Identification and characterization of a pyrantel pamoate resistant cyathostome population.

The research paper addresses the effectiveness of a drug called pyrantel pamoate (PP) against a specific type of worm (cyathostome) in horses on a farm in Louisiana, where the treatment was previously suspected to be failing. It was found that not only had the worms developed resistance to this drug, but also to another drug called oxibendazole (OBZ), pointing to the occurrence of multi-drug resistance.

Research Method

The researchers:

• Ran tests known as Fecal Egg Count Reduction assays (FECR) to assess the efficacy of the treatment. This process involves counting the number of worm eggs in feces both before and after treatment to determine how effectively the drug eliminates the worms.
• Performed additional treatments after two weeks to see if efficacy was affected by repeat treatment.
• Ran a “critical trial” in which three horses known to be free from the worms were exposed to the infested environment. These horses were then treated, and the worms present in their intestines were identified to see if any species were resistant to treatment.

Findings

The study found:

• The FECRs revealed varying effectiveness of PP, from a reduction of just 25% in mares to 83% in yearlings. Repeat treatments showed no improvement in results, indicating a consistent resistance to the drug.
• The critical trial found that 11 species of cyathostomes remained in the intestines of the treated horses, with seven species showing reduced responses to the treatment.
• Resistance was also discovered to another drug, oxibendazole. This drug had been over 90% effective on the same farm in the past, suggesting the parasites have developed resistance over time.
• In one experiment, dual resistance to both PP and OBZ was observed, raising the concern of multi-drug resistance.

Implications

These findings confirm the suspicion of deworming resistance to PP and OBZ at this location. The presence of multi-drug resistance will require more sophisticated parasite management strategies, including rotation of different classes of drugs, and careful monitoring of drug effectiveness. This study also highlights the importance of ongoing evaluation of drug efficacy in parasite control, as it illustrates the smooth pace at which resistance can develop.