Parasites of the Liver. 353 



showers may, however, act like the night dews and endanger tlie 

 sheep. 



Again in very wet seasons, certain habitually ' ' rotting lands ' ' 

 may prove comparatively harmless, while the sheep on higher 

 and comparatively less suspected pastures sufEer. The infested 

 snails dislike to be completely submerged, and migrate to the 

 adjacent higher level which is still damp but not submerged. 

 The sheep too eat off the tops of the grasses which project above 

 the water, and fail to take in the lower parts, on which the 

 cercaria may be more numerous. 



Harms and Michalik claim that they have seen the disease 

 transmitted by hay, the encysted cercaria retaining its vitality 

 though dried. 



Frost must be looked upon as a bar to infesting of the sheep. 

 The snails disappear or perish under frost, and it is only when a 

 thaw comes that these and the cercaria can resume their activity 

 so as to invade the system of the mammal. Animals may come 

 down with the disease during a long freeze, but it is from dis- 

 tomata taken in before it set in, or from those that are preserved 

 in dry fodder. 



As the cercaria may escape from the snail at any seaison when 

 the temperature is above freezing, and as the period from the egg 

 to the young fluke need be little more than 40 days, the conten- 

 tion of Gerlach and Johanni that sheep are never infested in 

 spring cannot be successfully maintained. With the advance of 

 the season, however, and the extensive encrease of the larvae in 

 the snails the cercaria become more and more abundant, and thus 

 late summer, autumn and early winter are preeminently the sea- 

 sons of distomatous invasion. 



Receptivity to distoma and distomatosis on the part of the 

 mammal is mainly associated with low condition. Lambs suffer 

 more severely than the mature sheep, and old ewes with worn out 

 teeth and weakened systems rather than the vigorous middle 

 aged. Debility from winter confinement and insufficient or un- 

 suitable food, and the disposition to devour the first growth of 

 spring in the low, damp and infested localities contribute to the 

 severity of the attack. Debility from other parasitism or any pre- 

 existing disease is equally predi.sposing. So it is with the lower- 

 ed vitality resulting from damp beds, dark, close buildings, and 



23 



