Occurrence of fenbendazole resistance in Parascaris spp. on breeding farms in Sweden.

Overview

• This study investigated the presence of fenbendazole resistance in Parascaris spp. (equine roundworms) on horse breeding farms in Sweden.
• The researchers also explored how farm management factors, especially the number of foals born annually, relate to the development of this drug resistance.

Background

• Anthelmintic resistance refers to the reduced effectiveness of drugs designed to eliminate parasitic worms.
• Parascaris spp. is a common gastrointestinal parasite in foals that has shown increasing resistance to widely-used drugs like macrocyclic lactones over recent decades.
• Recent reports indicate possible treatment failures with other drug classes such as tetrahydropyrimidines and fenbendazole, raising concerns about emerging resistance.

Study Objectives

• To confirm the presence and extent of fenbendazole resistance in Parascaris spp. on Swedish breeding farms.
• To investigate management-related risk factors associated with resistance, focusing on farm size and treatment practices.

Methodology

• The study was conducted over a span of three years (2021-2023) and included:
  • Eleven horse breeding farms in Sweden.
  • 92 foals testing positive for Parascaris spp.
• A faecal egg count reduction test (FECRT) was used to evaluate the efficacy of fenbendazole treatment following guidelines from the World Association for the Advancement of Veterinary Parasitology (WAAVP).
• Fenbendazole efficacy was measured by the reduction in parasitic egg counts in feces before and after treatment.

Key Findings

• Fenbendazole resistance was confirmed on four of the eleven farms, with drug efficacy ranging widely from 45% up to 96%:
  • Efficacy below 90-95% is generally considered indicative of resistance in such tests.
• One farm previously reported with reduced fenbendazole efficacy was confirmed to have established resistance, indicating persistence and potential spread of resistant worm populations.
• The largest farms, those with more than 40 foals born annually, had a significantly higher likelihood of harboring fenbendazole-resistant Parascaris spp., based on a generalized linear model (GLM) with strong statistical significance (t = 70.39, p < 0.001).
• There was a noted correlation between the number of foals on the farm and the frequency of parasite treatments given yearly:
  • Smaller farms (fewer than 20 foals) tended to treat foals twice during their first year of life.
  • Larger farms (more than 40 foals) were more likely to administer treatments at more frequent intervals.
• This treatment frequency difference may influence selection pressure for resistance development.

Conclusions and Implications

• Fenbendazole resistance in Parascaris spp. is now established on some Swedish breeding farms.
• The size of the farm (measured by number of foals born per year) is a significant risk factor for the development and presence of anthelmintic resistance.
• More frequent treatments on larger farms might contribute to increased selection pressure favoring resistant worm populations.
• The study highlights the urgent need for targeted parasite control strategies that consider farm size and treatment frequency to reduce the emergence and spread of resistance.
• It supports the importance of regular monitoring of drug efficacy using standardized tests like FECRT to inform sustainable parasite management on horse breeding farms.