20 SCIENCE BULLETIN, No. 21. 



water when food is succulent. Yet, as has been experimentally demon- 

 strated, the mere ingestion of large amounts of virulent bacilli is insufficient 

 to infect. As already noted, Fasciola hepaticum is very prevalent in 

 the affected paddocks. Whether the immature fluke (cercaria) becomes 

 contaminated whilst on the ground, or whether it becomes so during its 

 passage through the anterior part of the alimentary tract, is a matter for 

 speculation, but there is no improbability in the latter. 



A feature of the experimentally-induced disease is that if minimal lethal 

 doses of bacilli be injected, its course may be prolonged a day or so, and 

 the lesion at the point of inoculation is more pronounced. One can thus 

 inderstand that If the number of bacilli introduced into the liver by the 

 fluke is insufficient to overwhelm the defences of that organ, there is time 

 for a reaction on the part of the latter to occur, and for a definite local 

 lesion to become evident to the naked eye. That such a reaction on the 

 part of liver tissue does take place as a rule is evidenced by histological 

 examination. The result is that the bacilli are confined to the lesion and 

 no systemic invasion ensues, the general symptoms and lesions elsewhere 

 no doubt being due to an endo-toxin. 



The seasonal occurrence of black disease may be explained by taking into 

 account the habits of sheep and the life history of the liver fluke. Fluke 

 infestation of animals is generally considered to occur mainly in spring 

 and autumn, although it may take place at any time. In the spring there 

 is little need for sheep as a whole to .graze over the marshy places, which 

 are relatively small in area, in an infected paddock, as the animals drink 

 little water then, and the herbage elsewhere is, in general, succulent. In 

 spring and summer, therefore, the probability of the causal organism being 

 conveyed to the liver by the immigrating flukes is not so pronounced. The 

 disease first becomes noticeable in summer. At first, the death-rate is light, 

 but as the season advances the mortality gradually increases until in late 

 autumn it reaches its height, coinciding with the period at which the migra- 

 tion of flukes to the liver is also at its maximum. The onset of severe frost 

 is generally considered to put a stop to the ravages of the disease for that 

 season, but if the weather be mild, deaths take place throughout the winter. 

 In some places where the winters are not so severe it is stated that black 

 disease may be seen at any time of the year. This latter statement, how- 

 ever, is not to be accepted without reservation, for sometimes any death 

 of sheep is ascribed to black disease merely because the animals have been 

 found dead, no attempts at post-mortem examination having been made. 

 Furthermore, I have known men confidently say that they have known sheep 

 to die from black disease in their district, although they will admit that 

 they have never examined a case of that disease. 



In the districts where the disease in enzootic, the cessation or continuance 

 of infection may be explained if one considers the effects of frosts on fluke 

 embryos and cercariaB. Numbers of the former (not all, by any means) 

 will be killed off, thus lessening the number of cercarise to be swallowed 

 by the sheep, and consequently the chances of infection by the black disease 



