Anthelmintic resistance of small strongyles of horses in Upper Bavaria

Anthelmintic resistance of small strongyles of horses in Upper Bavaria was examined using the faecal egg counts of at feast 200 eggs per gl-am (epg) of faeces, as recommended in the guidelines of the World Association for the Advancement of Veterinary Parasitology, were included in the study. The horses were treated orally with the recommended doses per kg bodyweight as follows: 44 animals on 18 farms with 7.5 mg fenbendazole, 34 horses on 12 farms with 5-10 mg mebendazole, 34 horses on 12 farms with 6 mg febantel, 81 animals on 25 farms with 19 mg pyrantel embonate and 37 horses on 13 farms with 0.2 mg ivermectin. Foul-teen days after treatment of the 112 horses on 28 farms with (pro-)benzimidazoles a faecal egg count reduction of <90% was observed on each farm, in 104 horses (92.9%). The epg values in 25 horses on 13 farms were not reduced. Faecal egg count reduction of up to 19% was achieved in 10 horses on 9 farms, 20% to 39% in 14 horses on 10 farms, 40% to 59% in 19 horses on 14 farms, 60%-69% in 15 horses on 11 farms, 70% to 79% in 9 horses on 9 farms and 80% to 89% in 12 horses on 11 farms. Larval culture revealed only L3 of small strongyles with eight intestinal cells. In accordance with current guidelines, resistance is present if the percentage reduction in egg count is less than 90%. Therefore, (pro-)benzimidazole resistance of small strongyles is common and widely distributed in horses in Upper Bavaria. After the treatment of 81 horses with pyrantel embonate and 37 horses with ivermectin, faecal egg counts were reduced by 95% to 100% and 100%, respectively. The larval development assay was carried out using 30 horses from which small strongyles exhibited a (pro-) benzimidazole resistance, but were sensitive to ivermectin and pyrantel embonate in the faecal egg count reduction test. The evaluation of the concentrations of fenbendazole, pyrantel embonate and ivermectin preventing the development of 50% of L3 stages (LC 50) indicated that the LC 50 values of the three anthelmintics varied considerably. Given the extent of the deviation of the aver age LC 50 value, resistance to fenbendazole was observed, but could not be excluded for ivermectin and pyrantel embonate, even though there was only a small prevalence of horses with larvae.