THE MANUFACTURE OF VACCINES. 475 



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, aseptically, 

 from the carotid artery of a pig suffering from the disease, transferred to 

 the laboratory 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 "hyperimmunes" 

 should be healthy hogs, each weighing from 45 kg. to 136 kg. (100 

 to 300 pounds) and possessing either natural or acquired immunity 

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

 suspending the animal with the head down and after disinfecting the neck, 

 head, and nose, allowing the blood to flow from the jugular vein into 

 a sterile pan, or by drawing the fluid aseptically from the carotid artery, 

 after tying the animal on an inclined bleeding table. 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, the animals receive several injec- 

 tions at intervals of every few days, each succeeding dose of virus 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. One to two weeks after the hyper- 

 immune hog has received the last injection of virus, the animal is bled 

 from the tail, the end of the appendage being severed with a sharp 

 instrument, several hundred cubic centimeters 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 re-hyperimmunization. 



