18 SCIENCE BULLETIN, No. 21 



weighing- tha arguments presenting themselves, I have arrived at the con- 

 clusion that the solution of the problem of the natural method of infection 

 and the situation of the primary lesion in the liver is to be found in the 

 liver fluke. If the feasibility of such a hypothesis be accepted, then almost 

 all of the features concerning the transmission of black disease become 

 fairly clear. 



The factors that have been found to coincide with the prevalence of 

 black disease are just those which prevail in connection with the presence 

 of recent fluke infestation. Drainage of marshy areas around and along 

 the course of springs and watercourses, where carried out effectively, has 

 often been succeeded the following season by a greatly diminished mor- 

 tality from black disease. In fact, before I had demonstrated the cause of 

 the disease, I consistently recommended this plan. When owners now seek 

 advice regarding this condition I generally inform them that if they get 

 rid of the liver fluke they need not trouble very much about black disease. 

 This advice, when followed out by draining the infected places, has at times 

 had a not unexpected result, because paddocks where it was almost certain 

 one could get material in the shape of affected sheep during the season for 

 research work, have now become useless for investigations, as few cases are 

 seen in them, and these at only irregular periods. 



Drainage, as is well known, assists in keeping down fluke disease by 

 decreasing the number of water snails (the intermediate host of the fluke), 

 and providing an unfavourable nidus for the fluke embryos, &c. An instance 

 of the foregoing may be cited. A certain paddock running about 2,000 

 sheep has been notorious for black disease for the past twenty-six years, the 

 average annual loss in it from this condition being about 25 to 30 per cent. 

 The paddock contains several springs and the ground in their neighbour- 

 hood was swampy. Up to the end of 1918 fluke disease in this paddock was 

 very prevalent. At the end of that year the paddock was well drained. The 

 season of 1918-19 was very dry. During that year the paddock carried about 

 1,000 sheep. The mortality from black disease was about 3 per cent. In 

 another paddock, running about 2,000 to 3,000 sheep, where also the previous 

 annual losses from this disease had been considerable, the same conditions 

 obtained in 1918-19 as on the area just mentioned, viz., drainage of the 

 swampy spots and dry season. The percentage mortality from black disease 

 was lowered to about the same as in the previous instance. In both cases 

 the sheep during that season had been under closer observation than usual. 



Seeing that the causal organism is a sporing, and in every probability a 

 facultative, parasite, it is not probable that it could have died out in these 

 paddocks in such a short period. The explanation appears rather to be that 

 the intermediate host of the liver fluke, viz., the water snail, had become 

 considerably reduced in numbers, or had migrated to more congenial 

 localities not grazed over by the sheep, and also that many of the fluke 

 embryos, not finding their host, had also died, the consequence being less 

 fluke invasion of the liver of the sheep in these paddocks and less black 

 disease. 



