280 SWINE PEACTICE 



healthy susceptible swine where they were permitted to walk and 

 feed upon their eyes. 



Manner of infection. — Hog cholera may be transmitted by direct or 

 indirect infection. Direct infection may be affected by the actual 

 contact of a healthy, susceptible swine with one affected with cholera. 

 Direct infection may also occur in utero, the pigs becoming infected 

 before birth. This statement may be questioned by some, but the 

 fact that pregnant sows abort when they become affected with hog 

 cholera is common knowledge among swine breeders. It has also 

 been observed that abortion is of frequent occurrence in pregnant 

 sows that are simultaneously immunized. The transmission of the 

 virus from the sow to the pig in utero would not seem difficult when 

 it is remembered that this virus readily passes through porcelain 

 filters. 



Indirect infection also is of common occurrence in hog cholera. 

 This is accomplished by the infection being introduced on food stuff, 

 in water, in the inspired air, or on foreign bodies, such as splinters 

 or stubble. Indirect infection is infection that occurs when new cen- 

 ters are established without the introduction of a diseased animal. 



Susceptibility. — Young swine are apparently most susceptible to 

 cholera immediately after weaning. However, swine of all ages may 

 become affected. Thus sucTking 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 tub, upon contaminated 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 ob- 

 jects, 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 hypo- 

 dermic syringe. Injection for the purpose of hyperimmunizing, in 

 the production of anti-hog cholera serum, is done intravenously in 

 an ear vein. Infection may be successfully accomplished by intraperi- 

 toneal or intramuscular injections of the virus, but natural infection 

 rarely occurs in this way. 



Period of incuiation. — The period of incubation after natural in- 

 fection is variable, depending iipon the virulence of the virus and the 



