5O0 INFLUENCE OF CLIMATE ON ANIMAL DISEASE. 



thus dealt with in regard to the hfe-history and biology of H. 

 €outortus are, when broadly generalised, applicable in a general 

 way to most other nematode })arasites causing stock diseases, 

 and we find that vrarmth and moisture pla}- the same 

 general role in the development of the parasites causing them, 

 and hence in the prevalence of the diseases themselves. 



This therefore explains the reason for the greater preva- 

 lence of parasitic diseases of this nature on damp, marshv or 

 undrained pastures during wet weather, and es])eciall}' in those 

 parts of a coun.try where the wet and warm season happen to 

 coincide, as happens over the greater part of this country. 



It is not without interest, also, to note the association of 

 certain diseases during these wet summer months and their 

 increased prevalence. Thus the marked jjrevalence of 

 horse-sickness, blue tongue of sheep, ])ai-asitic diseases gener- 

 ally — but especially of infection of sheep by //. contortiis ( " wire- 

 worm" infection) — and of ephemeral fever or three-days sickness 

 of cattle during the early part of this year, and their similarly 

 marked prevalence in the summer of 1907. and similarly under 

 very wet climatic conditions ( e])hemeral fever then entering the 

 country from further north), is an association which lias been 

 observed. Such an association is, however, one which is cajiable 

 of explanation along the lines Avhich have been laid down in this 

 article, and it is hoped that the ex])lanation has been sufficiently 

 clear to allow of its appreciation. 



Having now seen from all of our ])revioi;s considerations 

 how the climatic and tellurical factors may exert their influence 

 on the distribution of diseases of bacterial or helminthic charac- 

 ter, and on those borne by either ticks or insects, we near the 

 conclusion of our consideration, for, although the subject of the 

 diseases caused by toxic plants is a fascinating one, we have not 

 yet obtained, so far as South Africa is concerned, sufficient data 

 to illustrate the jjoints with which it would 1)e of interest to deal. 



This point to which I refer is the effect of climate and soil in 

 ■determining the actual toxicity of the plant itself, and it is 

 regrettable that we have not more exact data concerning this 

 subject in South Africa, as it is a matter of general scientific 

 importance. There is little doubt, however that time and other 

 suitable conditions will remedy this state of affairs, and that our 

 Ivnowledge of this subject wall then become amplified. 



Ostertag and Zuntz, in Germany, have pointed to the produc- 

 tion of toxic substances in grasses grown under certain climatic 

 and tellurical conditions, which grasses were not toxic when the 

 conditions were altered, and, as I am sure you all know, an 

 ■explanation of the causation of lamziekte has been given follow- 

 ing similar lines by Theiler, and it is therefore of importance that 

 we should know under what exact conditions we may expect 

 such occurrences, and investigations into the matter will have the 

 highest interest. 



There are, however, a couple of observations which have a 

 bearing on the point, and to which reference may be made. 



