PAEASITES 437 



expellers, during which time they should be carefully tied 

 up, and their faeces destroyed by boiling water, or preferably 

 by burning. 



Very heavy losses from ' gid ' may occur in a, flock, but 

 inasmuch as there can be no ' gid ' without the pastures 

 being affected from the fseces of a dog suffering from tape- 

 worm, it is clear that the disease is well within the bounds 

 of prevention. 



The destruction of the brains of all sheep affected with 

 the disease should be carried out with the greatest care, for 

 if dogs have access to them they are certain to become 

 affected. 



The most important parasite belonging to the order of 

 Treinatodes is the Liver Fluke, Distovia hepaticum, of sheep. 

 This parasite may also be met with in cattle and horses, 

 but in sheep it causes widespread disease commonly known 

 as ' Rot: 



Fluke Disease. — It will be convenient in the first place to 

 deal with the parasite. The mature fluke lying in the bile- 

 ducts lays its eggs, which subsequently reach the intestinal 

 canal, and so find their way on to the land. As each fluke 

 is capable of laying several hundred thousand eggs, and 

 there are often scores and hundreds in one liver, it is easy 

 to understand how thoroughly the pastures may be strewn 

 with them. Fortunately, only a fraction ever pass beyond 

 the egg stage, as it is necessary for the further cycle of their 

 existence that they should pass into the body of a fresh 

 water-snail, Linmcea truncatula. Within this they undergo 

 a complex development, into what is finally known as 

 cercarice, and the water-snail with the contained cercarise, 

 or the latter already liberated in puddles or damp places, 

 are taken in with the food or water, and the mature fluke 

 in course of time develops. 



Fluke disease can only be produced in wet pastures. It 

 follows as the result of a long wet season, or keeping sheep 

 on undrained meadows or marshes. It does not occur on 

 salt marshes, for the presence of salt kills the intermediate 

 host. 



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