Combination Anthelmintics to Control Gastrointestinal Nematodes in Foals



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Two common nematodes that affect young horses are cyathostomes (small strongyles) and Parascaris equorum (ascarids). It has been recently found that populations of these nematodes are resistant to common anthelmintics used to control them. Small strongyles have been found to be resistant to pyrantel and fenbendazole, while ascarids have been found to be resistant to ivermectin. This represents a unique dilemma in controlling the gastrointestinal nematode population in the foal. It has been shown in other host species that combination anthelmintics can be used to successfully treat resistant nematodes. The current study utilized 28 foals and was conducted from April to November 2007. The foals were allocated into age cohorts and randomly assigned a treatment regimen. Group I was administered ivermectin at 0.2 mg/kg BW. Group II was administered ivermectin at 0.2-mg/kg BW and pyrantel pamoate at 6.6 mg/kg BW. Group III was administered ivermectin at 0.2-mg/kg BW and fenbendazole at 10 mg/kg BW. Group IV was administered pyrantel pamoate at 6.6 mg/kg BW and fenbendazole at 10 mg/kg BW. Fecal samples were collected at time of treatment and two wk post treatment to determine effectiveness in removing egg producing adult nematodes. Each age cohort was then treated 30 d later with a different anthelmintic or combination. That is, foals in group I were treated as those in group II, group II to treatment III, group III to treatment IV, and group IV to treatment I. Over a period of 4 mo, each foal received at least one treatment in consecutive order. The difference of egg counts (pre-treatment minus post-treatment) for small strongyles treated with ivermectin (IVM) was 29.39 eggs per g (EPG), 5.44 EPG for ivermectin with pyrantel (PRT), 3.85 EPG for ivermectin with fenbendazole (FBZ), and -8.32 EPG for pyrantel with fenbendazole. There was a significant difference when comparing IVM to IVM PRT (P = 0.0018), IVM vs. IVM FBZ (P = 0.0010), and IVM vs. PRT FBZ (P < 0.0001). IVM was more effective than each of the other treatments. There was no influence of treatment on ascarid EPG (P > 0.1184).