| Summary: | Horses have played an important economic, military and cultural role in South African history. However, disease has always posed a threat to their survival. Horsesickness, a viral infection transmitted by midges from the <em>genus culicoides</em>, is endemic in much of the country but has historically assumed epizootic proportions in certain years. In the lowveld and Zululand nagana (<em>trypanosomosis</em>), spread by tsetse flies, has killed both horses and cattle and affected the distribution of human settlement and agricultural activities. In addition, much of South Africa is very arid, yet has rich <em>floral taxa</em>. Several plants, such as <em>Senecio spp.</em> are highly toxic to horses, but in times of drought and fodder shortages, equines are faced with the choice of starvation or potentially succumbing to toxicosis by eating poisonous weeds. This paper considers the environmental impact of these three types of horse diseases in South Africa and explores the scientific and ecological investigations undertaken since the late nineteenth century to try to control them. Research into horse diseases brought together the laboratory and the field and raised important questions about the part played by environmental factors, as opposed to just germs, in the distribution of livestock infections and the ability of farmers and scientists to tackle them effectively.
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