Selective deworming of horses as a method to prevent parasite resistance to anthelmintics

The article discusses the current problem of parasite resistance to anthelmintics and proposes measures to prevent it. An analysis of the literature data showed that parasite resistance is formed due to excessive and incorrect use of drugs, as well as under the influence of their genetic characteristics. The current veterinary regulations do not provide sufficient systematization in the fight against helminthiasis, which necessitates a scientific assessment of the applied strategies. The results of the 2023—2025 study indicate a high prevalence of helminths in horses in the North-Western Economic Region, where macrocyclic lactones are the preferred drugs for deworming. The widespread use of the same groups of anthelmintics and a subjective approach to determining animal weight significantly increase the risk of resistance development. In this regard, it is proposed to switch to a more rational approach — selective deworming, based on diagnostics and regular monitoring of the extensiveness and intensity of the invasion by conducting FECRT (Faecal Egg Count Reduction Test) when the ineffectiveness of treatments is suspected. This approach will reduce the selective pressure on the parasite populations and slow down the process of the formation of resistance. The basis of the strategy to prevent resistance to anthelmintics is a diagnostic-oriented approach. It is necessary to identify «super-producers», as about 20 % of horse’s produce 80 % of the eggs, to deworm these animals, and to regularly monitor the intensity of the infestation using the FEC test to identify horses that produce more than the threshold value of eggs (200 eggs/g for parascarids and 500 eggs/g for strongylides). Fecal analysis should be carried out 4—6 times a year. Selective deworming may be risky for foals, as parascarids can cause obstructive ileus in young animals with high infection rates. In farms with a high prevalence of parascaridiasis, deworming should be performed even at low egg counts in feces to prevent acute cases and animal deaths. When rotating drugs, it is important to consider their mechanism of action and pharmacological group.

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