Effect of isometamidium on Trypanosoma congolense infectivity

Isometamidium chloride (Samorin, RMB, England) is a widely used and highly effective trypanocide for the treatment of bovine trypanosomiases. However, the appearance of isometamidium-resistant populations of T. congolense in Africa makes it necessary to develop methods for the rapid and reliable detection of drug resistance in the laboratory. Currently available tests are time-consuming and/or expensive. In the present study, the short-term in vitro incubation of trypanosomes in a range of isometamidium concentrations and the infectivity of the parasites in mice has been assessed. A series of T. congolense isolates were used which were known to differ in their in vivo sensitivity to the drug. The results showed a close correlation between the known level of resistance and the capability of trypanosomes to remain infective after incubation in isometamidium. Thus isolates displaying a high level of resistance in vivo remained infective following incubation in higher concentrations of drug. This assay may provide a simple and reliable method for detecting drug resistance in T. congolense.