618 REPORT OF THE COMMISSIONER OF AGRICULTURE. 



tious material or by subcutaneous iuoculation, presented tlirougliout 

 tliG lung tissue small liemorrhagic foci involving sevei-al contiguous 

 lobules. On section tlie}^ were of a dark-red color, and indicated 

 extravasation into the alveoli, wliicli were filled up witli coagulated 

 blood. These foci are very likely caused by plugs of bacteria grow- 

 ing in the capillaries, producing necrosis of their walls and consequent 

 extravasation. Taking into consideration the condition of the re- 

 maining viscera in sucui cases, it is highly })robable that the foci are 

 not primary, but secondary in character. They are not growths of 

 the bacteria introduced hj the inspired air, but carried there by the 

 circulation, the original place of entrance being the alimentary tract. 

 These centers of growth may gradually spread and involve the entire 

 lung substance, giving rise to that extensive hepatization occasionally 

 found. That infection may arise through the air in some cases, 

 especially in summer, when the bacteria are dried and carried away 

 as dust, is not necessarily excluded. The greater activity of the virus 

 in the warm seasons cannot be entirely duo to its dissemination in 

 this manner, since drying destroys the bacteria of this disease in from 

 one to two months, and it may reduce their pathogenic power in a 

 much briefer time. The greater diffusion and mortality must be 

 attributed to more favorable opportunities for the bacteria to multi- 

 ply outside the animal body in streams and in the soil on animal and 

 vegetable substances. In this connection it might be well to record 

 the following experiment: 



Two pigs placed in an air-tight box were subjected to the spray of an atomizer 

 containing 10™ of a Hquid culture of the hog-cholera bacterium diluted ^vith 40'^'^ 

 of distilled water. The spray was directed upon the faces of the animals, which they 

 could not avoid, owing to the small size of the box. They were removed at the end 

 of thu'ty minutes and placed in a disinfected pen. After a few days they seemed 

 somewhat dull, but both recovered. Several months after one o[ them ilied of 

 hog-cholera on being fed with infectious material. It is highly probable that the 

 bacteria were carried into the lungs at the time. 



In view of the fact that another bacterium has been recently found 

 associated with severe lung lesions, and is probably the cause, it be- 

 comes necessary to re-examine diseases of the lungs, whether associated 

 with true hog-cholera or occurring independently. The subject is 

 fully discussed further on. 



Cycles of virulence. — The variation in the severity and extent of 

 epidemics of infectious diseases has been a subject for observation 

 and comment by all who have studied them more carefully. It is 

 characteristic of infectious diseases attacking man as well as those to 

 which animals are subject. 



The change in virulence is indicated both by the number of ani- 

 mals affected in a given time and by the suddenness with which 

 they are struck down after infection. Tlie record of cases of hog- 

 cholera kept at the experimental station for more than a year is very 

 instructive, in showing clearly how the virus of a specific disease may 

 become very much attenuated, then suddenly regain its virulence, 

 sweep aAvay a large number of animals very rajiidly, then again lose 

 its virulence until it has scarcely any efl'cct upon the animal system. 

 This change in virulence is indicated in various ways. When atten- 

 uated the virus produces a chronic disease, characterized by local 

 ulcerations of the mucous membrane in the large intestine. The 

 affected animal lives for four or more weeks. This is the way in 

 which the disease manifests itself most commonly. When the viru- 

 lence is great the disease is rapid, the lesions hemorrhagic in charac- 



