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
diminazene 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; one of the goats was cured. All of the second group, which received no treatment, became parasitaemic. The third group of goats received the same dose of
drug as the first group but three days after all of them were first detected parasitaemic; trypanosomes reappeared in all the five goats. When this third group was treated, the frequency of trypanosomes resistant to the
drug dosage was estimated to be less than 1 in 10(3). The parasites which reappeared after the treatment of these animals were used to infect two additional groups of five goats intravenously. 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 contrast, the five goats in the second, untreated, group became parasitaemic. Finally, when the goats in which the
infections had relapsed were retreated with
diminazene aceturate at the same dose rate, the level of parasitaemia 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 for a population resistant to the dose of
drug used.