THE CAUSE OF BL\CK DISEASE. 



bacillus also, until next season. Furthermore, provided there is food else- 

 where, there is no inducement for sheep to frequent wet places in the 

 winter. 



It hardly needs stating that if the causal organism is not present either 

 contaminating the soil, or mixed with the stomach contents of the sheep, no 

 amount of fluke invasion will produce black disease. 



Another interesting feature perhaps unimportant in itself, but forming 

 one of the links in the chain of evidence is that for several years past the 

 seasons in general have been on the whole dry, and several large sheep- 

 owners have informed me that during the past few years black disease has 

 by no means has been so prevalent on their stations as in former years. 



Although it may be admitted that the evidence put forward to incriminate 

 the liver fluke in its migration from the exterior of the body to the liver of 

 the infected sheep, as being the mechanical agent in the transmission of 

 the causal organism of black disease is largely circumstantial, yet this 

 explanation of the natural method of infection and the situation of the 

 primary lesion or lesions appears more satisfactory than any other advanced 

 so far. 



Immunisation. 



After the determination of the causal agent it was a natural sequence 

 to endeavour to devise a method of prevention against black disease. Cura- 

 tive treatment, in view of the acuteness of the disease and the conditions 

 under which sheep are kept in Australia, is impracticable. One aspect of 

 prevention has already been dealt with, viz., the rendering of the moist 

 areas unsuitable breeding-grounds for the intermediate host of the fluke 

 (the water snail) by drainage. If this is not practicable such areas could 

 be fenced off. The question of preventive vaccination has been considered, 

 and an attenuated vaccine prepared. It was first tried on small animals, 

 then on sheep, at the laboratory. About sixty of the latter, in batches of 

 three or four, were vaccinated at various periods, and apparently the 

 immunisation was efficacious, as the animals subsequently resisted fatal 

 amounts of virulent bacilli. No reaction to the latter appeared at times, 

 and a moderate local tumifaction developed at others, if a large dose (for 

 example, 3-5 c.cs. of virulent culture) were injected. There were no deaths. 



This success determined me to try the value of preventive vaccination in 

 the field. During the seasons 1918-19 and 1919-20 between 5,000 and 6,000 

 .sheep were vaccinated (about 2,600 each season), at such a period as would 

 permit the immunity to be fully established before the onset of black 

 disease. The animals were placed in two notoriously infected paddocks 

 where the annual loss for years had been from 20 to 30 per cent., and 

 several hundred uiivaccinated sheep of the same age, sex, &o., were placed 

 with them as controls. The results have been not altogether expected. If 

 the vaccination were unsuccessful one would expect both vaccinated and 

 control sheep to die in the same proportion, whereas if it were successful, 



