Combination deworming for the control of double-resistant cyathostomin parasites – short and long term consequences.

This research article investigates the efficacy of combination deworming against equine cyathostomin parasites resistant to benzimidazole and pyrantel drugs. The results suggest that while the combination treatment shows initial effectiveness, this is not sustained in the long term.

Research Background and Purpose

• Equine cyathostomin parasites are commonly found within the gastrointestinal system of horses and have developed resistances to common deworming medications globally.
• Using a combination of two drugs, oxibendazole and pyrantel pamoate, has been proposed as a potential solution to managing this resistance. Though this approach was effective in one previous study, no research has evaluated the long term effects of this treatment regimen.
• The goal of this research was to evaluate the effectiveness of this combination therapy over a one-year period in a horse population already resistant to these drugs.

Research Methodology

• This study involved a herd of 21 horses, all of whom were tested for parasite egg counts in their fecal matter at start of the study, two weeks later and then every eight weeks throughout the treatments.
• The initial efficacy rates for the individual drugs, oxibendazole and pyrantel pamoate, were 66.7% and 63.3% respectively. In a sequence of four treatments administered every eight weeks, the combined drug treatment was applied, after which the individual drug treatments were once again given.

Research Findings

• Although initially, the combination therapy demonstrated an additive effect, the efficacy declined over time. The reduction in mean fecal egg count was significantly greater for the first combination treatment (76.6%) in comparison to the second (42.6%), third (41.6%), and fourth (40.7%) treatments.
• In terms of individual drugs, the final single active mean fecal egg count reductions were 42.3% for oxibendazole and 42.7% for pyrantel pamoate, with no significant difference from their initial efficacies.

Conclusion

• The study concludes that while combination deworming therapy might show initial success, it fails to sustain its effectiveness over a longer period.
• The strategy of using combination of drugs with decreased starting efficiencies does not represent a sustainable solution for the control of double-resistant equine cyathostomin populations.