IXFLUENCK OF CLIMATK ()\ ANIMAL DISKASK. 475 



latter is only found in a (yroiipinj^- of diseases acconlin^- to their 

 wtiolog}'. 



If, to make this point clearer, we enquire still further into 

 the matter, taking the collection of animal diseases met with in 

 South Africa to illustrate our remarks, we are met hy the fact 

 that certain of the diseases <jn our list occur ec|uall}' commonly 

 in other parts of the world siiuatefl in tnore temjierate and colder 

 regions. Examples of stich diseases are the hacterial affections, 

 anthrax, hlackquarter. glanders, one due to a virus of the filterahle 

 rvj^e, namely, contap;ious ])lem-o-j)neumonia of hovines, and 

 numerous parasitic helminthic affections. It may also l)e re- 

 called that a disease fortunately not on our present list, l)Ut still 

 existing in other parts of the world and in some places under the 

 conditions referred to immediately ahove. namel}-, rinderpest, 

 when introduced into this country some years ago. soon 

 showed hx its rapid and wide spread that the conditions of this 

 subcontinent were very favourable to its existence. 



On our list, however, we note the existence of a certain col- 

 lection of diseases exem]^lihed amongst the groups ijabesiosis. 

 Theileriasis, Anaplasmosis. Nuttalliosis, Spirochretosis, and Try- 

 panosomiasis, the causal agents of which are ])rotozoan organ- 

 isms, and by a disease known ])opularly as heartwater. due to a 

 filterable virus. Now, a peculiarity common to all these dis- 

 eases is found in the requirement for their transmission from 

 one animal to another of the in.terniediary agency of some animal 

 carrier, this being either an arachnid (tick) or an insect (fly), 

 and it is to this group of diseases transnn'tted indire(^tly in this 

 manner, that we particularly refer when we speak of the tropical 

 or subtropical diseases of stock. That diseases transmitted in 

 this way are. however, not j)eculiar to the tropical or subtropical 

 climates is soon proved in examining the distribution oi one of 

 them, namely, redwater. a tick-borne disease of cattle of ver)^ 

 widespread occurrence, and found under the varying climatic 

 conditions of such countries as Africa. North and South Ame- 

 ricas, India, I^ast and West Indies, Australia. Caucasia, Italy, 

 Turkey, Roumania, Germany, France, Finland, Denmark, (jreat 

 Britain and Ireland and other ])laces ; but that as a group they 

 have come to be specially regarded as diseases of tropical or sub- 

 tropical climates is simply de])endent upon the fact that it is in 

 these climates they tend to be most prevalent, the explanation of 

 this being that it is in regions situated in these climates their 

 animal transmitting agents find the environment best suited to 

 their existence and develoj^ment. This (juestion of suitability of 

 environment to the re(|uirements of their transmitting agents is 

 tlius the chief factor regulating the distribution of these tropical 

 or subtropical diseases, and that this etivironment slionld be .so 

 favourable to these carrying agents is not ditlicult to understand 

 when we remember that outside of food su))]'!}- the principal con- 

 ditions demanded In- them for their existence is the jiresence of 

 a suitable amount of heat and moisture, both of which are 

 >U])plied in oi>timum degree in tropical and sub-troijical regions. 



