Activity of ivermectin long-acting injectable (IVOMEC(®) GOLD) in first-season grazing cattle exposed to natural challenge conditions in Germany.

The persistent activity of ivermectin long-acting injection (IVM LAI; IVOMEC® GOLD, Merial; 3.15% ivermectin w/v) against nematode infections of cattle was evaluated under natural challenge conditions. Seventy nematode-free Brown Swiss calves were blocked by pre-treatment bodyweight and allocated randomly to seven groups of 10 animals each: saline (control) at 1 mL/50 kg bodyweight once on day 0 or IVM LAI at 1 mL/50 kg bodyweight (630 mcg IVM/kg) on either days 0, 7, 14, 21, 28, or 35. After housing until day 35, calves were grazed as one herd on a naturally contaminated pasture for 42 days. Calves were then weighed and housed for 4 weeks before being necropsied for parasite counting. Treatment with IVM LAI prevented the establishment (>90%, p < 0.05) of Dictyocaulus viviparus (100%), Bunostomum phlebotomum (100 %), Haemonchus contortus (98.6%), Ostertagia ostertagi/lyrata (94.9%), and Oesophagostomum radiatum (93.3%) for at least 77 days; Ostertagia leptospicularis (99.1%) for 63 days; Cooperia punctata (97.7%), Trichostrongylus axei (96.5%), and Ostertagia spp. inhibited larvae 4 (93.3%) for 56 days; Cooperia oncophora/surnabada (96.9%), Trichuris discolor (93.6%), and Cooperia spp. inhibited larvae 4 (98.8%); and Nematodirus spp. inhibited larvae 4 (97.1%) for 42 days. Calves of groups treated with IVM LAI had significantly (p < 0.001) higher days 0 to 77 weight gains than the saline-treated controls (28.40-39.25 vs 2.60 kg); the weight gains of the IVM LAI-treated groups, however, were not different from one another (p > 0.3). This study demonstrated a very high efficacy of IVOMEC® GOLD in preventing the establishment of a wide range of bovine nematodes for extended periods of time which was associated with a significant benefit to productivity in terms of weight gain.