336 AGRICULTURAL BACTERIOLOGY 



produced that will protect the animal from a serious in- 

 fection for from six to ten weeks. If a small quantity of 

 virulent blood is introduced into the animal at the same 

 time the serum is injected, active immunity will be pro- 

 duced, which will generally protect the animal for life. 

 The introduction of the virus at the same time as the anti- 

 serum may result occasionally in a fatal case of cholera. 

 In order to avoid this risk, the protective serum may be 

 injected, and about a week or ten days later the virus may 

 be given, together with a second dose of the anti-serum. 

 The first method is known as the serum-alone method, the 

 second as the simultaneous method, and the last as the 

 double or combination method. Each has its advantages 

 and disadvantages which must be considered in determining 

 which to apply. The serum-alone is safe, but protects for 

 only a short time, unless the animals coma in contact with 

 infectious material soon after treatment, in which case the 

 results are substantially the same as those obtained in the 

 simultaneous treatment. The method is of small value iii 

 the protection of breeding animals. It does allow the 

 farmer to protect his herd for a short time when the dan- 

 ger of infection is great. 



In the simultaneous method, some of the treated animals 

 may die from cholera, because not sufficient serum was 

 used to protect against the virus administered. The ani- 

 mals in which acute cholera is thus produced may serve 

 as centers of infection from which the disease may spread 

 to other herds. This danger has led many to advise 

 against the use of the simultaneous method. In herds in 

 which the disease already exists, only the serum should be 

 used. The combination method avoids the danger of tho 

 simultaneous treatment, since rarely are any animals lost 

 by cholera due to the treatment, It is more expensive, 

 since serum must be given twice. 



