Consumption and effectiveness of anthelmintic-medicated block supplements for the treatment of trichostrongyle nematodes in a northern Great Plains Bison Herd.

In high numbers, trichostrongyle nematodes decrease productivity and cause health problems in ruminants including North American bison (Bison bison). Treatment strategies used to limit their impact in cattle are also being used by bison producers, including free-choice group-treatments either as the sole option or in combination with squeeze chute treatments. There have been no published studies evaluating the efficacy of free-choice anthelmintic strategies for bison under pasture conditions, and so the present study evaluated the effectiveness of fenbendazole-medicated multinutrient blocks using 56 pastured bison from South Dakota that were naturally infected with trichostrongyles. Consumption comparisons between non-medicated protein-based multinutrient blocks and those based on molasses found that bison eventually consumed roughly similar amounts, but that consumption increased more quickly for the protein-based blocks. Therefore, Safe-Guard® (fenbendazole) Free-choice Protein Blocks were used for the efficacy study and were added to the pasture for 10 days. Consumption rates were similar to non-medicated rates and consistent with recommendations for cattle. Based upon 154 pre-treatment and 168 post-treatment fecal samples collected daily for 7 days before and 7 days after treatment, there was a 95 % reduction (with 90 % confidence interval ranging from approximately 90-98 %) in fecal egg counts resulting from the treatment. This study showed that substantial trichostrongyle reductions can be achieved among grazing plains bison herds through the use of Safe-Guard® blocks. While effective, the increased risk for developing anthelmintic resistance from chronic underdosing requires that free-choice products be used only occasionally to solve health issues until management strategies can be improved.