Recommendations for a sustainable control of gastro-intestinal worm infections in horses in Germany

Abstract

Horses kept on pasture are principally always exposed to infections with gastro-intestinal helminths. In Germany, according to their prevalence respectively their pathogenicity, the most important species are the ubiquitously occurring small strongyles (cyathostominae), the roundworm Parascaris equorum, especially in foals the threadworm Strongyloides westeri and the tapeworm Anoplocephala pedoliata. The conventional recommendation for the control of gastro-intestinal nematodes in horses is the strategic, i.e. according to a predetermined time-schedule, application of anthelmintics to all animals of the farm or age group. In practice the strategic use of anthelmintics is usually being applied without any previous or accompanying coproscopic diagnostic. Recent investigations have shown that an increasing number of parasitic horse nematode populations have developed resistance against single or multiple antiparasiticides. This accounts mainly to the small strongyles, for which on more than three quarters of recently examined horse farms in Northern Germany resistance against benzimidazoles was detected. Recently, also pyrantel was found not to be sufficiently effective on approximately one third of the tested German farms. In contrast to this, concerning the macrocytic lactones a satisfying efficacy was observed on all tested farms. However, obviously this drug class often is not anymore unaltered effective against the horse ascarid R equorum.The increasing problem of anthelmintic resistance requires a reorientation of worm control strategies with the aim of a sustainable use of anthelmintics or at least a postponement of resistance development. To this end the routine monitoring of parasite infections should consequently be performed and based on the monitoring findings targeted (and not calendar based) anthelmintic treatments should be applied. Furthermore, the infection intensity on the farm and thus the required treatment frequency should be reduced through the regular control of treatment success, avoidance of importation of resistant parasite populations by using quarantine treatments on all horses newly introduced into the farm, stable and pasture hygiene, where possible, a reduction of pasture contamination by pasture cleaning or by previous pasture use. Routine, that is multiple per year, worm monitoring can be done by examining individual or pooled faecal samples. The latter allowing a cost efficient as well as reliable qualitative assessment of the infection situation on the respective farm. This should be an ultimate requirement for any treatment decision. The present paper outlines concrete recommendations for the conduct of an integrated worm monitoring and treatment concept targeted at the respective horse groups. This includes a table with specific practical procedure schedules for foals, breeding mares, yearlings and adults.