Summary

Trypanocidal drugs remain the principal method of animal trypanosomiasis control in most African countries. However, there is growing concern that their future effectiveness may be severely curtailed by widespread drug resistance. This document presents an overview of the current situation of resistance to drugs for the chemotherapy of trypanosomiasis in African livestock. Although the number of case reports on drug resistance is increasing, there is a lack of reliable data at the regional or national level on the true prevalence and impact of drug resistance.

In order to compare data on a temporal and spatial basis across Africa there is an urgent need for better standardization of tests for the detection of drug resistance. The advantages and disadvantages of the currently available assays are briefly reviewed and measures suggested to improve the situation.

Finally, some guidelines on delaying the development of drug resistance are proposed and measures which may be adopted to control drug resistance when it occurs are recommended. Although there is still a lack of knowledge about the mechanisms of resistance and the factors responsible for the development of drug resistance, urgent measures are needed to maintain the efficacy of the existing drugs. Based on experiences of the control of resistance to other drugs such as antimalarials, antibiotics and anthelmintics it is suggested that reliance on the "sanative pair" guideline might not be sufficient to control resistance to trypanocides. This guideline needs to be accompanied by the following additional measures:

• Reduction in the number of treatments. The most efficient way to delay the development of drug resistance is to reduce the selection pressure caused by these drugs. Exclusive reliance on drugs for the control of trypanosomiasis, especially in areas of high challenge, and mass treatments at short intervals should be avoided. More attention should be given to integrated control measures involving the vector as well as the parasite.
• Avoidance of underdosing. Underdosing commonly occurs in the field and is an important cause of resistance development. Measures should be adopted to minimize underdosing. Better formulations of the existing prophylactic drugs may help to avoid subtherapeutic concentrations, which exert a strong selection pressure for resistant clones.
• Quinapyramine should no longer be used in cattle. Cross-resistance with the other available trypanocides has now been clearly demonstrated at the level of individual trypanosomes. The use of this drug in cattle is therefore contraindicated.

Acknowledgements

The authors would like to thank J.R.A. Brandt, M.C. Eisler and A.S. Peregrine for their critical review of the manuscript, and others who commented on the paper via PAAT-Link. Thanks also to C. Mattelaere and J. Van Hees for their help in preparing the manuscript. Financial support was received from the INCO-DC programme of the EU (project No. IC18-CT95-0006).