Principles for the use of macrocyclic lactones to minimise selection for resistance

Objective To provide principles for the appropriate use of avermectin/milbe mycin or macrocyclic lactone (ML) anthelmintics in sheep, to ensure effecti ve worm control and to minimise selection for ML resistance. Strategy The principles were based on an assessment of the information curr ently available. The MLs were categorised into three groups (ivermectin [IV M], abamectin [ABA] and moxidectin [MOX]) based on structural differences, persistence and efficacy against ML resistant strains. The reported order o f activity or efficacy against ML resistant worm strains was IVM<ABA<MOX. G eneral treatment schemes were considered for Australian conditions and were divided into the following situations: 1. quarantine treatment, 2. treatme nt on/to clean pasture, 3. treatment on/to safe pasture, 4. treatment on/to moderate/heavily contaminated pasture. For each of these situations a stra tegy was considered for farms where ML resistance was present or absent. It was assumed that resistance commonly occurs in some or all other broad spe ctrum anthelmintics, and even where ML resistance has been detected, the ML group remains the most effective. The guidelines provided are general and it is expected that state agencies and sheep/veterinary advisers would give specific advice to suit their environments and drug resistance/worm proble ms. Conclusions The primary recommendation is to use a mixture of effective dru gs when treating sheep. However, unless the combination treatment is highly effective it is unlikely to delay selection for ML resistance if sheep are being treated and moved to a clean or safe pasture. Where possible, relian ce on the ML anthelmintics should be reduced by not using them every year, not using them in low risk stock or by using narrow spectrum and low effica cy drugs such as naphthalophos when appropriate. Anthelmintic treatment sho uld be given as part of a strategic worm control program. It is suggested t hat IVM-oral and IVM-capsules should not be used when ML resistance is pres ent. In this situation MOX or ABA should be used in combination with other drugs, provided that the chosen MIL is effective against the resistant para site. It is essential to monitor the efficacy of ML and drug combinations b y post-treatment worm egg counts, particularly when ML resistance has been detected.