Why did tsetse flies limit the use of horses in southern Yorubaland?

In southern Yorubaland, as in many other parts of sub-Saharan Africa, the presence of the tsetse fly significantly limited the use of horses because the flies act as vectors for a debilitating disease known as nagana.1

Nagana is a form of trypanosomiasis caused by Trypanosoma parasites, which the tsetse fly transmits to domestic animals, including horses and cattle, through its bite. The infection leads to severe symptoms such as fever, anemia, muscular wasting, and eventual paralysis, which often proved fatal to livestock. Because horses were highly susceptible to this disease, their survival and utility in the tsetse-infested woodlands of the region were severely restricted, preventing their widespread use for transportation or military purposes.123