Drug treatment for lungworm in bighorn sheep: Reevaluation of a 20-year-old management prescription

We conducted a 1-year management experiment to examine the annual effects o f alternative management treatments, including anthelmintic administration, on lamb survival in Rocky Mountain bighorn sheep (Ovis canadensis canadens is) herds in southcentral Colorado. Beginning in December 1991, 4 bighorn h erds were managed under alternative treatment regimes: baiting with alfalfa hay and apple pulp treated with fenbendazole (BT), baiting with alfalfa ha y and apple pulp without fenbendazole (B), placing fenbendazole-treated sal t blocks on winter ranges (T), and both withholding bait and fenbendazole ( control; C). Treatments were rotated annually under a predetermined, random ly-selected schedule. Mean lamb production, survival, and recruitment rates did not differ among herds or years. None of the management treatments pro duced demonstrable improvements in lamb production, survival, or recruitmen t. Estimated effects (+/-SE) of treatments on lamb production and survival were small (less than or equal to 0.09 +/- 0.18). Annual recruitment rates (lambs/marked ewe) through October ranged from 0.13 to 0.88, but overall re cruitment rates (mean; 95% CI) for B (0.72; 0.55-0.90), T (0.71; 0.47-0.95) , and BT (0.52, 0.08-0.97) were indistinguishable from C (0.62; 0.19-1.0). Sick and coughing lambs were observed every summer in 1 herd and 1 summer i n a second. Our results demonstrate that annual parasite treatment is not p rerequisite for acceptable lamb survival among southcentral Colorado's wild bighorn populations, and that annual parasite treatment may not prevent ca tastrophic losses of lamb cohorts. Based on these data, we question the nee d for annual baiting and parasite treatment in the herds studied here and r ecommend such practices be reevaluated elsewhere.