GROUP OF SPORE-BEARING ANAEROBES 357 



The swelling increases rapidly in size, and the emphysema soon 

 manifests itself by the crackling sound produced when the thumb 

 is drawn firmly across the part. After death the organisms con- 

 tinue to grow and the body becomes distended with gas. The 

 subcutaneous tissues of the infected part are edematous, even 

 gelatinous, with blood and gas-bubbles. The underlying muscles 

 are dark brown or even blackish, whence the name, blackleg. 

 The disease usually results fatally in cattle in from one to three 

 days after the first appearance of the lesions. 



Immunity. The production of true toxins by Bacillus chauvcei 

 is not well understood. According to some authors, no toxin can 

 be demonstrated. Others believe that there is a relatively thermo- 

 stabile toxin produced which will endure a temperature of even 

 115. Grassberger and Schattenfroh, who have made the most 

 careful study of this problem, have succeeded in producing broth 

 cultures containing a toxin that, in doses as small as those em- 

 ployed with diphtheria toxin, will kill laboratory animals. This 

 toxin is produced by certain strains of the organism only, but these 

 they believe are the more pathogenic. This toxin they have shown 

 to be thermolabile. They have worked out methods of standardi- 

 zation closely resembling those of Ehrlich for diphtheria. Anti- 

 toxin may be produced by the injection of increasing doses into 

 suitable animals, particularly cattle, and this antitoxin has a 

 protective influence when injected into other animals. This 

 method of immunization has never come into general use. 



Animals that have recovered from an attack of the disease 

 acquire immunity to a recurrence. Very young cattle and aged 

 cattle have a considerable degree of natural immunity. To 

 what the immunity developed may be due is not well understood. 

 Probably it is in part opsonic. 



Active immunization of animals is extensively practised. 

 Various methods of attenuation of the organism for the vaccine 

 have been developed. That in common use in the United States 

 is the one adopted by the Bureau of Animal Industry, and is 

 essentially that developed by Kitt. Fresh material is secured 

 by macerating in a mortar the muscle tissue from a blackleg 

 tumor and squeezing the fluid through a linen cloth. This is 

 spread in a thin layer, and dried to a brown scale at a temperature 



