176 SWINE DISEASES 



on foreign bodies, such as splinters or stubble. Indirect 

 infection is the manner of infection that occurs when new 

 centers are established without the introduction of a 

 diseased animal. 



Susceptibility. Young swine are apparently most 

 susceptible to cholera immediately after weaning. How- 

 ever, swine of all ages may become affected. Thus 

 sucking pigs or old brood sows or boars may contract 

 cholera and die of its effects. 



Channels of entrance of the infection. The virus 

 of hog cholera most frequently gains entrance to the body 

 of the swine by way of the digestive tube, upon contami- 

 nated food or water, or in various substances that swine 

 are likely to chew or ingest. Infection may also be 

 introduced through the respiratory tract, but infection 

 by this avenue of entrance is not common. The virus 

 may be introduced through the skin by means of barbs, 

 awns, splinters, or similar objects, but this avenue of 

 entrance is not common in natural infection. 



In the production of a permanent immunity by the 

 use of the virus and serum, the virus is introduced through 

 the skin by a hypodermic syringe. Injection for the 

 purpose of hyperimmunizing, in the production of anti- 

 hog-cholera serum, is done intravenously in an ear vein. 

 Infection can be successfully accomplished by intraperi- 

 toneal or intramuscular injections of the virus, but natu- 

 ral infection rarely occurs in this way. 



Period of incubation. The period of incubation 

 after natural infection is variable, depending upon the 

 virulency of the virus and the resistance of the infected 

 swine. In most instances there will be a rise of tem- 

 perature from the fifth to the seventh day, and the 

 animals will be visibly sick by the tenth day after infec- 

 tion. The period of incubation in inoculation experi- 

 ments varies from four to seven days. 



Lesions. The anatomical changes observed in hog 

 cholera are quite variable because of the multiform types 

 that it assumes, probably due to complications. The 

 following types have been described: pure hog cholera 



