunity, but in a herd of 

 this kind, although all the animals are exposed, 

 some may not become infected in time to secure 

 this result, because hog cholera does not always 

 spread rapidly through herds that are at pasture 

 or in other large runs. 



Let us select, as an instance, one shoat in the 

 herd and assume that the animal has received 

 serum alone to-day. If in the course of the next 

 three or four weeks the usual duration of immu- 

 nity due to serum alone it chances to take up 

 virus sufficient to infect, it will undergo a reaction 

 and thereafter be permanently immune to hog 

 cholera ; but if the event of infection is delayed 

 much longer, it will find the animal susceptible to 

 the disease. In other words, if infection takes 



