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For the control of worm infections, the strategic use of anthelmintics, often accompanied by additional farm and/or pasture management procedures, is currently applied on most horse farms in industrialized countries. However, the particular effects of the specific worm control procedures are often unclear and have only been investigated to a limited extent. We examined faecal egg count (FEC), faecal egg count reduction (FECR) and questionnaire data on farm and pasture management procedures. The aim of this study was to determine whether specific worm control practices reported to be applied in European horse farms affect worm prevalence.
This study involved 20 German, 26 Italian and 16 UK horse farms for each of which FEC were performed on a minimum of 16 horses. In total, 2029 horse faecal samples were quantitatively analysed for helminth eggs, resulting in 56.3% of the faecal samples being positive for strongylid eggs. The prevalence in the 742 German horse samples (48.1%) was significantly lower than that in the 737 Italian (61.1%) and the 550 UK (60.9%) samples. As expected, a significant effect of horse age on the infection prevalence was observed, with adult horses showing lower prevalences and lower mean FEC than foals and yearlings. The majority of the participating farms were stud farms (n = 29), followed by riding stables (n = 27) and racehorse stables (n = 6). The prevalence of strongyle infection by farm type differed between countries. While in Germany, horses on riding farms were significantly less often strongyle positive, in the UK horses on stud farms showed the lowest strongyle prevalences, whereas in Italy, no significant difference between farm types were seen. On all farms, horses received routine/preventive anthelmintic treatment. An effect of treatment frequency on strongyle prevalence was only encountered with adult horses. On farms performing more than one annual treatment, faecal samples were significantly less often positive. Furthermore, by comparing the FECR results of individual horses with their pre-treatment FEC, it was found that high pre-treatment FEC were associated with a significantly higher probability for a FECR below 90%.
Overall, age-dependent strongyle infection patterns and general worm control approaches were found to be similar on horse farms in the three countries. Also, a negative association of pre-treatment FEC and treatment efficacy was consistently found in all countries. However, mean strongyle prevalences and frequencies of anthelmintic treatments were considerably different. In addition to the age-dependent prevalence patterns, the finding of a possible negative association between high FEC and reduced FECR might argue for a focus on horses showing high pre-treatment FEC when monitoring anthelmintic treatment efficacy in the field.