Anthelmintic resistance in gastrointestinal nematodes of cattle owned by smallholders in Southern Chile

Helminth infections present a significant challenge to rural and indigenous smallholders. In a municipality of Los Lagos Region, Chile, ivermectin has been the preferred method of anthelmintic control in cattle, but its efficacy against gastrointestinal nematodes after years of usage had not been evaluated. In the spring of 2021, 30 smallholder cattle farmers and 128 bovines were selected using a one stage cluster sampling. During the first visit, fecal samples were collected, and animals were treated with subcutaneous ivermectin (0.2 mg/kg). Samples were sent to the Parasitology Laboratory of the Universidad Católica de Temuco. Fourteen days after the first sampling, a second fecal sample was taken from the same animals. Fecal eggs count (FEC) was performed in a Whitlock universal chamber. Animals with initial FECs ≤100 eggs per gram (epg) were excluded from the FEC reduction test (FECRT). The classification of resistance, suspected resistance, or susceptibility was established based on the guidelines provided by Denwood et al. (2023) and the WAAVP (2023). Among the 128 animals sampled and treated in the first sampling, 61 (47.7 %) had FEC ≥100 epg, with 85.2 % being ≤18-month-old. Considering animals with initial FEC ≥100, the mean farm adjusted FECR percentage was 55.0 % (95 % CI 34.3 % - 87.9 %) and the treatment failed to reduce the egg count per gram by more than 95 % in 65.6 % of the animals sampled. Additionally, animals ≥18-month-old had 48 times higher odds of achieving a 95 % FECR percentage than animals <6 month-old. This study provides evidence of gastrointestinal nematode's resistance to subcutaneous ivermectin treatment in smallholders' cattle in a municipality of Los Lagos region, Chile.