METHODS OF USING ANTI-HOG-CHOLERA SERUM 155 



cannot be depended on to protect more than four 

 weeks. 



' ' Virus breaks" are no more likely to occur 

 when follow-up treatment is administered than 

 they are following simultaneous treatment, and, 

 as with the latter method, their prevention con- 

 sists wholly of giving full doses of virulent virus, 

 and using due care not to treat pigs too young. 

 When these precautions are observed, we can 

 vouch for the fact that follow-up treatment pro- 

 duces a permanent immunity. We have used it 

 since 1912 in maintaining many immune herds, 

 usually administering the final dose, simultaneous 

 treatment, when the pigs were about twelve weeks 

 old. We have not had a " virus break" during 

 the nine years the system has been employed; 

 that is, no pigs that survived the immediate reac- 

 tion following the final serum-virus treatment sub- 

 sequently developed hog cholera. It is our custom 

 to select hypers from these herds, and so far none 

 of them have developed hog cholera as a result 

 of hyperimmunization. In the East many veteri- 

 narians use the follow-up system in maintaining 

 herds immune to cholera, and " virus breaks" are 

 not common. 



Experimentally we have tested the effects of 

 giving follow-up treatment using various inter- 

 vals between the time of administering serum 

 alone and that of administering serum and virus, 



