Pre-established anthelmintic protocols in horses: low efficacy and unnecessary treatments highlight the need for strategic deworming

Abstract

Context Deworming all horses with commercial anthelmintics on a rotational basis every 90 days is still popular worldwide. Aims To investigate anthelmintic resistance and therapeutic requirements of adult horses on the basis of faecal examinations after a rotation between commercial anthelmintics commonly used in Brazil. Methods We followed up faecal egg counts and coprocultures of 29 horses for 7 months, after pre-established anthelmintic treatments, every 3 months (August and November 2019 and February 2020). The following commercial anthelmintics were used respectively: moxidectin (1), ivermectin + praziquantel (2) and trichlorfon + mebendazole (3). Faecal egg count (FEC), FEC reduction tests (FECRT) and coproculture were performed before (Day 0) and after 14 days for each of the three treatments. Wilcoxon test (95% confidence level, P < 0.05) was used to test the efficacy of the treatment on the FECRT and the occurrence of resistance to the treatments used. Results The FEC results and percentage of individual FECRT indicated parasite resistance, and 82 of 87 (94%) deworming treatments were unnecessary. The efficacy rate was 78%, 34% and 53% for Treatments 1, 2 and 3 respectively. In the coprocultures before deworming, 98.5% were small strongyles, 1.1% Strongylus equinus and 0.4% Gyalocephalus capitatus. After deworming, 98.7% of parasites were small strongyles and 1.3% Dictyocaulus arnfieldi. FEC values at Day 0 and 14 days after anthelminthics were different (lower) only for the moxidectin treatment (U = 271.5; P-value <0.01). For both ivermectin + praziquantel (U = 373; P-value >0.05) and mebendazole + trichlorfon (U = 391; P-value >0.05) treatments, there was no difference in FEC between Day 0 and Day 14 at 95% confidence level. Conclusions The anthelmintics used in the deworming protocol of this study had reduced efficacy against the parasites identified in the coprocultures and the pre-established treatments were probably unnecessary in 93% of the horses with a low level of infection. Implications This study has highlighted not only the issue of the indiscriminate use of anthelmintics regarding parasitic resistance, but also the exposure of horses to unnecessary treatments. Such widespread use is a costly waste of resources and further reinforces the discontinuation of schedule rather than strategic deworming protocols.