The effect of risk factors of sheep flock management practices on the development of anthelmintic resistance in the Czech Republic

The in vivo faecal egg count reduction test was used for the evaluation of the benzimidazole and macrocylic lactone drug efficacies against gastrointestinal nematodes in 10 flocks of sheep. The same samples were tested concurrently in vitro for benzimidazole resistance using the egg hatch test, and for resistance to ivermectins using the microagar larval development test. The conformity of obtained results between in vivo and in vitro tests was recorded. When the selected methods were applied, anthelmintic resistance to benzimidazoles was detected at four (40%) farms while resistance to ivermectins was evident at one (10%) farm. At one farm (10%) ivermectin resistance was suspected. Moxidectin was effective at all surveyed farms. Teladorsagia was recognized as the only benzimidazole resistant genus in post-treatment coprocultures, whereas Haemonchus larvae were resistant to ivermectins. This represents the first recorded occurrence of ivermectin resistance in gastrointestinal nematodes of sheep in the Czech Republic. A linear mixed-effects model demonstrated that the majority of evaluated management practices have a significant effect on resistance to benzimidazoles. While application of preventive practices like quarantine and smart drenching maintains low levels of anthelmintic resistance, others like the dose-and-move strategy, administration of the same family of drugs over extended periods of time, and number of treatments per year are responsible for the increase of resistance to anthelmintics at evaluated farms. Only targeted selective treatments approaches had no effect on resistance status. This study indicates the importance of farm management practices in anthelmintic resistance development.