influkncl; of climati-: hx axi.mal disease. 493 



and we shall see the importance of the factors which determine 

 this distribution as we proceed to examine the manner in which 

 the parasite is transmitted from one l;ost to another. 



Now in the case of this disease the imi)ortcmce of both sets 

 of factors with which we are concerned is brought out. but the 

 necessity of certain tellurical conditions for the development of 

 the ])arasite is made very evident, for it hap])ens that wherever 

 the disease is met with, the conditions under v,hich it exists 

 always resemble each other in su])plyino" water in the forms of 

 pools, streams, etc.. and thus it is a disease associated Avith pas- 

 tures of a marshy, undrained. or oenerally wet character. This 

 fact is, indeed, so striking that in many places where this disease 

 occurs pastures of this nature are often referred to by farmers 

 as being " flukey " of " fluke-strucl<."' It is also, however, recog- 

 nised that ])astures of this nature favour the development of 

 helminthic ])arasites generally, and hence, to sec the especial im- 

 portance which they have in relation to the development of the 

 river fluke, we must enter into some consideration of its life- 

 historv, which may be as briefly as possible i)Ut as follows : — 



The adult fluke is met with as an inhabitant of the bile- 

 ducts of affected animals, and in this ]K)sition lavs the eggs pro- 

 duced by it. which eggs, i^assing to the intestine, leave this in the 

 fccces of the host animal, to be distributed on the pasture. The 

 next thing that happens is that these ova so de]X)sited now hatch 

 under suitable conditions within a Aariable time, usually a few 

 weeks, and from them emerges the next stage in the life-cycle, 

 known as the ciliated embryo or miracidium. So far the life- 

 history is, comparativel}' S])eaking, uneventful, but at this mira- 

 cidium stage a crisis occurs in the life of the liver fluke, for if 

 at this point a certain intermediate host animal is not met with, 

 no further development can occur, an.d the embryo under these 

 conditions may die in about 36 hoiu-s. I'rovided, however, that 

 the suitable host, which hai)pens to be a species of fresh-water 

 snail, is present, then the further development ])roceeds. In 

 this case the embryo bores its wav into the body of the snail, and 

 there passes to the next stage in its development, which is known 

 as the redia stage; this redia stage ma}- now in turn give rise to 

 another generation of redia forms ( daughter-redia ) . but eventu- 

 ally these redia forms give rise to the last stage occurring within 

 the body of the snail, a tailed form which is known as cercaria. 

 The next thing to happen is that these cercaria; leave the snail 

 host, move about until they meet with the stem of some aquatic 

 plant or with blades of herbage, up which the}- crawl, and, hav- 

 ing lost their tail-like appendage, they there become encysted. 

 It is now finally this encysted form ])resent on the lierbage 

 which, being swallowed by a suitable animal — the sheep in this 

 instance — develops within this animal to the mature adult stage, 

 and in this form, and having reached the bile-ducts of its host, 

 it there again lays its eggs to start the life-cycle afresh. 



These facts, broadly stated, will thus show what an interest- 

 ing life-history the parasite possesses, but for us they bring out 



