140 THE HOESE IN HEALTH AND DISEASE 



nitrate. Some poisons exert their destructive influence by causing 

 a coagulation of the tissue-juices; others, by causing a liquefaction 

 of the tissue-cells. 



Animal parasites may transmit as well as cause disease. In 

 later pages the harmful influences exerted by them will be shown. 

 It has been proved that some diseases can be absolutely controlled 

 and prevented if proper measures are taken to destroy the parasites 

 which carry the infectious organisms. 



Vegetable parasites include certain molds, rusts, and smuts. 

 They are found infesting grain and forage which has been im- 

 properly harvested or stored. When conditions have been favor- 

 able for their growth they may cause that fatal disease of horses 

 known as forage-poisoning. 



Bacterial parasites are the cause of all infectious diseases. 

 However, all bacterial parasites of the body are not pathogenic — 

 that is, disease-producing in their action. Many micro-organisms 

 exert a distinctly beneficial influence in the body and serve the 

 host in keeping down the growth of harmful bacterial parasites. 

 When bacteria infect the body they may remain at or near the site 

 of infection; they may spread through the tissues by direct growth, 

 or be carried by the body fluids to distant parts ; they may multiply 

 in the blood, or they may set up metastatic infections by forming 

 local lesions in other parts of the body. Disease may be produced 

 by the action of their toxins or poisons, or by the mechanical inter- 

 ference which their presence exerts. 



NATURAL RESISTANCE AGAINST DISEASE 

 Horses and all other animals are constantly exposed to infec- 

 tions with the various forms of parasites just mentioned, many of 

 them in a high state of virulence. Nature has provided a number 

 of very efficient defensive mechanisms to protect the body against 

 these micro-organisms. It is through these agencies that our ani- 

 mals escape disease, even after prolonged exposure. 



The protection afforded the body against disease may be due 

 to any one or more of the following defences : 



I. The Antibodies in the Blood-serum. — These substances are 

 similar in character to enzymes and appear to act in a chemical 

 manner. They are stimulated to production through the intro- 

 duction of foreign substances. They prevent bacteria from multi- 

 plying and neutralize the poisons produced by them. The major- 



