Isometamidium, diminazene and the homidium salts have been in use for more than 35 years and it is estimated that about 35 million doses per year are currently used in Africa. These drugs remain popular with livestock owners and veterinarians because they are generally affordable and effective. Since there is no indication that new products will become available in the near future, it is of utmost importance that measures are taken to avoid or delay the development of resistance and to maintain the efficacy of the currently available drugs.
The repeated use of chemicals as pesticides or chemotherapeutic agents inevitably leads to the development of resistance in the target organisms. Figure 1 clearly illustrates that resistance systematically occurs within approximately ten years following the introduction of antimicrobials, insecticides, trypanocides and anthelmintics to the market (Waller, 1994). This also occurred with the trypanocidal drugs, such as isometamidium chloride (ISMM), the homidium salts and diminazene aceturate, which were introduced during the 1950s; the first reports of acquired resistance were published during the 1960s (Finelle and Yvore, 1962; Jones-Davies and Folkers, 1966; Na'Isa, 1967; Jones-Davies, 1967). Quinapyramine was marketed earlier, but was withdrawn in 1976 because of resistance and toxicity problems. It was later reintroduced for use in camels and horses and may still be used in error in cattle in some locations.
This document gives an overview of the current state of knowledge on trypanocide resistance, the current methods of its detection and how these may be improved, and more refined guidelines to delay the development of resistance and to control resistance when it occurs.