544, REPORT—1905. 
it occurs in epidemics, with intervals of ten to twenty years. It is undoubtedly a 
disease which prevails chiefly ix low-lying localities and valleys, and is but rarely 
met with in elevated exposed positions. It, however, is met with now and then 
in river valleys up to an elevation of some thousands of feet. Season has also a 
remarkable influence on its development, being exceedingly common in summer 
and disappearing on the appearance of the first frosts of winter. 
Ten years ago various theories were held as to the cause of this disease. Some 
people thought that it was due to eating poisonous herbs; others, to some 
peculiarity or state of the night atmosphere; others, to eating grass covered with 
dew ; and still others, to the eating of the spiders’ webs which may be seen on the 
grass in the morning. It was known at that time not to be contagious in the 
ordinary sense of that term; that is to say, a horse could be stabled alongside 
a case of Horse-sickness without incurring the disease, or a horse might be 
placed without danger in the same stall in which a horse had recently died of 
Horse-sickness. 
Nature of the Disease.—A horse which has been exposed to infection shows 
no signs of the disease for about a week. Its 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 noticing anything amiss. He then became rather dull in his move- 
ments, and I handed him over to the groom to lead. He died that evening imme- 
diately 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 case 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 naturally 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 everyone who has to do with horses in South Africa is perfectly familiar 
with it, and everyone 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 Virus 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 capable of giving 1ise 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 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 ex- 
ample, 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 
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 
