A field evaluation of anthelmintics in horses in Sweden.

A field evaluation of anthelmintics in 336 horses on 37 farms was conducted between February and May 1986 in Sweden. The herds, each comprising at least eight horses, had histories of grazing on permanent pastures and receiving regular treatments against parasites at least three times a year. Small strongyles were refractory to (pro)-benzimidazole drugs in all but one of 23 herds examined. There was an average reduction in egg output of approximately 60%, and approximately 30% of 205 horses examined were shedding less than 100 eggs g-1 7 days after treatment. There was great between-herd variation of both the faecal egg-count depression (6.4-90.4%) and drug efficacy (0.0-80.0%). The non-benzimidazole drugs under investigation were fully effective against mature small strongyles.