148 HOG CHOLEKA 



are due to inert virus, insufficient doses, and, very 

 probably, to giving simultaneous treatment to 

 pigs too young. If virus is inert infection is not 

 produced, active immunity is not established, and 

 if hogs chance to be exposed to cholera after the 

 passive immunity due to the serum has disap- 

 peared, they readily contract the disease. If a 

 pig more than twelve weeks old receives a full 

 dose of virulent virus as a part of simultaneous 

 treatment and remains well during the following 

 four weeks, his immunity to cholera may be ac- 

 cepted as a fact ; if a herd that is given simultane- 

 ous treatment passes the first four weeks without 

 incident and later " breaks " with hog cholera, we 

 may safely assume that inert virus was used, or 

 that doses ridiculously low were administered. 

 Sometimes inert virus is sent out with impotent 

 serum that will protect against no other kind; 

 sometimes virus is used too long after being 

 drawn; and if it is heated we do not believe it 

 should be there is some danger of killing it. 



Prevention of " virus breaks," when one under- 

 stands their causes, is simple. Provided one 

 treats pigs more than twelve weeks old, a full dose 

 of virulent virus in conjunction with the serum 

 used in simultaneous treatment is all that is re- 

 quired. A breeder who is familiar with the bene- 

 fits and hazards of simultaneous treatment will 

 not be greatly disturbed if a shoat or two should 



