MANUFACTURE OF VACCINES 733 



The product is kept in the refrigerator in hermetically sealed vials. 

 It is claimed that the material so prepared maintains its original 

 strength or infectivity several months. 



Cummings'* method consists in the dialysis of the rabic material 

 in standard suspensions. Dialysis for twelve to twenty-four hours 

 possesses the advantage of destroying the infectivity of the virus, 

 without disturbing its immunizing properties. 



DORSET-NILES (HOG-CHOLERA) SERUM. f To prepare the material 

 for this process of immunization it is first necessary to secure a virulent 

 strain of hog cholera virus. This may be obtained from any typical 

 outbreak of the disease. A specimen of blood may be drawn, asep- 

 tically, from the carotid artery of a pig suffering from the disease, and 

 tested for activity. Frequently a given strain of virus may not produce 

 the acute form of hog cholera. In attempting to raise the virulence of 

 a relatively weak virus it may be passed through a series of young pigs 

 until it uniformly produces symptoms after four to six days' incubation 

 and death in less than fifteen days. None except a virus having this 

 degree of activity should be used in manufacturing the hyperimmune 

 serum. 



The virulent blood used in the process of hyperimmunization 

 should be obtained from susceptible pigs weighing from 25 to 50 kg. 

 (50 to 100 pounds) each. The animals to be used as the"hyperim- 

 munes" should be healthy hogs, each weighing from 100 kg. to 150 kg. 

 (220 to 330 pounds) and possessing either natural or acquired immunity 

 to the disease. The blood is best secured from a diseased pig by 

 suspending the animal with the head down covered with a shroud 

 wet with a disinfectant solution. The neck is shaved and disinfected. 

 A small incision is made on the median line through which a specially 

 devised bleeding knife, properly sterilized, is introduced. The blade 

 of this knife severs the large vessels at the base of the heart and the 

 blood flows through the hollow handle into sterile containers. After 

 the blood is obtained it is defibrinated, the serum separated from the 

 clot, and the clot discarded. The number of pigs necessary to furnish 

 sufficient virus for the hyperimmunization of one hog depends upon the 

 weight of both the virus pigs and the immune hog. 



The immune hogs may be hyperimmunized either by the "slow" 



* Proceedings isth International Congress on Hygiene and Demography, Washington 

 D. C., 1912. 



t U. S. Bureau of Animal Industry, Bull. No. 102. 



