^2 THE PROIJLEM OF HORSE SICKNESS. 



not susceptible, or, at least, less susceptible than horses uot 

 immune to the disease. 



It is, however, on the other hand, an equally well-known 

 fact that salted horses may again sicken from horse-sickness 

 (which fact is expressed by the term aanmaaning or "relapse ''). 

 This relapse is generally of a less severe nature than the primary 

 attack, many horses recovering again. Furthermore, the ex- 

 perience has been made that salted horses can die of the disease, 

 and that they die more in some localities and in some seasons 

 than in others. 



The term aauuiaaning or " relapse " is open to misinter])re- 

 tation. It is usually explained by analogv to the experience in 

 malarial fever in man. There is a difference. In malarial 

 fever in man the germs are still ])resent in the apparently 

 healthy man, and develop again under some external influence 

 such as cold, change of climate, etc., and at any time of the 

 year, whereas in horse-sickness aainiiaaniuc/s are only noted 

 during the horse-sickness season. Indeed, it is not a relai^se 

 in the true sense of the word; it is a new infection. This has 

 experimentally been proved. 



As a result of our investigations into immunity. I have 

 been able to devise a means of protection by a combination of 

 serum and virus injections, which wdll be explained later. A 

 great number of mules and a number of horses have been im- 

 munised and sul)iected to tests on their immunity. In this 

 way definite conclusions were arrived at as to the extent tc 

 which immunity is conveyed b}- an attack of horse-sickness. 

 For our initial experiments we used a virus obtained from a 

 horse in Pretoria. We soon found that some of the mules 

 exposed in various parts of the country again contracted the 

 disease — it is true only to a small extent ; some recovered, whilst 

 others died. Material was obtained from these cases of iMxak- 

 down, and used for further investigations. With the object of 

 giving the salient facts, without going too much into detail, I 

 have selected the two chief viruses that have been used through- 

 out my investigations, being the Pretoria ordinary strain re- 

 ferred to above, and the Tzaneen strain obtained from the first 

 authenticated case of a breakdown in an immunised mule. The 

 experiments relate to horses only. The following figures have 

 been compiled from the results of a number of experiments in 

 which the immunity in particular was studied from the injection 

 of the two viruses. A total number of 1,078 animals come into 

 consideration, of which 327 horses were first injected with 

 ordinarv virus, and 7=;i with Tzaneen virus. With the object 

 of facilitating comparisons, percentages have been used through- 

 out : — 



