502 



NA TURE 



September 14, 1905 



from their nostrils, due to the effusion of the liquid part 

 of the blood into the lungs and trachea. 



Nature of the Virus which causes this Disease. — There 

 can be no doubt that this disease, like the Tsetse-fiy 

 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 capable 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 

 10 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 com- 

 paratively 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 examination 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 

 visible 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 

 Chamberland filter. What the nature of this parasite is 

 one cannot tell. It behaves in many curious ways. For 

 example, Horse-sickness blood which is simply dried and 

 pounded into po^-der is found to be perfectly inert. On 

 the other hand, blood kept in the moist condition remains 

 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 

 infectivity 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 

 injected under the skin, and the large quantity required 

 before the disease can be conveyed through the stomach, 

 for a long time it has been supposed that it must be 

 conveyed from sick to healthy horses by means of some 

 biting insect. Experiments have been made within the 

 last few years by Watkins-Pitchford and others in order 

 to clear up this aspect of the question. Horses have been 

 placed in fly-proof shelters in exceedingly unhealthy places, 

 and it was found that in no case did any of these pro- 

 tected horses incur the disease ; whereas horses allowed 

 to feed in the same place, but without any shelter, soon 

 succumbed to the disease. But, up to the present, so far 

 as I am aware, the particular biting fly, mosquito, or 

 other insect which is the carrier of this disease has not 

 been discovered, and there can be no doubt that one of 

 the most important facts to make out in the etiology of 

 this disease is the discovery of the particular insect which 

 conveys the disease from the sick to the healthy. By 

 this discovery a flood of light may be thrown on the 

 causation of the disease, and some means discovered of 

 combating the disease through the insect, as has been 

 successful in some instances in regard to the case of 

 human malaria. 



Prof. MacFadyean also suggests that experiments are 

 needed to show what is the *' reservoir " of the virus. 



rrevcniion. — .Although we have been unfortunate up to 

 the present in not being able to make out the exact nature 

 of the parasitic cause of this disease, or to discover the 

 exact insect which carries it, a large amount of patient 

 persevering work has been done within the last ten 

 years in regard to its prevention by protective inocula- 

 tion. 



In this important work Bordet, Edington, Koch. Thoiler, 

 Watkins-Pitchford, and others have laboured for many 



NO. 1872, VOL. 72] 



years, and, according to recent reports, with some measure 

 of success. 



Dr. Edington, for example, who has been working at 

 this problem for several years, reports that Heart-water 

 is identical with Horse-sickness, and that by inoculating 

 mules with Heart-water blood he has been able to salt 

 them against Horse-sickness. He says that experiments 

 testing this vaccine show it to be an ideal one. It gives 

 a high protection to the animals inoculated. Its keeping 

 powers are excellent. No animal has died as the result 

 of this inoculation nor has any dangerous symptom been 

 produced. He states that he is not in a position to supply 

 a vaccine for Horse-sickness in horses, but has every hope 

 of attaining this successful end very shortly. 



We must congratulate Dr. Edington on his results, and 

 trust that this method of conferring immunity may prove 

 itself to be successful when put to practical use. For my 

 part, I am somewhat sceptical of Dr. Edington's methods 

 of immunising against Horse-sickness. I am sure he will 

 forgive my expression of scepticism when I recall to his 

 memory the various methods he has already brought for- 

 ward, just as optimistically, which have all been tried 

 and found wanting. 



Dr. Koch has lately recommended a method of 

 immunisation against Horse-sickness. This is the artificial 

 establishment of an active immunity in susceptible animals 

 by gradually increased doses of virulent blood, alternated 

 in the early stages of treatment with the injection of 

 serum prepared from the blood of highly fortified salted 

 horses. Mr. Gray reports that the experiments already 

 conducted on these lines show that the process as laid 

 down by Koch requires important modification before the 

 process of establishing immunity against Horse-sickness 

 can be of any practical use. 



Mr. Watkins-Pitchford in Natal is also hopeful of 

 succeeding in producing immunity against Horse-sickness. 



Dr. Theiler, too, reports that he has succeeded in pro- 

 ducing a serum which can be utilised in connection with 

 virulent blood to confer active immunity. He informs me 

 that his method is a subcutaneous injection of serum 

 and an intra-jugular injection of virus carried out simul- 

 taneously. The death rate in mules, from the effect of 

 the inoculation, he states to be about 5 per cent. It is 

 higher in horses, but he expects shortly to attain the same 

 result in them. During the last Horse-sickness season 

 he exposed 200 immunised mules to natural infection in 

 various parts of the country. Of that number only one 

 died with symptoms of Horse-sickness. As Dr. Theiler 

 is himself communicating his method in detail to the 

 Association, I need not enter more fully into it. 



The man who discovers a practical method of dealing 

 with Horse-sickness will be one of the greatest benefactors 

 of this country. There has always been a tradition that 

 a large money reward is awaiting this discovery. I do 

 not know whether this is well founded or not, but certainly 

 such a work would well deserve the highest possible re- 

 ward. The best reward is to give the successful investi- 

 gator more opportunity and more assistance in pursuing 

 his beneficent work. The reward given by the French 

 people to Pasteur was the Pasteur Institute; by the 

 German Government to Koch, the Imperial Hygienic 

 Institution. 



Catarrhal Fever of Sheep: Blue Tongue. 



This disease was first described by Hutcheon, the Chief 

 Veterinary Surgeon of Cape Colony.' It is very similar 

 in many respects to Horse-sickness. Both these diseases 

 occur most often in low-lying, damp situations, such as 

 river valleys and the coast plain. They also occur at the 

 same time of the year ; that is, from January to .April. 

 Blue Tongue, like Horse-sickness, is probably carried from 

 the sick to the healthy by means of some night-feeding 

 insect. .\t the same time the diseases are not identical, 

 since the inoculation of Horse-sickness blood into a sheep 

 does not give rise to Blue Tongue, nor the blood of the 

 sheep injected into the horse give rise to Horse-sickness. 



To Mr. Spreuill, Government Veterinary Surgeon in 



1 It is to Mr. Hutcheon that South Africa owes its knowledge of many 

 stock diseases. For the last twenty-five years he has laboured with the 

 utmost earnestness inCape Colony, often under IryinE conditions, and hi.* 

 description of the various diseases formed the basis of all the modern work 

 done on the subject. 



