8o4 bacteria: their work 



give rise to characteristic diseases : such bacteria are spoken of 

 as pathogenic. 



Before a particular organism can be considered as the cause 

 of a specific disease, it must be found in the blood, lymph, or 

 tissues of all animals suflFering from the disease in question; 

 moreover, the organism must be isolated and cultivated outside 

 the animal body, and the introduction of such pure cultures 

 into healthy animals must give rise to the same disease. 



The manner in which bacteria cause disease is not in every 

 case quite clear. It is, however, known that in some cases the 

 bacteria, during their growth in the body of the animal attacked, 

 produce certain substances which act as poisons, and it is to the 

 direct action of the latter that the diseases are due. 



These poisons are generally designated toxins ; some of them 

 are elaborated by the bacteria when the latter are cultivated in 

 artificial media outside the body, and may be obtained from 

 the culture by filtration and other means. It is found that such 

 toxins, quite free from bacteria, when injected into the system 

 of an animal immediately produce the symptoms of the disease. 



In a text-book of the present capacity we cannot do more than 

 merely mention two diseases due to the attack of parasitic 

 bacteria, namely, anthrax and tuberculosis, both of which are 

 prevalent among farm animals. 



The organism, which causes anthrax or 'splenic fever' among 

 sheep and cattle, is Bacillus anthracis. It is one of the best 

 known bacteria, of comparatively large size, and easily detected 

 in the blood of all parts of animals which have died from the 

 disease. 



Infection, or the introduction of the bacillus into the body of 

 any susceptible animal such as a horse, cow, sheep, pig, or goat 

 is usually followed by death in two or three days. 



On the farm the bacillus is taken in by stock from water 

 and grass which have been contaminated by blood and other 

 discharges from diseased animals. 



