CONTAGIOUS DISEASES OF DOMESTICATED ANIMALS. 393 



by not affording favorable conditions for development and propagation, 

 or suflSciently frequent changes from within to without, and vice versa., 

 of the animal organism, as seems to be the case, or whether they have 

 Gn\y reduced the number of those microscopic parasites by causing a 

 great many to perish, or denying them an opportunity to reach their 

 proper nidus or place of development in the body of a hog, will be very 

 difl&cult to decide, and is practically immaterial. ^ 



At first, it appeared that the disease was milder only in Henderson 

 County, and I thought the sandy soil, the hilly or somewhat broken sur- 

 face near the Mississippi Eiver, aud the, therefore, more i)erfect drainage 

 might have something to do with it; but this probably is the case only 

 to a very limited extent, because reliable people have assured me that 

 the disease was last year (1878) just as malignant in Henderson County 

 as in any other place. Still, the sandy soil, good drainage, &c., is prob- 

 ably not altogether without influence, especially if the season is inclined 

 to be dry, for, even during the present winter (1879-'80), the disease 

 proved to be more malignant in the eastern parts of the county, in the 

 vicinity of Biggsville, where the soil is darker and heavier and the sur- 

 face less broken than further toward the Mississippi. 



One other circumstance may also have contributed somewhat in caus- 

 ing swine plague to be more lenient this year than a year ago. All con- 

 tagious and infectious diseases, in order to aiiect an animal, seem to 

 require in the latter a certain degree of predisposition ; in other words, 

 the disease-producing Schizomycetes, in order to be able to produce mor- 

 bid changes, seem to require certain conditions which do not exist in the 

 same degree in every animal, and which, to all appearances at least, 

 may even be entirely absent in some few animals, or may become par- 

 tially or fully exhausted, or completely destroyed under peculiar circum- 

 stances; for instance, by a previous attack. Further, it is well known 

 that on the first appearance of almost every contagious or infectious 

 disease those animals, as a rule, become affected first and succumb 

 soonest which possess the greatest predisposition or offer the most 

 favorable conditions for the development and the effectiveness of the 

 infectious principle. Swine plague does not seem to make an exception. 

 "Wherever it prevailed very extensively a year ago, it may be presumed that 

 the hogs and pigs which possessed a special predisposition, or offered very 

 favorable conditions, and became exposed to the influence of the infectious 

 principle, contracted the disease and have since died, aud consequently 

 are out of the way ; that most, if not all, of the older hogs at present 

 existing, especially as the disease prevailed last year almost everywhere, 

 are animals with comparatively little predisposition ; and that the pigs born 

 since last spring and now living are mostly the offspring of sows which 

 were not much predisposed, or did not offer very favorable conditions 

 for the development of the disease. That such a difference as to predis- 

 l)Osition must exist becomes patent by the fact that in nearly every 

 affected herd, no matter how malignant the disease may prove to be, 

 one or a few animals will either remain exempted altogether or will con- 

 contract the disease only in a very mild form, and recover. It receives 

 also some additional confirmation by the fact that wherever swine 

 plague makes its appearance for the first time it usually proves more 

 malignant than at places at which it has been prevailing year after year, 

 provided the quantity and intensity of the infectious principle are about 

 the same. In Henderson County the disease has been an almost regular 

 visitor for twenty-seven years, and in Southern Wisconsin it is a com- 

 paratively new disease. According to a letter received in December 

 (1879) from a reliable person in Bloomiugtou, Grant County, Wiscon- 



