Therapeutic activity of isometamidium chloride in Boran cattle against a tsetse-transmitted clone of Trypanosoma congolense with a low level of drug resistance

Experiments were conducted with a clone of Trypanosoma congolense, IL 3580, which exhibited a low level of resistance to isometanidium chloride. Five cattle were treated intramuscularly with isometamidium chloride at a dose rate of 0·5mgkg-1 body weight (BW) and challenged 28 days later with 5 Glossina morsitans centralis infected with T. congolense IL 3580. All 5 cattle and 15 untreated steers challenged on the same day became parasitaemic by day 15 post-infection. Thus, at a dose of 0·5 mg kg-1 BW, the prophylactic action of isometamidium chloride did not extend to 28 days following treatment. Subsequently, the 20 steers were divided into 4 groups of 5 animals each and treated with isometamidium chloride at one of the following dose rates; 0·5 or 1·0 mg kg-1 BW intramuscularly and 0·5 or 1·0 mg kg-1 BW intravenously (Groups A, B, C and D, respectively). Group A consisted of the 5 animals that had previously been treated with isometamidium chloride. Animals relapsed in all groups except those in Group B, treated intramuscularly with isometamidium chloride at a dose of 1·0 mg kg-1 BW. Four of the 5 animals in Group A, treated intramuscularly with isometamidium chloride at a dose of 0·5 mg kg-1 BW relapsed following a mean interval of 16 days post-treatment. Similarly, infections in all animals in Groups C and D, given intravenous injections of isometamidium chloride at a dose of 0·5 and 1·0mg kg-1 BW, respectively, were not eliminated as a result of treatment. The mean intervals to first detection of parasitaemia in these 2 groups following treatment were 14 and 20 days, respectively. The results therefore indicate that intravenous administration of isometamidium chloride does not enhance the drug's therapeutic efficacy in the treatment of a T. congolense clone which expresses a low, but significant, level of resistance to isometamidium. © 1992 Centre for Tropical Veterinary Medicine, University of Edinburgh.