DISEASE IN GENERAL 195 



spread through the tissues by direct growth (actinomycosis); 

 some are carried by the body fluids to distant parts (strangles) ; 

 still others may multiply in the blood and locate in various 

 organs where they set up infections by forming local lesions 

 (tuberculosis). 



The multiplication of infectious organisms is more rapid after 

 a prolonged disturbance of some kind. For example, if the 

 kidneys have not been performing their work properly, the heart 

 and liver both show the effects of the retention of the harmful 

 products of metabolism by being reduced in efficiency. When 

 these important organs are not working efficiently, any infectious 

 organism in the body would multiply very rapidly. 



The elimination of infectious organisms from the body is with 

 the various se- and excretions. For example, the milk is infectious 

 in tuberculosis of the udder; the saliva in foot-and-mouth disease; 

 the urine in cases of hog cholera; the feces in Johne's disease; 

 the nasal discharge in glanders and strangles; the vaginal dis- 

 charge in contagious abortion; the discharge from wounds in 

 anthrax; the blood in Texas fever and other parasitic blood 

 infections. This list shows how essential it is properly to handle 

 animals that are suffering from infectious diseases to prevent 

 others from becoming infected. 



The transmission of infectious organisms can be understood, 

 now that we have considered the ways by which disease-produc- 

 ing germs gain entrance to the body and the avenues by which 

 they leave. The vehicles that usually carry infectious germs 

 from animal to animal or place to place are the air, water, food, 

 and animal parasites. Transmission by direct contact with 

 infected se- and excretions, by rubbing, licking, and biting, and 

 through common feed mangers and drinking places are far more 

 frequent methods of spreading disease than is usually realized. 



RESISTANCE TO DISEASE 



Resistance to disease is possessed by all animals in some 

 degree. It is more marked in some individuals than others, 

 depending upon age, sex, and breed. The study of disease 

 resistance in animals is one of the most important and interesting 

 subjects that the student of live stock husbandry can pursue. 

 Only a few scientific facts were known about it until the develop- 

 ment of experimental research a few decades ago. Even now 

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