Dealing with double trouble: Combination deworming against double-drug resistant cyathostomins

Abstract

An alternative control regimen for drug-resistant parasites is combination deworming, where two drugs with different modes of action are administered simultaneously to target the same parasite. Few studies have investigated this in equine cyathostomins. We previously reported that an oxibendazole (OBZ) and pyrantel pamoate (PYR) combination was not sustainable against a cyathostomin population with high levels of OBZ and PYR resistance. This study consisted of a field study and two computer simulations to evaluate the efficacy of a moxidectin-oxibendazole (MOX-OBZ) combination against the same cyathostomin population. In the field study, anthelmintic treatments occurred when ten horses exceeded 100 eggs per gram. Fecal egg counts and efficacy evaluations were performed every two weeks. The two simulations utilized weather data as well as equine and parasite population parameters from the field study. The first simulation repeated the treatment schedule used in the field study over a 40 year period. The second evaluated efficacies of combination treatments using selective therapy over 40 years. In the field study, efficacies of MOX and both combination treatments were 100%. The egg reappearance period for MOX was 16 weeks, and the two combination treatments were 12 and 18 weeks. The first (46.7%) and last (40.1%) OBZ efficacies were not significantly different from each other. In the simulation study, the combination treatment delayed MOX resistance development compared to when MOX was used as a single active. This occurred despite the low efficacy of OBZ. The second set of simulations identified combination treatments used with selective therapy to be the most effective at delaying MOX resistance. Overall, this study supports the use of combination treatment against drug-resistant cyathostomins, when one of the actives exhibits high efficacy, and demonstrates benefits of this approach despite substantially lowered efficacy of the other active ingredient.

Keywords: Combination; Cyathostomin; Deworming; Equine; Moxidectin; Oxibendazole.

Graphical abstract

Fig. 1

Graphical representations of the single active moxidectin (MOX) treatment and the combination treatments of oxibendazole (OBZ) and MOX. Figure A shows fecal egg counts as eggs per gram (EPG). Error bars represent 95% confidence intervals (α = 0.05). Figure B shows the percent efficacy of the treatments using the fecal egg count reduction (FECR) test calculated using the total herd fecal egg counts pre- and post-treatment.

Fig. 2

Results from the first simulation study implementing treatment parameters used in the field study and the effect on the rate of MOX resistance development. Treatments were administered one time per year either on January 1st (gray bars) or March 31st (black bars), or the annual treatments rotated between the dates (white bars). Treatments were a combination of MOX with varying BZ efficacies, or MOX single active.

Fig. 3

Results from the second simulation study implementing selective therapy at various treatment thresholds and the effect on the rate of MOX resistance development. Treatments administered were either a combination of MOX and BZ (50% efficacy) as in the field study, or MOX single active.