Laboratory mice infected with a Trypanosoma congolense isolate of water-buffalo origin did not respond when treated with diminazene aceturate (Berenil, Hoechst, West Germany) at 3.5 mg/kg and 7 mg/kg body weight (b.wt.) and also isometamidium chloride (Samorin, May & Baker, Dagenham, England) at doses less than 3 mg/kg b.wt. Effective cure was achieved with isometamidium and diminazene at 3 mg/kg and 14 mg/kg b.wt. respectively; doses in excess of those recommended for use in the field to treat animal trypanosomiasis by the manufacturers. Irregular use of the two drugs on infected water-buffaloes while improperly restrained may be one of the factors responsible for the dual drug resistant T. congolense isolate emerging.

Laboratory examination of blood smears from water buffaloes in an area of high fly challenge indicated an infection rate of 51.11 % caused by both Trypanosoma congolense and T. vivax. A similar study in cattle raised in a low fly challenge area showed an infection rate of 22.73% caused by T. congolense only. Diminazene aceturate administered as a single dose at 3.5 mg/kg body weight cured the infection in cattle, while a similar dose at 7 mg/kg body weight failed to cure the water buffaloes. Cattle were further protected by a single dose of isometamidium chloride administered at 0.5 mg/kg body weight for 6 weeks of this study. On the other hand, isometamidium chloride administered at 1 mg/kg body weight cured transiently 90% of the infected water buffaloes and 10% persisted with a T. congolense infection. It was concluded that the lower dose rates approved by the manufacturers for the two trypanocides treated adequately infected cattle in a low challenge area, but such results were not achieved with the higher dose rates for infected water buffaloes in a high fly challenge area.