DISEASES PRODUCED BY ULTRA-MICROSCOPIC ORGANISMS 483 



intravenously. The animal thus hyperimmunized is bled, from 

 the tip of the tail usually, the blood allowed to clot, and the serum 

 used for immunization. It is customary to preserve this serum 

 from consecutive bleedings until a considerable quantity has ac- 

 cumulated, when it is tested for potency. For testing it is cus- 

 tomary to use young pigs, weighing from 30 to 60 pounds. A 

 serum of satisfactory potency should protect, in a dose of 15 c.c. 

 or less, one of these animals against an injection of 2 c.c. of virulent 

 blood. The serum injections of 20 c.c. are commonly used to 

 protect pigs against infection. If the animals are exposed to in- 

 fection, they may have a light attack of the disease, and are there- 

 by rendered permanently immune. Where it is desired to im- 

 munize, and exposure to infection is not certain, a much more 

 lasting .immunity is conferred by the use of the serum simul- 

 taneous method. In this method virulent blood (2 c.c.) is in- 

 jected at the same time as the immune serum. This results in 

 the development of an active immunity, which is relatively per- 

 manent in comparison with the immunity of two to three weeks 

 conferred by antiserum injection alone. The use of this serum has 

 been found to be highly successful in practice. 



It is not known what property of the antiserum is thus effec- 

 tive, whether it is antitoxic, opsonic, or bactericidal. There is 

 some evidence that it is the last, but proof is difficult to secure. 



Transmission. The virus may be demonstrated in the blood, 

 the tissues, and the urine of infected animals. It is probable that 

 infection commonly takes place through ingestion. 



Virus of Horse Sickness 



Disease Produced. African horse sickness, or Pferderpest. 



This disease, known from southern Africa for more than a 

 century, was first shown by MacFadyean in 1900, and later in 

 1901 by Xocard, to be due to an ultramicroscopic, filterable virus. 

 The disease is characterized as an acute or subacute disease of 

 solipeds, that appears in epizootics during the hot months of the 

 year. The principal lesions are edematous swellings and hemor- 

 rhages of the internal organs. The virus will pass through a 

 Berkefeld or a Chamberland porcelain filter if the serum is diluted 

 with physiological salt solution. 



