MANUFACTURE OF HOG-CHOLERA SERUM 335 



more powerful or virulent at the commencement of an outbreak 

 than it is later on. As a result, if we take the first few animals that 

 appear real sick when the disease first appears on a farm and 

 bleed them, we obtain a virus that is usually of high quality. This 

 method for securing virus is a practical one when the laboratory 

 is conducted by a state experiment station, and men are con- 

 stantly in the field who are trained in the manner of drawing and 

 handling of virus. In the state of Ohio this plan is followed to a 

 very large extent, and it is the opinion of the officials there that 

 they are getting a virus which is of very high quality. This plan 

 has another advantage, in that it is comparatively cheap. By 

 selecting those animals that are very sick at the time the farm is 

 visited by the state inspectors, it is usually possible to secure the 

 consent of the farmer to kill them, as he recognizes that they are 

 almost certain to die anyway. 



Injection Method. — The third and, I believe, the most practical 

 method of securing hog-cholera virus for this work is to secure 

 a virus that is of the highest obtainable quality, and then repro- 

 duce this strain of virus by injecting it into the body of a number 

 of small shoats that possess no immunity. In this manner the 

 virus is given an opportunity to reproduce in the body of the pigs, 

 and is recovered a few days later in its most powerful form. Virus 

 obtained from young shoats is always more powerful than that 

 obtained from older animals, and for this reason it is desirable 

 to use animals ranging from 60 to 100 pounds for this purpose. 



By this method a virus is obtained that is of known potency, 

 and also it can be drawn under conditions which ensure its being 

 of the highest standard of purity. By this frequent passage of 

 the virulent material through the body of young animals the 

 potency of the virus is kept at a high standard, and this ensures 

 a powerful serum. In the United States Bureau of Animal In- 

 dustry Serum Plant at Ames, Iowa, Dr. Niles has used the same 

 strain of virus for the last ten years, and produces a serum of un- 

 questionable strength and protective power, as shown by the re- 

 sults of repeated field trials. 



The greatest disadvantage to this method of securing virus 

 blood is the cost. Young shoats of this size and age are rather 

 hard to obtain, especially if the serum plant is not located at some 



