ETIOLOGY 49 



that in the upper air passages of healthy swine, cattle, horses, 

 cats and dogs* there are bacteria not distinguishable in their 

 cultural characters and their effect upon rabbits from the swine- 

 plague bacterium. The presence of this organism in the trachea 

 of healthy pigs has been suggested as the source of the cause 

 of sporadic cases of swine plague and it may explain the fre- 

 quent association of this form of pneumonia with hog cholera. 

 What the conditions are by which these bacteria are enabled 

 to produce disease in their host have not been clearly pointed 

 out. 



If the rabbit is taken as the animal on which to test the 

 pathogenesis of the bacteria belonging to the swine-plague 

 group, we find that those from different sources are very simi- 

 lar. In nature, the bacteria of swine plague, rabbit septicaemia, 

 fowl cholera, and those located in the normal upper air pas- 

 sages of the various species of animals mentioned possessed 

 of marked variation in virulence exist, that is, those which 

 will kill a rabbit when inoculated subcutaneously with a pure 

 culture in from i6 to 24 hours to those which require from 3 

 to 10 days, or even weeks, to destroy life. With the variations 

 in the time period, we have corresponding differences in the 

 lesions. The virulent forms produce septicsemia while the 

 attenuated varieties excite a severe purulent infiltration about 

 the place of inoculation and exudates on one or more of the 

 serous membranes. Conversely, it has been shown that rab- 

 bits possessed of a certain amount of natural or artificially 

 produced resistance will, when inoculated with a virulent cul- 

 ture, die after the same period of time and with lesions similar 

 to those produced by the attenuated virus in the susceptible 

 rabbit. 



The fact that this organism is not frequently described in 

 our works on bacteriology and because of some coniusion 

 existing concerning it and the bacillus of hog cholera, it is 

 deemed best to insert a short description of it here. 



*The investigations thus far made show these bacteria to be present 

 in 48 per cent, of healthy swine, 80 per cent, of cattle, 50 per cent, of 

 sheep, 16 per cent, of horses, 90 per cent, of cats, and 30 per cent, of dogs. 



