488 INFLUENCE OF CLIMATE ON ANIMAL DISEASE. 



•i 



that of 1780-81 in Ca])e Colony. Others are those of 1801, 1819, 

 1839. 1854-55 in Cape Colony, of 1888 in Natal, of 1891 and 

 1892 in Cape Colony again, and 1893-94 in the Transvaal and 

 Orange Free State. In the Cape in 1854-55, according to Eding- 

 ton. the nuniher of horses alone dying from the disease was 

 64,850. representing a value of £525,000, at this time a commis- 

 sion being ai)pointed to en(]nire into the disease; and in the 

 ejMclcmic of 1891 and 1892 it was estimated that 13.979 horses 

 and i4(j nudes succiunl)ed. 



Apart, however, from such great epizootics, it is recognised 

 that in certain localities the disease shows a well-marked annual 

 seasonal ])revalence, although in certain tropical areas and also 

 stibtropical 1)elts the disease may be met with m11 the year round. 

 In the first-mentioned localities the greatest ])revalence is met 

 Avith in the warm wet months of summer and early autumn, and 

 the areas of these localities which are most affected are the lo\v- 

 Iving ones. 



It must be ])ointed out. however, that it is not the absolute 

 altitude of a r(?gion tha.t is so inijiortant in this connection, but 

 rather the com])arative altitude, and that the disease may occur 

 in the low-lying portion of a region which lias an elevation above 

 sea-level of even several thousand feet. It is true, generally 

 S])eaking, that very high altitude, such as that of the W'itwaters- 

 rand, may, in ordinary years, give a certain amount of protec- 

 tion against the disease, but that even such places are not pro- 

 tected !)}• their altitude in years when the disease is very preva- 

 lent is shown by the heavy mortality occurring on the Rand area 

 during the years 1907 and 1915. 



Tlie more im])ortant factors in determining the ])resence of 

 the disease, howe\-er. seem to be the ]->resence of \vater and also 

 moisture. That this is so is indicated by a number of facts such 

 as the following. This disease outside of certain regions before 

 referred to shows its greatest prevalence during the wet summer 

 months, and then it is most common in the lower lying areas 

 and in the neighbourhood of rivers, spruits, vleis, ]:)ans, or dams. 

 It is known also that the prevalence of the disease varies with the 

 quantity and distribution of the rainfall — that the earlier the 

 rains apjx^ar, the earlier the disease ap]:)ears. It has also been 

 remarked that if two seasons in which the total rainfall is about 

 equal in amoimt are taken, the disease is not so prevalent in the 

 one in which the precipitation is fairly ec|uall}- distribttted 

 throughoitt the season, as in that in which the maximum rainfall 

 occurs inside a short i:)eriod of time — an occurrence noted as more 

 usually happening in the later ])art of the horse-sickness season. 

 It is also generally known that the ravages of the disease in any 

 given horse-sickness season cease with the a]:)pearance of the 

 first frost, and that cases occurring after this time are the ex- 

 tremely rare occurrences, which, however, have been met with. 



These are facts which are generally recognised, and that a 

 number of them were known to the farmers of Sotith Africa 

 even as early as t8ti is shown in a letter quoted by Edington 



