INFECTIOUS DISEASES. 541 



lands dry out during the heat of summer and are then covered with 

 light rains. Decaying vegetable matter seems most favorable for 

 nourishing and preserving the virus. 



The direct cause of anthrax is always infection of a previously 

 sound animal, either directly from a diseased animal or through 

 various media which contain excretions or the debris from the body 

 of one previously infected. 



The specific virus of anthrax was first discovered by Davaine in 

 1851. He recognized microscopic bodies in the form of little rods in 

 the blood of animals suffering from anthrax. It was not, however, 

 till a quarter of a century later that Pasteur defined the exact nature 

 of the bacillus, the mode of its propagation, and its exact relationship 

 to anthrax as the sole cause of the disease. In the animal body this 

 bacilli have a tendency to accumulate in the spleen, liver, and else- 

 where, so that these organs are much more virulent than the muscles 

 or less vascular tissues. When eliminated from the animal in the 

 excretions, or when exposed to outside influences by the death of the 

 animal and the disintegration of the tissues, the body of the rod is 

 destroyed and the spores only remain. These spores, which may be 

 called the seeds of the bacilli, retain their vitality for a long period; 

 they resist ordinary putrefaction ; they are unchanged by moisture ; 

 and they are not affected by moderate heat. If scattered with the 

 debris of a dead animal on the surface of the ground, they may re- 

 main around the roots of the grass in a pasture or may be washed to 

 the nearest low-lying ground or marsh. If buried in the body of an 

 animal dead from anthrax, they may be washed deep into the ground, 

 and in later years (in one proved case 17 years) be brought to the 

 surface and infect other animals. They are frequently brought to 

 the surface of the earth, having been swallowed by earthworms, in 

 the bodies of which they have been found. 



This accounts for the outbreaks at the time of the first rains after a 

 dry season. During the latter the earthworm goes deep in the ground 

 in search of moisture ; it finds the spore which has been washed there 

 in past years, swallows it, and afterwards brings it to the surface. 

 The virus is carried with the wool from infected sheep and remains in 

 it through the process of manufacture into cloth. The spores remain 

 in the hides of animals which have died of anthrax and retain their 

 vitality throughout months of soaking in the tanners' pits, the work- 

 ing of the harness maker or the cobbler, and after the oiling of the 

 completed leather. The dried spores in the dust from any of these 

 products may be carried by the atmosphere. 



Infection of an animal takes place through inoculation or contact 

 of the bacillus or its spores with an abraded surface or mucous mem- 

 brane, on a sound animal. In an infected district horses may eat with 

 impunity the rich pasturage of spring and early summer, but when 



