494 INFLUENCE OF CLIMATE ON ANIMAL DISEASE. 



something more than this, and fin-nish ns with the explanations 

 for the peculiarities in regard to its distribution, for we have 

 seen that Avithout the intermediate snail host the complete 

 development of the parasite cannot occtn\. and hence the distri- 

 bution of this snail host must regulate the distribution of the 

 parasite worm. 



In this fact. then, we have the key to the importance of the 

 tellurical conditions and the presence of water in regard to the 

 distribution of the disease, and in knov^ing that the snail host 

 is incapable of existing in the absence of water, although its 

 habits are amphibious rather than aquatic. In Etn^ope this snail 

 host is Liiiuva fniiicatiilns or miiiittits, a fresh-water snail which 

 is found in the neighbourhood of ponds, ditches, and streams. 

 According to Theobald, it is to be met wdth in elevated as well as 

 in low-lying places, occurring in the Pyrenees at a height of 

 4.000 feet, and he also remarks that the smallest natural collec- 

 tions of water can serve for its development. This snail, how- 

 ever, though of very wide distribtition. is not a species met with 

 in everv coimtry where the disease due to the parasite worm is 

 encotmtered. In these in.stances, however, and where the in- 

 termediate snail host has been determined, this host has been 

 foimd in a snail possessing habits in regard to w^ater similar to 

 those of Liiniid-a tntucatula. Thus in the Sandwich Islands 

 Limncca cahiiciisis is mentioned as the intermediate snail host, 

 wdiilst in North and South America L. limiiilis and L. viator 

 respectively figure in this role. 



In Sottth Africa we find the same tellurical conditions influ- 

 encing the distribution of the disease as we find elsewhere. It 

 is a disease associated with marshy places or vleis, or along the 

 edges of motmtain streams and the pools formed by them, where 

 these streams occur, and is especially prevalent in these places 

 during very Avet periods. Dixon states that it is especially preva- 

 lent in the Cape, Stellenbosch, Paarl, and Worcester dis- 

 tricts of the Cape Province, and also that in very wet 

 seasons it causes great trouble and losses to owners of 

 sheep in the Cathcart and Stutterheim districts, and on the 

 Stormberg and Sneeuwberg ranges. That it has been 

 present in the Cape Province for a long time is shown by the 

 fact that the Commission appointed to enquire into the diseases 

 of cattle and sheep in that Colony in 1877, even then met with 

 the established disease from the Wodehouse district, through the 

 Stormberg range, to as far as the Molteno district. At that time 

 also the association existing between the presence of water and 

 the prevalence of the disease vvas recognised, and it is interesting 

 to note that the Commissioners state in their recommendations 

 that, in order to prevent the occurrence of the disease, the sheep 

 in the places where ft occurred would have to be kept away from 

 marshy or boggy places or neighboiU"hood of streams or pools 

 of water. 



So far we do not know the details of the development of the 

 parasite or the nature of the intermediate host in this country. 



