4^6 INFLUENCE OF CLIMATE ON ANIMAL DISEASE. 



the adult stage, and that the onl}' effect of this temperature was 

 to increase the jieriod occupied by the moult occurring in this 

 change. He also studied the influence of low tem])eratures on the 

 larvse of the blue tick, and it is interesting to record the results 

 obtained as showing the eft'ects of the temperatures used. He 

 noted that Rhipiccphalus larvfe did not die when exposed to 

 minus i8° C. for 15 minutes, but that exposiu'e to the same tem- 

 perature for 30 minutes did cause their death, and also that 

 whilst the larvje of the same sj^ecies resisted a temperature of 

 minus 5° C. for 24 hours, the majority of them died when sub- 

 jected to the same tem'perature for 48 hours. 



We may, however, finish now with this part of the subject 

 in referring again to that disease of cattle, sheep, and goats 

 known as heartwater, and trarismitted by the bont tick. In speak- 

 ing of the habits of this tick, it has been mentioned already that 

 in the Transvaal it is in the bush veld that it has its home, and 

 does not seem to hnd the conditions of the middle or high veld 

 suitable for its existence and development, although it has been 

 introduced into these areas on several occasions bv cattle return- 

 ing there in summer from their winter bushveld pastures. The 

 high-veld farmer has long ago had these lial)its of the bont tick 

 impressed u]Kin him as a result of his i)ractice of transhumance 

 in trekking with his stock in winter to the low bushveld pastures, 

 where he meets with the disease it transmits. He has also long 

 recognised that in trekking back to the high veld in summer he 

 leaves the disease behind him, but it is only as a result of com- 

 ])aratively recent researches that we now find an exj^lanation for 

 these observations. Here, then, again we have had an example 

 of how climatic and tellurical conditions will influence the dis- 

 tribution of a disease through the influence exerted by them on 

 the transmitting tick agent, and as the eft'ects of these factors 

 on ticks and the diseases carried by them have been sutticiently 

 indicated, we may now pass to consider how these same factors 

 may influence the diseases carried by insects. 



Insect-i'.orxk Diseases. 



It may here be exjx-cted of the writer that he should deal 

 in some details with such diseases as are well kn<)\vn to be fly- 

 borne, examples of which are to be found amongst the trypano- 

 somiases, but let it here be stated that such is not his intention. 

 What has already been written in other ])laces concerning the 

 bionomics of the flies transmitting these diseases is enough to 

 fill several large-sized monogra])hs, and the writer wishes more 

 particularly to refer to certain well-known South African dis- 

 eases which, though not definitely known to be insect-trans- 

 mitted, are suspected of being carried in this way. For this 

 ]'>urpose special reference will be later made to such well-known 

 South African diseases as horse-sickness and l)lue tongue of 

 sheep, but, in passing, brief reference may be made to trypano- 

 somiasis as far as this ])ortion of South Africa is concerned. 



It is, for instance, interesting to note that in Zuhiland, where 



