250 AGRICULTURAL BACTERIOLOGY 



new susceptible host, The exits from the host by which or- 

 ganisms find their way to new hosts vary in the different 

 diseases. In intestinal diseases, as hog cholera, the excreta 

 serve as a mode of exit. In tuberculosis the secretions, 

 as milk and saliva, function as carriers of contagion. In 

 some diseases the blood from wounds caused by biting in- 

 sects or the discharges from abscesses on the surface of the 

 body serve as channels of transmission. 



The transfer of the causal organisms from one animal to 

 another may take place in a multitude of ways. In the 

 same herd a healthy animal easily comes in direct contact 

 with the infectious material from a diseased animal. The 

 spread of the disease to other herds may take place through 

 the transfer of an infected animal or of infectious material, 

 such as milk or contaminated objects. The direct methods 

 of transfer can be readily guarded against, but the more 

 indirect modes of transmission are much more difficult to 

 detect. Thus hog-cholera virus can be readily transferred 

 by dogs, crows, or persons carrying the virus on their feet. 



The opportunity for the transfer of organisms for any 

 considerable distance is dependent on the resistance of the 

 organism, which is largely determined by the fact as to 

 whether it produces spores or not. Most of the non-spore- 

 forming organisms can not persist for any long period out- 

 side the animal, since they succumb quite readily to such 

 unfavorable environmental influences as drying, sunlight, 

 and the action of saprophytic bacteria. The spore-forming 

 organisms, on the other hand, can persist for long periods, 

 owing to the resistance of the spores to all ordinary environ- 

 mental conditions. 



The prevention of the transmissible diseases involves the 

 keeping of the causal organisms from coming in contact 

 with healthy animals. This can be accomplished by isola- 

 tion of diseased animals, by the disinfection of their secre- 



