100 Report S.A.A. Advancement of Science. 



An irreconcilable difference, however, appeared to exist in the 

 presence microscopically in one disease (malaria), of a large and 

 easily demonstrable organism, which rendered the parallel between 

 the diseases a difficult one to accept. The similarity in some points 

 between horse-sickness and the human disease, yellow-fever, did 

 not at the same time escape notice, for, while the paludal influence 

 was existent in both diseases, an ultra-microscopic cause was common 

 to both horse-sickness and yellow-fever, i.e., the materies morhi 

 was invisible to the most careful microscopic scrutiny, but w'as, in 

 both diseases, capable of passing through the pores of a porcelain 

 filter fine enough to arrest the passage of all known microbes. 



In view of certain parallels in the incidence and morbid anatomy 

 of malaria and the yellow-fever of man, I hazarded the suggestion 

 that the causal factor of horse-sickness was a flying insect (probably 

 a mosquito), and this opinion received strong support from the sub- 

 sequent discovery that yellow-fever also was due to the attack of 

 a mosquito (the Stegomyia fasciata), and that horse-sickness (as an 

 insect-borne disease, with an ultra-microscopical organism), thus 

 found a further parallel in the yellow-fever of man. 



It was not, however until 1901 that I was able to put this 

 theory to the test, for the visitation of rinderpest and the late 

 war prevented the carrying out of any adequate lines of experi- 

 mental work. Field experiments, however, conducted in T891, 

 resulted in my being able to demonstrate the agency of a fiying insect 

 in the production of the disease, and in being able to suggest a 

 practical method of preventing the same ; by keeping stables, horse- 

 lines, etc., and their immediate environs, enveloped from sunset to 

 sunrise in a haze of smoke, generated by smouldering fires of damp 

 hay, stable litter, etc. Where such measures have been adopted, 

 the best results have been secured, but where the precaution has 

 been taken in a half-hearted and perfunctory manner, or where it 

 has been necessary to use horses after sundown, or even leave them 

 standing still during sultry days in dangerous localities, the disease 

 has manifested itself, even though such animal might have been 

 stabled at night in a smoky atmosphere. 



The actual species of mosquito or blood-sucking fly, concerned 

 in the production of the disease, is still a matter for future research, 

 but the decision of this point cannot, I think, have any very great 

 weight in regard to the question of the -prevention of the disease, 

 which is the a 11 -important question from the point of view of the 

 horse-owner, as well as of the practical investigator. 



The problem of protection, or the production of an immunitv, 

 to this disease, has long exercised the minds of scientific workers in 

 South Africa, particularly during the last decade. Efforts towards 

 this end have been made also by scientists even in London, Paris, 

 and elsewhere, but though such men as Koch. Bruce. Danvsz, and 

 Bordet, of the Pasteur Institute, Theiler, of the Transvaal, and 

 Edington, of the Cape, have brought their experience to bear on the 

 problem, no adequate means of immunising, and so preventing the 

 horse contracting the disease, have been devised. 



