56 



B. typJii murium (Loeffler) is fatal to mice ( Mus musculus), but not 

 to rats. M. Danysz isolated a bacillus during an epidemic of field 

 mice which was indistinguishable from the above, except that its 

 virulence was capable of being raised to a point where it would 

 destroy a relatively large percentage of rats inoculated with it by feed- 

 ing. We see from this that, strictly speaking, it is not a natural dis- 

 ease among rats, still there are cases where its virulence has for a 

 time remained high enough to infect a considerable per cent of rats 

 exposed to those that have sickened of it. Not only is this true in 

 cage experiments, but probably it sometimes occurs in nature after 

 the virus is once thoroughly introduced (an article by M. Danysz; 

 also experience of this service in plague in San Francisco, 1903 to 

 1905), and may therefore be grouped under the list of " natural" 

 infections. This bacillus is unfortunately of a very unstable nature, 

 in so far as its virulence is concerned; some cultures appearing to be 

 avirulent, while others cause an all but absolute mortality among 

 the rodents eating it. 



The duration of the disease is variable and appears to depend 

 somewhat on the size of the dose received, as well as virulence of the 

 culture. We have seen death in thirty-six hours or less following 

 ingestion. On the other hand, it may occur in two weeks. Usually 

 it occurs in from six to twelve days. In a typical case when the animal 

 has lived ten or twelve days it is much emaciated, its tissues are dry, 

 and intestinal hemorrhages are sometimes met with. When the dis- 

 ease is much prolonged a pustular eruption may be present over the 

 skin. The organism can often be isolated from the heart blood by 

 plating, such isolation alone affording means of diagnosis. The only 

 present interest this organism has is as a means of destroying the rat. 

 It was believed to be harmless to man, but more recently cases of 

 human illness have been reported that were believed to have been 

 caused by infection with this bacillus. 



PNEUMONIA. 



We have recently seen a case of lobar pneumonia in a rat in which 

 a diplococcus was present in pure culture. Possibly connected with 

 this is a condition of abscess of lung, which is not very uncommon. 

 The cavity is filled with a creamy or cheesy matter composed of 

 broken-down cells. Often these cavities break into the pleura. 

 Several morphological types of organisms are found, but from their 

 variation this laboratory has regarded them as secondary or acci- 

 dental, especially as we have failed to demonstrate that this material 

 was infectious. 



STAPHLOCOCCUS ABSCESSES. 



These are rather common and may occur subcutaneously or in the 

 superficial muscles of any part of the body. 



