DISEASES OF. SWINE 449 



notably yellow fever, contagious pleuro-pneumonia, South African 

 horse sickness, and foot-and-mouth disease. (F. B. 379.) 



Predisposing Causes. While the specific cause of hog cholera 

 is the minute micro-organism or germ just referred to, there are 

 many factors which may render a herd more susceptible to the dis- 

 ease. In general, anything which tends to lower the health of the 

 animals may be regarded as a predisposing cause. Among such pre- 

 disposing factors there may be mentioned improper feeding, an in- 

 sanitary condition of the hog lots, damp or cold sleeping places, and 

 dirty drinking and feeding troughs. Of course insanitary surround- 

 ings and poor feed can not in themselves produce hog cholera, but 

 they lower the vitality of hogs to such an extent that they become 

 comparatively easy victims of any disease-producing germs to which 

 they are exposed. (F. B. 379.) 



Manner of Infection. Although the conditions just mentioned 

 undoubtedly exert considerable influence upon the relative resisting 

 powers of hogs to hog cholera, the disease can be started in a herd 

 only by introducing the germ which causes it. This germ is always 

 present in the bodies of sick hogs, and is thrown off from them in 

 large numbers in the feces and urine, thus contaminating the yards 

 or pens in which sick hogs are kept. The most dangerous factor in 

 spreading hog cholera is, therefore, the sick hog; but any agency 

 which might serve to carry a particle of dirt from infected yards may 

 be the means of starting an outbreak of the disease. 



Sick hogs may get onto a farm (1) by escaping from a neigh- 

 boring herd, (2) by the purchase of new stock which may show no 

 symptoms of sickness until some days after purchase, (3) by return- 

 ing show hogs to the herd after visits to fairs or stock shows, (4) by 

 purchase of hogs which have apparently recovered from hog cholera. 

 The risk incurred by purchase of new hogs or the return of hogs 

 which have been shown at fairs is chiefly due to the fact that such 

 hogs are generally transported by rail, unloaded in public stock 

 yards, or driven along public roads. It is well known that sick hogs 

 are frequently shipped by rail, and the roads over which they are 

 driven, the stock yards, and the railroad cars thus become contam- 

 inated with the germs of hog cholera. 



When healthy hogs are placed in such cars or yards or driven 

 along public roads they almost always have the opportunity to con- 

 tract the disease, but the intervals between shipment and delivery to 

 the purchaser is so short that the symptoms do not appear until a 

 week or more after delivery, when it is usually too late to prevent 

 the spread of disease to hogs already on the farm. Obviously, the 

 only safe plan in such cases is to place all new arrivals in lots en- 

 tirely separated from those occupied by the main herd and to keep 

 them isolated until all danger of their developing hog cholera has 

 passed. 



Aside from the danger of introducing infection through the hogs 

 themselves it must be remembered that the germ of the disease, which 

 as already stated is infinitesimally small, may be transported in a 

 minute particle of dirt on the feet of attendants or neighbors who 



