Reemergence Of Cyathostomins Species Demonstrates Anthelmintic Resistance Following Drug Administration In Horses

Abstract Arbitrary administration of anthelmintics to control equine gastrointestinal worms has led to increased resistance to the three broad-spectrum drug classes; benzimidazoles, tetrahydropyrimidines, and macrocyclic lactones. With little promise of new drug classes to target cyathostomins being introduced to the market in the near future, anthelmintic drugs must be administered judiciously to prevent complete anthelmintic parasite resistance. The objective of this study is to determine the reemergence rate of cyathostomins following three commercial horse dewormers during summer. Nine horses housed at two locations were enrolled to the study and was repeated June–September through 2017–2019. Horses were removed from the study if sequencing failed due to low egg recovery for more than 50% of the timepoints. Ivermectin (macrocyclic lactones; n = 6), Moxidectin (macrocyclic lactones; n = 8) and Strongid (pyrantel pamoate; n = 8) were administered to horses and fecal samples were collected every 14d for 98d. Samples were tested using fecal egg counts with a modified McMaster technique and 18S rRNA profiling of the V5.8 and ITS1 regions. Sequences were clustered and taxonomy was assigned against a custom NCBI Blast+ database with the aligned sequences of 19 cyathostomins. Data were analyzed using presence/absence methods in R studio. Treatment and Day significantly impacted the average number of species present (P < 0.001). Moxidectin had the lowest number of species present followed by Strongid then Ivermectin (7.14, 10.17, 11.09, respectively). Equine shedder status had no effect on the average number of species present (P > 0.05). Six species, CO. labiatus, CS. catinatum, CY. auriculatus, CY. elongatus, CT. goldi and CT. longibursatus, showed resistance to the three treatments (P > 0.05). Moxidectin was the most effective at eradicating cyathostomins infestations (P < 0.05). Identifying resistance patterns at the species level will enable mechanistic molecular approaches to determine anthelmintic resistance in cyathostomins.