Equine Review: May–June 2023

This was more a question of when, not if, as resistance in equine cyathostomins to all the major classes of available anthelmintics has been reported previously. In the first report of macrocyclic lactone resistance in the UK, Bull et al (2023), faecal samples from four Thoroughbred studs in England were collected and faecal egg counts (FECs) were performed using the sensitive mini-Flotac method before and at intervals after treatment with ivermectin, moxidectin (each drug alone or in conjunction with the cestocide praziquantel) or pyrantel.

Yearlings in the largest stud had FEC reductions ranging from 36–81%, consistent with resistance to moxidectin, ivermectin and pyrantel. Yearlings on the other three studs did not appear to have resistance, but yearlings on all four studs had egg reappearance times of 4–6 weeks, shorter than the original published values of 9–13 weeks, an early indicator of developing resistance. Samples from mares on studs A and B also had a reduced strongyle egg reappearance time (7–10 weeks), but resistance was not confirmed by FEC reduction.