DISEASES OF SWINE 413 



affected. Pleuro pneumonia in cattle is also a contagious disease and 

 at one time had a foothold in this country. It is a disease in which 

 the germ does not live long outside the body and is only carried 

 by contact or artificial means. It was eradicated by destroying all 

 that were affected and disinfecting the places where it had been. 

 Glanders among horses is also contagious and it is only spread by 

 contact or close association. All strictly contagious diseases are con- 

 trollable and could be exterminated by united effort. The drastic 

 measures used to stamp out pleuro pneumonia would stamp out sheep 

 scab in a short time. 



Infectious Diseases. These are caused by some special agent 

 or parasite and the cause may live and multiply outside the body. 

 Infectious diseases may be and frequently are contagious. Some in- 

 fectious diseases however are not contagious. The line separating 

 contagious and infectious diseases is not very clear. The distinc- 

 tion is largely one of degree. Among the types of infectious dis- 

 eases we have lumpy jaw and blackleg of cattle; distemper and 

 influenza of horses ; cholera and swine plague in hogs, and roup in 

 poultry. True lumpy jaw of cattle is always caused by ray fungus. 

 The fungus is obtained upon the food which the animal takes, the 

 disease is rarely spread by the discharges from the wound. Black- 

 leg is obtained from the pasture or forage, the germs being known 

 to live for a long time outside of the body. Influenza and strangles 

 occur in epidemics because the germs live outside of the body and 

 under favorable climatic conditions develop generally, thus causing 

 widespread outbreaks at the same time. Hog cholera and swine 

 plague are both infectious and contagious, the germs live outside 

 the body and no amount of separation of herds will ever stamp 

 out the cliseases. It only decreases the number of cases. The germs 

 of tetanus or lock jaw are to be found growing in the soil, but do 

 not cause trouble unless accidentally introduced into a closed wound. 

 Infectious diseases can not be wholly controlled because the ocqur- 

 rence in an animal is not essential to the life of the germ. Some 

 may be prevented by vaccination, as blackleg, and some have been 

 greatly reduced by learning the habitat of the germs outside of the 

 body and making those places uncongenial for their growth. (Ind. 

 B. 100.) 



CAUSES OF DISEASE. 



The causes of disease are the indirect or predisposing causes and 

 the direct or exciting causes. The predisposing causes are any fac- 

 tors which tend to render the body more susceptible or to favor the 

 presence of the exciting cause. The exciting cause is the specific 

 agent or thing that induces the diseases. To illustrate, an animal 

 having a narrow, pinched chest may be in health but when sub- 

 jected to the same conditions as its companions, it contracts disease 

 while they do not. The lessened lung capacity has rendered the 

 animal susceptible. Hogs pastured on high dry ground and fed 

 on clean feeding floors are generally free from intestinal worms, 

 while hogs pastured upon low wet ground and fed in the mud are 

 frequently infested with worms. In the first case the predisposing 



