Strongyle egg reappearance periods following moxidectin treatment in horses in Southeast England

Background
A reduction in the egg reappearance period (ERP) has been suggested to be an early indication of emerging anthelmintic resistance in strongyles.

Objective
To measure the strongyle ERP following moxidectin treatment of horses in the southeast of England.

Study design
Prospective study.

Methods
Horses with a faecal egg count (FEC) of ≥400 strongyle eggs per gram (EPG) in a routine screening sample were enrolled into the study. Moxidectin (400 μg/kg) was administered per os and FEC tests repeated every 2 weeks for 16 weeks.

Results
Forty-eight horses completed the study. The mean EPG prior to treatment was 1047 (range 375–2137 EPG). In all but two horses, FEC was 0 EPG 2 weeks after moxidectin administration. In the remaining two, the FECs were each 12.5 EPG (97.8%–98.3% reductions compared to pre-treatment FEC). At 4 weeks post-treatment, 6 horses had positive FECs (96.6%–99.2% reductions). At 6 weeks, 11 horses had positive FECs (83.8% reduction in one horse; >90% reduction in 10). At 8 weeks, 21 horses exhibited positive FECs (<90% reduction in 2). At 10 weeks, 27 horses had positive FECs (<90% in 6). At 12 weeks, 31 horses had positive FECs (<90% reduction in 11). At 14 weeks, 34 horses had positive FECs (<90% reduction in 13). At 16 weeks, 38 horses had positive FECs (<90% reduction in 17).

Main limitations
Weights of some horses were estimated using weigh tapes rather than a weighbridge. Dosing of the horses with moxidectin was carried out by owners.

Conclusions
The results indicated acceptable efficacy of moxidectin at 14 days after treatment; however, the ERP pattern measured across the group suggests that this anthelmintic has a considerably shorter suppressive effect on strongyle egg shedding than measured when it was first introduced (12–16 weeks).