Strongyle Egg Reappearance Periods following Moxidectin Treatment in
Horses in Southeast England
Abstract
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 mcg/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 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). 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 suggest that this anthelmintic has a
considerably shorter suppressive effect on strongyle egg shedding than
measured when it was first introduced ( >13 weeks and up to
24 weeks).