INFLUENCE OF CLIMATE ON ANIMAL DLSI'.ASE. 495 



There is ])ractically no doubt, however, that the development is 

 along- similar lines to that described above, and the occurrence 

 of the disease is indicative of the habits which the intermediate 

 host must be expected to possess. Gilchrist would appear to be 

 the only worker who has entered into this qtiestion to any depth, 

 and it is ver\- interesting to note that his work indicates the 

 intermediate snail host as being most ])robably existent in the 

 snail Physa tropica. In this species he claims to have found the 

 redia and cercaria stages of the fluke, but the non-detection of 

 the encysted stage and the consequent alisence of infection ex- 

 periments unfortunately leave the matter awaiting final settle- 

 ment. 



Here, then, having seen from this example how the climatic 

 and tellurical conditions may influence the distribution of a 

 parasitic disease in what we may speak of as a rather indirect 

 manner, let us now. in considering the other disease we have 

 selected, study the more direct influences of these conditions on 

 helminthic development. The particular exam])le which we 

 have here chosen is the disease recognised as occurring in sheep 

 consequent upon the presence in this animal of a parasitic nema- 

 tode worm, formerly known as Slroiif/yliis coutorfns. but now 

 designated Hccmonchus contortus. 



This worm, though met with in other ruminants, is most 

 commonly a parasite of the abomasus of the sheep, and from this 

 facts it has in many |)laces earned the popular name of thC'sheei)- 

 stomach worm." In this country, however, the common name 

 ap])lied to it is that of " wireworm " or '" haarworm," and under 

 these synonyms is known to sheep-farmers throughout the whole 

 Union as causing a disease in sheep, and especially young sheep, 

 which, under certain conditions, may be very serious. In some 

 places where the pastures are of a wet or damj:) character, it is 

 more or less always prevalent, whilst in other j^laces its prevalence 

 is usually most marked during the annual wet season. During very 

 wet seasons, however, it becomes more generally and markedly 

 l)revalent than usual, and may then cause serious losses, especially 

 in younger animals, unless preventive or curative measures are 

 adopted. 



The importance of the disease in this country, therefore, 

 lends a review of the observations on the development of the 

 parasite, an interest additional to that which it has for us in our 

 broader considerations. In order to review these observations, 

 however, we must, as w^e have already done in the case of liver 

 fluke, first consider the life-history of the worm, and this is 

 briefly the following : — 



Commencing with the adult sexually mature parasites, we 

 fin.d tiiese occurring in the abomasus or stomach of the sheep, 

 and in this position they may be encountered in a large proportion 

 of animals examined. The numbers, however, in which the para- 

 sites are found vary in difl'erent individuals and with certain con- 

 ditions into which we shall not enter here. All animals carrying 

 the parasite, however, do not necessarily show symptoms of 



