APPARENT RARITY OF DIMINAZENE-RESISTANT TRYPANOSOMES IN GOATS INFECTED WITH A DIMINAZENE-RESISTANT POPULATION OF TRYPANOSOMA-CONGOLENSE

Experiments were carried out in goats to determine the frequency with which diminazene-resistant trypanosomes occur in parasite populations before and after the intramuscular treatment of the goats with diminaz ene aceturate. Trypanosoma congolense IL 3274, a diminazene-resistant clone, was used to initiate infections in three groups of five goats. The goats in the first group were treated with diminazene aceturate at a dose of 7.0 mg kg(-1) bodyweight within 10 seconds of infection; on e of the goats was cured. All of the second group, which received no t reatment, became parasitaemic. The third group of goats received the s ame dose of drug as the first group but three days after all of them w ere first detected parasitaemic; trypanosomes reappeared in all the fi ve goats. When this third group was treated, the frequency of trypanos omes resistant to the drag dosage was estimated to be less than 1 in 1 0(3). The parasites which reappeared after the treatment of these anim als were used to infect two additional groups of five goats intravenou sly. The goats in one group were treated with the same dose of drug as before, within 10 seconds of infection and were all cured. In contras t, the five goats in the second, untreated, group became parasitaemic. Finally, when the goats in which the infections had relapsed were ret reated with diminazene aceturate at the same dose rate, the level of p arasitaemia temporarily decreased by at least 10(3) trypanosomes ml(-1 ). These findings suggest that diminazene-resistant T congolense occur at low levels in trypanosome populations despite attempts to select f or a population resistant to the dose of drug used.