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SCIENCE. 



[N. S. Vol. XXII. No. o.-iO. 



temperature then goes up rapidly, and it 

 dies after four or five days' illness. Very 

 often the horse appears perfectly well until 

 within a few hours of death. For example, 

 my horse was the last one to die on the 

 Ingwavuma expedition. On the day of his 

 death I rode him until noon without no- 

 ticing anything amiss. He then became 

 rather dull in his movements, and I handed 

 him over to the groom to lead. He died 

 that evening immediately after we .got into 

 camp. It is, therefore, a very rapidly fatal 

 disease, and almost every horse which is 

 attacked by it succumbs. I have never 

 seen a ease of horse-sickness which had 

 been brought on by artificial inoculation 

 recover. But there can be no doubt that 

 a small percentage of horses infected nat- 

 urally do recover, and these recovered 

 horses are, more or less, immune in future 

 to the disease. There is no necessity for 

 me to describe the symptoms of this well- 

 known disease, as every one who has to do 

 with horses in South Africa is perfectly 

 familiar with it, and every one has seen 

 dead horses with the characteristic mass of 

 white foam issuing from their nostrils, due 

 to the effusion of the liquid part of the 

 blood into the lungs and trachea. 



Nature of the Vmis which causes this 

 Disease.— There can be no doubt that this 

 disease, like the tsetse-fly disease, is caused 

 by some form of blood parasite. A small 

 quantity of fluid taken from any part of a 

 horse suffering from horse-sickness is ca- 

 pable of giving rise to the disease if injected 

 under the skin of a healthy horse. For 

 example, the thousandth part of a drop of 

 blood from a sick horse will, in many cases, 

 give rise to the disease if injected under 

 the skin of a healthy horse. It must be 

 admitted, however, that some horses require 

 a larger dose than others, but it may be 

 said that no horse has yet been found to 

 withstand more than a comparatively small 



quantity of infective blood thrown under 

 the skin. Now, although every drop of 

 blood must contain many of the organisms 

 of this disease, yet the most careful exam- 

 ination of such blood under the highest 

 powers of the microscope reveals nothing. 

 Again, if we filter horse-sickness blood 

 through a porcelain filter — a filter which is 

 capable of keeping back all the known vis- 

 ible micro-organisms— the filtrate is found 

 to be virulent. It is evident, then, that we 

 are here dealing with a blood parasite so 

 small in size as to be absolutely invisible 

 to the highest powers of the microscope, 

 and also so minute as to readily pass 

 through the pores of a Chamberlain filter. 

 What the nature of this parasite is one can 

 not tell. It behaves in many curious ways. 

 For example, horse-sickness blood which is 

 simply dried and pounded into powder is 

 found to be perfectly inert. On the other 

 hand, blood kept in the moist condition re- 

 mains virulent and capable of giving rise 

 to the disease for years. Or, again, the 

 germ of horse-sickness is so resistant to 

 external agencies that if, as described by 

 MacFadyean, a part of the liver of a horse 

 dead from horse-sickness be buried in the 

 ground and subjected to putrefaction, it is 

 found that the liver tissue retains its in- 

 fectivity for months. Although a very 

 small quantity of blood introduced under 

 the skin of a horse will almost certainly 

 give rise to the disease, it is quite different 

 if the blood is introduced into the stomach. 

 In the latter case a small quantity of blood 

 has no effect, and the horse requires to be 

 drenched with a pint or more before the 

 disease can be given in this way. 



The question now arises as to how horses 

 are infected by this disease in nature. On 

 account of the small quantity of blood 

 which will give rise to the disease if in- 

 jected under the skin, and the large quan- 

 tity required before the disease can be con- 



