Parasites in Horses Kept in A 2.5 Year-Round Grazing System in Nordic Conditions without Supplementary Feeding

SIMPLE SUMMARY: Grazing horses year-round may be a means to increase biodiversity. In this study, parasite occurrence was documented on a monthly basis in 1- to 3-year-old Gotlandsruss stallions grazed year-round for 2.5 years. Horses became infected by several parasites and, when needed (>200 strongyle eggs/gram feces), horses were dewormed with the anthelmintic drug pyrantel, which has low or no ecotoxic impact on soil fauna. This strategy failed to control small strongyle occurrence. Horses excreted larger amounts of small strongyle eggs during summer–autumn than during the rest of the year, and the number of excreted eggs increased year-on-year. High small strongyle egg excretion did not seem to affect the body condition of the horses. Some horses were also infested with chewing louse, but did not scratch more than unaffected horses. We found that to keep egg excretion below 200, pyrantel was not sufficient and a substance known to be toxic to dung fauna and freshwater invertebrates had to be used on some occasions. ABSTRACT: Horse grazing can be favorable from a biological diversity perspective. This study documented the occurrence of endo- and ectoparasites and sought to reduce parasite egg excretion with the anthelmintic drug pyrantel in 12 Gotlandsruss stallions maintained in a year-round grazing system for 2.5 years. Feces samples were collected monthly and all horses were treated with pyrantel, the anthelmintic drug of choice in biological diversity preservation, at study population mean cyathostomin eggs per gram (EPG) of >200. The relationship between cyathostomin EPG and body condition was studied, as was horse behavioral response to Bovicola equi (chewing louse) infestation. Eggs of cyathostomins (small strongyles), Parascaris spp. (roundworm), Oxyuris equi (pinworm), Anoplocephala perfoliata (tapeworm), and Gasterophilus spp. (botfly) were detected at least once during the trial. Excretion of cyathostomin eggs was highest during summer–autumn and increased year-on-year. No relationship was found between cyathostomin EPG and body condition. Infestation with B. equi did not affect the number of scratching sessions compared with unaffected horses. Therefore, in this year-round grazing system, pyrantel treatment had to be complemented with moxidectin to reduce excretion of cyathostomin eggs, thus compromising biological diversity.