Application of field methods to assess isometamidium resistance of trypanosomes in cattle in western Ethiopia.

This study assessed the degree of isometamidium resistance of trypanosomes infecting cattle in the upper Didessa valley of western Ethiopia. An initial prevalence study was conducted to identify sites with a high risk of trypanosmosis in cattle. The trypanosome prevalence varied widely, with two sites, Kone (21.3%) and Village 1 settlement (15%) having a relatively high prevalence based on the phase-contrast buffy-coat technique (BCT). In the highest risk area, the Kone settlement, an isometamidium block treatment study was conducted from April to June 2001. A total of 300 cattle were included in this study, 100 from each of three villages (Cheleleki, Kolu and Burka). At day minus 14 of the study, all 300 cattle were treated with diminazene aceturate at 7 mg/kg body weight. Subsequently, these cattle were ear-tagged and randomly assigned into two groups, 50 controls and 50 for isometamidium treatment in each village. Fourteen days later (day 0), the 50 treatment cattle were given isometamidium chloride at 1 mg/kg body weight. Both groups of cattle were then examined for trypanosome parasites using BCT every 14 days until 84 days. The two indices used in assessing isometamidium resistance, namely the proportion of infections during an 8-week follow-up period and the ratio of mean hazards in an isometamidium treated versus untreated group, provided consistent results across the three villages. In Burka village, both indices demonstrated the presence of isometamidium resistance trypanosome infections while in Cheleleki and Kolu villages, both indices did not indicate significant levels of resistance. There were significant differences between the Kaplan-Meier survival estimates of the control and treatment groups in Cheleleki (P < 0.01) and Kolu (P < 0.05) but not in Burka (P > 0.05).