Ecotoxicological effects of the emerging contaminant ivermectin on Rhinella arenarum: A comparative study of active ingredient and commercial formulation

Ivermectin (IVM) is a broad-spectrum veterinary antiparasitic used worldwide in cattle breeding. The aim of this study was to evaluate the lethal effects of the active ingredient and a commercial formulation of IVM (1 % active ingredient) in the embryonic stage (S. 4–6) and larval stage (S. 25) of the South American amphibian Rhinella arenarum through chronic standardized bioassays. Also, behavior analysis and oxidative stress and cholinergic effects biomarkers were analyzed at 1, 10 and 100 μg IVM/L concentrations. For the embryonic stage, the active ingredient (96 h- LC₅₀: 15900 μg/L) was more toxic than the commercial formulation (96 h-LC₅₀: 51230 μg/L) during the acute period, while at chronic exposure the commercial formulation was more toxic (504 h-LC₅₀: 10.25 μg/L), compared to the active ingredient (504 h-LC₅₀: 312.80 μg/L). For the larval stage, in acute exposure, the active ingredient (96 h-LC₅₀: 800 μg/L) was more toxic than the commercial formulation (96 h-LC₅₀: 1550 μg/L). In the chronic exposure, the commercial formulation (504 h-LC₅₀: 77.33 μg/L) was more toxic than the active ingredient (504 h-LC₅₀: 195.25 μg/L). Overall, larvae exhibited greater sensitivity to both the active ingredient and the commercial formulation. However, during chronic exposure, embryos were more sensitive to the commercial formulation than larvae. The commercial formulation primarily induced oxidative stress, and both forms of the compound affected behavior and cholinergic effect biomarkers, even at low environmentally relevant concentrations (1 μg/L). These results highlight the potential impact of IVM on aquatic ecosystems.