DISEASE IN GENERAL 213 



before the natural recuperative powers of the body have been 

 exhausted from conflict with the forces of disease. 



(3) The Elimination of Disease Carriers. — If diseases are to 

 be prevented from spreading, all agents that are capable of 

 carrying germs from place to place must be watched. In this 

 connection may be mentioned dogs, birds, and all animals com- 

 ing from infected premises, whether they themselves are sick or 

 apparently healthy; insects such as the flies, which lay eggs from 

 which bots and grubs develop, and the Texas fever tick; hay and 

 grain from quarantined areas; stock cars, wagons, pens, and show 

 yards in which animals from infected farms have been confined. 



Probably, the water supply is more often a means of spreading 

 diseases of farm animals than any other inanimate object. The 

 public watering trough, the stream, the puddle, and even the well 

 in the farmyard should be carefully guarded against infection. 

 In general, any disease which can infect animals by entrance 

 through the mouth into the alimentary canal may be transmitted 

 by contaminated water, for example, glanders, strangles, internal 

 parasites, and infectious abortion. By providing clean, fresh 

 water, preferably from deep wells, losses from diseases conveyed 

 by water may be largely eliminated. 



The unrestricted dog has been shown to be responsible in 

 some instances for the spread of foot-and-mouth disease, hog 

 cholera, and other diseases, not only from one farm to another, 

 but from one state to another. Dogs are the sole carriers of 

 the virus of rabies, a fatal disease of both man and the domestic 

 animals. As hosts of certain parasites which produce great 

 damage in other animals, such as "gid," "measles," tapeworm 

 and roundworm, dogs must be carefully considered when an 

 attempt is made to eliminate those agents that spread disease. 



No system of disease-prevention can be successful unless recog- 

 nition is taken of those animals which have had an infectious 

 disease. For example, cows which have had contagious abortion 

 and apparently recovered, may yet actually be giving off in their 

 se- and excretions the germs of the disease. Such individuals 

 are known as "carriers." It is not only very difficult but often 

 impossible to discover these "carrier cases." However, recently 

 perfected biologic laboratory tests, which make use of the blood 

 as a diagnostic agent, are proving very helpful in picking out 

 those animals that are dangerous to their stable-mates. 

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