A field evaluation of pro-benzimidazole, benzimidazole, and non-benzimidazole anthelmintics in horses.

: The effectiveness of 1 pro-benzimidazole (pro-BZD) drug, 3 benzimidazole (BZD) drugs, and 3 non-benzimidazole (non-BZD) drugs in keeping fecal egg counts below 50 eggs per gram 2 and 4 weeks after treatment at 6-week intervals was compared in groups of brood mares and yearlings at 2 Standardbred farms. In a preliminary study (December 1978 to April 1979) as well as major study (April to November 1979), horses were kept in the same groups in the same areas. In the major study, treatments were arranged in a Latin square design. On farm 1, which had a history of repeated use of BZD drug since 1964, the non-BZD drugs, dichlorvos and pyrantel pamoate, rated 83%-100% in their ability to suppress egg counts below 50 eggs per gram. They were significantly better (P less than 0.05) than pro-BZD (febantel) or BZD (cambendazole, fenbendazole, mebendazole) drugs, which rated 13%-58%. Phenothiazine-piperazine-carbon disulfide rated 60%-77% on farm 1 and also was significantly better (P less than 0.05) than pro-BZD or BZD drugs. On farm 2, which had a history of limited use BZD drugs, there was no significant difference between the 3 classes of anthelmintics, which all rated between 67% and 100%. Results of larval culture showed small strongyles to be the major source of high egg counts. In all groups of horses, irrespective of the treatment, there was a marked increase in fecal egg counts by 6 weeks after treatment.