BUREAU OF ANIMAL INDUSTRY. 619 



ter, involving nearly all tlie vital organs. Thcso are found to con- 

 tain the speciiic bacterium in large quaiitities. 8ul)Cutaneous inocu- 

 lations of cultures derived from svicli cases will in general produce 

 a disease as severe and as rapidly fatal. When the pigs are fed with 

 the viscera from these same cases even more severe local and general 

 lesions are the result. From such a height of virulence there is a 

 gradual descent. Animals which have become infected in the natu- 

 ral way are attacked by a milder and more protracted disease. Inoc- 

 ulations of cultures become less successful or fail altogether. _ Even 

 when fed with the viscera of animals which have died of the disease, 

 pigs will after a time become affected with a slow chronic malady. 

 In these milder forms of the disease comparatively few l^acteria pene- 

 trate into the vital organs to multiply there. _ Cover-glass prepara- 

 tions of spleen pulj) do not show a single microbe in many fields, 

 while the same j^reparation from hemorrhagic cases rnay show from 

 50 to 100 in every field of a i\ homogeneous objective. Nor does 

 the quantity of virus introduced materially change the result. When 

 the disease is at its highest point of virulence cases of natural infec- 

 tion are frequently as severe as those fed with large quantities of 

 virus. On the other hand, when animals are fed with cultures of 

 more or less attenuated virus, the local destruction of tissue in the 

 intestines may be very grave and cause speedy death, but the internal 

 organs remain more or less intact. 



This change in virulence has not been observed in experiments 

 upon mice, rabbits, and guinea-jDigs. The same peculiar lesions have 

 appeared throughout a period of fourteen months. The duration of 

 the disease may vary, but this depends ujDon the quantity of virus 

 introduced into the system. The lesions in the jjrotracted cases due 

 to the inoculation of very small quantities are if anything more pro- 

 nounced. 



What agencies are at work in bringing about this variation in 

 virulence is a problem still to be solved. There is no clew to an ex- 

 planation, and we simply record the facts as observed. There seems 

 to be a sufficient reason for regarding the increase of virulence as due 

 to climatic and meteorological conditions affecting the bacterium 

 outside of the body, for our own oliservations show that the success- 

 ive x)assage of the virus through the body of pigs by feeding dimin- 

 ishes its virulence and may finally destroy it. 



The relation of the virulent and attenuated bacteria to the ani- 

 mal organism is expressed by the statement that the former are 

 capable of living and multix)lying in the blood vascular system of the 

 infected pig, while the latter are unable to do so. Their destructive 

 action is limited to the intestinal tract. There may be two properties 

 by whose change a virulent bacterium becomes attenuated, and vice 

 versa — the capacity of living with a limited supply of oxygen, and 

 the power of forming a poison or ptomaine which is more or less de- 

 structive to cellular life. Either or both of these properties, Avhen 

 augmented or diminished, may bring about the differences which are 

 observed between malignant and mild types of the disease. 



EXPERIMENTS UPON OTHER ANIJMALS WITH THE HOQ-CHOLERA BACTERIUM. 



Mice infected hy feeding. — It was desirable to determine how far the bacterium of 

 hog-cholera was infectious to other animals besides pigs when introduced with the 

 food into the alimentary canal. Mice, being susceptible to inoculation, might con- 

 tract the disease about the pens, and being eaten by pigs would quite naturally be- 

 come a source of infection. To determine this point bread and spleen were thor- 



