farmers' institutes, 817 



n suseoptible animal. The iuociilation li;is the cffi'd oT iiiodiiciiiu ;i mild 

 lorin of the disease wliich would sufiiic t(( prutcct :i;i:iinst a natural in- 

 fection. In localities where the danjjn- is yreat an inoculation is lirst 

 made with material that has been heated to a liigh decree, and after ten 

 days again inoculated with material that has been heated Itut to a lesser 

 degree but which if used in the first case would cause serious illness 

 and i)0ssibly death in many cases. The first inoculation produces a 

 slight immunity :ind the second greatlj' increases it. 



Heating is not the only method of decreasing or attenuating the viru- 

 lence of germs. It may also be accomplished by growing them in differ- 

 ent media, and at different tempei-atnres, for different periods of time, 

 and by ehemicals. All of these methods have been employed in securing a 

 virus for protecting against hog cholera and, like vaccination, without 

 success except in a comparatively limited number of cases. Botli vaccina- 

 tion and inoculation are objectionable in that living germs are emi)loyed 

 and in the event mild cases are induced the germs will be passed from the 

 body and may Ijecomo the center of infection for an epidemir in n lierd 

 or neighborhood. 



Inoculation is also made by taking blood from an animal suffi'ring 

 with the disease and injecting it into a susceptilde animal. This is the 

 method used in immimizing against southern cattle fever. Tlie dose is 

 small. Attempts have been made to use the sterile serum from affected 

 animals but without success. In the case of southern fever the hypo- 

 dermic syringe takes tlie place of the tick, whicii does the same thing under 

 natural conditions. By the artificial method the size of tlie dose is reg- 

 idated. The same metliod has l>een tried with hog cholera. 



Inoculation differs from vaccination in that the material is placed di- 

 rectly into the tissue of the body with a liypxiderniic syringe and not by 

 ^scratching the surface. 



Feeding.— Imnnmity may also be acquired by feeding small quantities 

 of germs that have had their virulence reduced. The quantity and viru- 

 lence are both gradually increased until the aiumal can successfully with- 

 stand what would ordinarily produce a serious <ii- fni.il illness. 'I'liis 

 method has been found successful in some experimental work and is 

 used by a few l)reeders with ai»parent success. This is in reality at the 

 basis of a method that has receivi'd a lirejit d'al of attention in the i»ast 

 few years. It was accomplished in a crude way by feeding swine -on 

 pieces of the carcass of one that had died of cholera. The quantity was 

 i^radii-illy increased and the periods between the feeding shortened. The 

 animals selected for the purpose were gcneially old ones whose resistance 

 were already strong. Its use was especially recommended for breeding 

 sows near the time of farroAving as it was l)elieved that immunity would 

 result in utcro. The process was continued .after farrowing to iidensif.\- 

 the immunity through the effect on the mother's milk. The natural prod- 

 uct not being always available and necessarily of v.-iriable character. 



52-Afc'ri. 



