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Liver Rot in North Wales, 1920-21. 



[May, 



host snail, has been discussed : the epidemic of 1920-21 has 

 illustrated some of these points in a most interesting manner. 

 The snail, as already mentioned, has normally a distribution 

 coincident with shallow ditches and pools, more particularly 

 on clays and silts. From these centres, distribution takes place 

 on to ill-drained adjacent land, and, in fact, anywhere where 

 suitable conditions can be found, and it is surprising how soon 

 such movements take place, the animals appearing to move 

 almost automatically against any slight flow of water and so 

 penetrating steadily during suitable weather from the centres 

 to considerable distances in a few weeks. 



Such extensions of range undoubtedly occur each winter, 

 and indeed throughout the year should wet weather prevail. 

 The natural check to this distribution is drought, and normally, 

 during the average spring and summer such periods occur, 

 killing off those snails which have reached the least suitable 

 (most readily dried) situations; such process being progressive 

 as long as the dry weather lasts. Although the amount of 

 annual rainfall is of great importance here, nevertheless, its 

 distribution throughout the months is of hardly less importance. 

 In West Wales all winters may be regarded as wet from this 

 biological standpoint. It is those years in which rainfall is 

 general and persistent throughout the spring and summer that 

 lead up to marked extensions of range of L. truncainla, and 

 to its further increase by uninterrupted breeding on the ground 

 gained. Given the infection of such snails by means of the 

 normally present chronic or mild cases of Fluke infection usually 

 present among the flocks, we have the conditions which precede 

 and cause an epidemic of " Rot." The following diagram shows 

 the rainfall in months for North Wales during the period 

 1920-21, illustrating the points mentioned. It will be seen that 

 we have a period comprising the autumn and winter of 1919-20, 

 the wet and sunless summer of 1920, and the winter of 1920-21; 

 a period of eighteen months during which distribution and in- 

 fection could proceed simultaneously. The snails became 

 remarkably abundant, particularly on some of the low^-lying 

 heavy land, some limited areas yielding up to 130 to the square 

 foot, as on the Malldraeth Marsh in Anglesey. In some 

 instances several hundred acres became heavily stocked, in 

 others only certain limited spots were invaded. 



Effects of the Drought of 1921.— The long wet period de- 

 scribed above was succeeded by the remarkable drought of 1921. 

 This drought afforded an opportunity for studying the effects on 



