CHAPTER XXI 

 COMMUNICABLE DISEASES 



All communicable diseases are of infectious origin; that is, 

 they are produced by the entrance into the body of pathogenic 

 micro-organisms. These diseases are easily communicated to other 

 animals of the same or other susceptible species by direct contact. 

 They are the chief source of loss to the horse industry. Each is 

 recognized as being caused by a specific or single organism, to which 

 the body reacts in a definite manner by showing characteristic 

 symptoms and lesions. As a rule, the bacteria of these diseases do 

 not produce wound infections following introduction into the tis- 

 sues. Although our present methods of study have failed to dis- 

 cover a germ for rabies and a few other infectious diseases, it is a 

 foregone conclusion that some form of germ life is responsible 

 for each. The term "virus" is employed to designate the cause 

 of all infectious diseases for which no organism has yet been 

 isolated. 



It is important to remember that all diseases caused by infec- 

 tious organisms are not contagious, although all contagious (com- 

 municable) diseases are infectious. 



As with the non-communicable diseases, secondary infections 

 are often implanted upon or associated with the primary causes of 

 communicable maladies. Most of the secondary infections are 

 caused by pus organisms. In many cases they gain admission to 

 the blood-stream and are carried to all parts of the body. When an 

 organ with a weak resistance is met, they inunediately start to 

 multiply and cause serious interference with its functions, com- 

 plicating the original disturbance, rendering diagnosis more diffi- 

 cult, and hastening death. 



The time which elapses between the exposure of the animal to 

 infection and the appearance of visible symptoms of the disease 

 is called the 'period of incubation. It varies from a few days in 

 very acute diseases to several weeks in chronic affections. 



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