THE CAUSE OF BLACK DISEASE. 



In connecting the nuke with the transmission of black disease it is note- 

 worthy that in all undoubted cases of that condition there has been fluke 

 infestation of the li\er, and the invasion has been recent ; in some cases, how- 

 ever, it was only slight. The dnimals have all been in good condition. I 

 have never seen a case of black disease in which the nuke disease was very 

 advanced. This indicates that infection occurs very shortly after the 

 carrier fluke gains admission to the liver, and agrees with experimental 

 evidence, the latter showing that the condition is not a chronic one and that 

 the period of incubation is not a matter of several days. In natural cases 

 the period of incubation, of course, would not commence until the bacilli 

 had been lodged in the liver. A greater or few r er number of flukes may reach 

 the liver prior to the advent of the carrier, for it is not suggested that 

 every fluke is an actual carrier of infection, although it is a. potential one. 

 This is shown by the fact that a large percentage of sheep on infected 

 country, dead from causes other than black disease, have fluke-infested 

 livers. As a matter of fact, it is possible that relatively few of the total 

 number of fluke parasites reaching the liver carry with them the causal 

 organism of black disease, seeing that the iiecrotic foci ir> the liver, which 

 are the primary lesions, are usually so few in number. There may be only one 

 such lesion. In view of the relatively small number of bacilli required to 

 infect artificially, the great number of those present in a single hepatic 

 lesion would be quite adequate to account for the symptoms and death in 

 naturally-infected cases. In the few instances where careful search has 

 failed to reveal a definite local lesion in the liver, but where bacteriological 

 or experimental investigation has shown the bacteria to be present in that 

 organ, it is suggested that such are cases of mass infection, the defences of 

 the organ and the body in general having been overwhelmed before any 

 reaction that is, a. local lesion has had time to become visible to the naked 

 eye. In the early days of the investigations (1914-15) the liver was not 

 specially examined by making numerous incisions for these necrotic foci, 

 and no doubt those situated in the depths of the organ escaped observation 

 during the usual routine post-mortem examination. When, however, it 

 was found that the absence of these particular lesions was more unusual 

 than their presence, special attention was paid to the liver. 



In connection with fluke invasion itself, it has been noted by other 

 observers that bacteria (for example, B. coli) have been mechanically con- 

 veyed to the liver from the intestinal tract by that parasite. 



In discussing the role of the liver fluke in the transmission of black 

 disease, there is no doubt that the causal organism of the latter is numer- 

 ously present in certain' paddocks, probably scattered over the whole surface 

 by dead black-disease sheep allowed to rot where they die and other 

 agencies, but, being a facultative parasite, one would reasonably expect 

 it to be more abundant in the situations more favourable to its 

 saprophyti mode of existence, viz., in moist localities. It may be accepted 

 that sheep in such paddocks will swallow varying amounts of the bacilli in 

 question with their food and water, although such animals drink little 



