MANUFACTVRE OF VACCINES 569 



munes " should be healthy hogs, each weighing from 100 kg. to 1 50 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" 

 or "quick" method. In the former, now seldom used, the animals 

 Receive several injections at intervals of every few days, each succeeding 

 dose being increased in proportion to the weight of the animal. In the 

 "quick" method the virus is injected in one large dose, the amount being 

 determined by the weight of the animal. The virus may be injected 

 intramuscularly, intraperitoneally or intravenously, the latter method 

 now being used almost exclusively. Ten days to two weeks after the 

 h3^erimmune hog has received the last injection of virus, the animal 

 is ready for bleeding. When bled from the tail, the end of the appen- 

 dage is severed with a sharp instrument, several hundred cubic centi- 

 meters of blood are collected aseptically, defibrinated, a preservative 

 added and the material placed in the refrigerator. This process is 

 repeated several times, at intervals of one week to ten days, when the 

 animal is ready for final bleeding. 



By this procedure all the blood is secured from the animal according 

 to the method described for bleeding virus pigs. The "slaughter" 

 method, used in many laboratories, consists of only the final bleeding, 

 thus eliminating tail bleedings. As a rule the different lots of serum 

 representing the different bleedings from several h)T)erimmune hogs 

 are mixed and the whole subjected to test. In order to test the 

 potency of the product eight susceptible pigs, each weighing about 23 

 kg. (50 pounds) are inoculated subcutaneously, each with 2 c.c. of 

 virus. Six of these pigs are simultaneously injected with graduated 

 doses (15 to 25 c.c.) of the serum under test. If the hyperimmune 



