INFECTIOUS DISEASES 121 



lose their vigor and may show unthriftiness. This 

 is no doubt one cause for the disease being so 

 prevalent in the corn-belt. 



Period of Incubation.^The shortest period of 

 incubation is two days, the longest three or four 

 weelcs. This depends on the susceptibility of the 

 animals, the activity of the germ and the manner 



FIG. 12 — A COLERA HOG 



of infection. An acute attack indicates a short 

 period and a mild or chronic one a long period. 

 Usually, from one to two weeks will elapse from 

 the time the animal is exposed until it comes 

 down with the disease. 



Symptoms. — Three different forms of hog chol- 

 era occur, and for this reason the disease does not 



