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TRANSACTIONS OF SECTION I, 545 
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, Experi- 
ments 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 protected 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, as 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. 
Professor MacFadyean also suggests that experiments are needed to show what 
is the ‘reservoir’ of the virus, 
Prevention.—Although we have been unfortunate up the present in not being 
able to make out the exact nature of the parasitic cause of this disease, or to dis- 
cover 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 
inoculation. 
In this important work Bordet, Edington, Koch, Theiler, Watkins-Pitchford, 
and others have laboured for many 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 inocu-~ 
lating 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, [ am somewhat sceptical of Dr. Edington’s methods of immunising 
against Horse-sickness. I amsure he will forgive my expression of scepticism 
when I recall to his memory the various methods he has already brought forward, 
just as optimistically, and 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 producing 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 simultaneously. The death rate in mules, 
1905. NN 
