372 SPECIFIC MICRO-ORGANISMS 



spherical structures of variable size and number may be seen. 

 The exact nature of the Negri bodies is uncertain. Some stu- 

 dents of rabies regard them as protozoa, while others consider 

 them to be products of cell degeneration. The evidence to de- 

 cide the matter is not yet at hand. They seem to occur only in 

 rabies and to be constantly present in this disease. 



Lyssa or rabies 1 is primarily a disease of dogs but it occurs in 

 other mammals as well, usually as a result of dog bites. In ani- 

 mals inoculated directly into the brain with the most virulent 

 material (fixed virus), the symptoms of rabies appear in 4 to 6 

 days and death occurs on the seventh day. Inoculation with the 

 saliva or nervous tissue of a mad dog (street virus) rarely causes 

 symptoms before three weeks and the onset may be delayed for 

 a year. In fact many persons and animals bitten by rabid dogs 

 may fail to develop the disease at all. This variability depends 

 upon the virulence and the amount of virus and especially upon 

 the part of the body into which it is introduced. Bites upon the 

 face or hands, because of the rich nerve supply and the lack of 

 protection by clothing, are especially dangerous. After the dis- 

 ease has developed so as to cause symptoms, death is inevitable 

 in the present state of our knowledge. 



Rabies may be diagnosed in an animal by observing the course 

 of the disease, by autopsy and by inoculation of test animals and 

 observation of the course of the disease in them. If the sus- 

 pected animal be caged, the question of rabies may be settled in a 

 few days, for, if he is mad, the raging stage will be quickly followed 

 by the characteristic paralysis and death. If the animal has been 

 killed, a careful autopsy may reveal the absence of food from the 

 digestive tract and the presence there of abnormal ingested ma- 

 terial (grass, wood or stones), highly suggestive of rabies. Mi- 

 croscopic examination of the central nervous system may reveal 

 the Negri bodies, characteristic of the disease. For confirmation 

 of the diagnosis a portion of the brain or spinal cord, removed with- 



1 For a general discussion of rabies see Gumming: Journ. A. M. A., 1912, Vol. 

 LVIII, pp. 1496-1499. 



