Survey of anthelmintic resistance on Danish horse farms, using 5 different methods of calculating faecal egg count reduction

Abstract

Summary This study reports on the prevalence of anthelmintic resistance in strongyles of horses in Denmark. Of 5 methods used for the calculation of faecal egg count reduction (FECR) the method recommended by the World Association for the Advancement of Veterinary Parasitology, for the detection of resistance in sheep was the most sensitive procedure for detecting resistance. Using this method benzimidazole resistance was detected on 33 of 42 farms (79%) examined. Pyrantel was tested on 15 farms and FECR tests indicate resistance on 3 (30%) farms. On 2 farms on which resistance to pyrantel was detected resistance to benzimidazoles was also detected. On one of 16 farms examined ivermectin resistance was indicated at Day 14 but not at Day 19. On the 15 remaining farms ivermectin was effective. Due to the high prevalence of anthelmintic resistance in Danish horse herds it is recommended that tests of anthelmintic efficacy be conducted routinely to monitor the effectiveness of the strongyle control programmes.

Anthelmintic resistance in nematodes of horses

“…Initially, most reports of BZ resistance were from single farm observations in isolated geographic areas. More recently, multi-farm prevalence studies in the United States and Europe have demonstrated a prevalence of resistance to BZ of 75% or greater [19,25,40,51,81,82,90,101,106]. In many areas, resistance to BZ anthelmintics is now so common that it is difficult to find populations of cyathostomes that are not BZ-resistant.…”

“…Pyrantel-resistant cyathostomes have been reported throughout the southeastern United States [12,71,90,106] as well as in Norway [51] and Denmark [19]. Since lower anthelmintic efficacies should delay the selection for resistance [3], it may be that the relatively lower efficacy of pyrantel [18,67] has delayed the appearance of resistant worms [21].…”

“…This is the only anthelmintic drug class used in horses in which there is still no resistance. In fact, FEC reductions at two weeks post treatment with ivermectin continue to be virtually 100% [19,90,101,106], even though many farms have long histories of using ivermectin four to six times a year. A recent controlled efficacy study with ivermectin confirmed that ivermectin efficacy against equine nematode parasites remains extremely high (99-100%); there is no indication that ivermectin efficacy has decreased with time [55].…”

“…A review of methods used in recent anthelmintic resistance prevalence studies in horses demonstrates that the WAAVP recommendations are not being widely accepted or utilized by current researchers. In some reports the WAAVP standard for sheep and goats was used (< 95% reduction, LCL < 90%) [19,51], in others the WAAVP standard for BZ resistance in horses was used (< 90% reduction) [82,101], and in other studies treatment was categorized as effective (worms sensitive) if FEC reduction was > 90%, equivocal (suspected resistance) if the FEC reduction was between 80 and 90%, and ineffective (resistant) if the FEC was reduced by < 80% [81,90,106]. In these studies, the same standard for declaring resistance was used for all drugs tested.…”

“…Disparities in the standards used for declaring resistance will have a direct impact on the prevalence of resistance that is reported. Craven [19] reported a prevalence of resistance to pyrantel on Danish horse farms of 20%. However, if the 90% cutoff was used, prevalence decreased to 6.7%, and if the 80% cutoff was used, pyrantel resistance would not have been detected.…”