Macrocyclic lactone endectocides include two chemically distinct compo
unds moxidectin, a milbemycin, and ivermectin, an avermectin. The sign
ificance of the chemical differences between these compounds in relati
on to nematode resistance remains to be established. Reported studies
indicate that moxidectin at the recommended dose rate of 0.2 mg/kg con
trols identified strains of nematodes, isolated from sheep and goats,
with demonstrated resistance to ivermectin. This reflects the signific
antly greater potency of moxidectin against the 3 genera of nematodes
most commonly involved in anthelmintic resistance, Haemonchus, Osterta
gia and Trichostrongylus. Moxidectin, in recommended strategic treatme
nt programmes, should reduce the risk of further development of resist
ance to the macrocylic lactone endectocides.