Ecological Thresholds for a Fenthion-Based Veterinary Pharmaceutical in Tropical Soil Using Species Sensitivity Distribution (SSD) Modeling

Fenthion is a veterinary pharmaceutical (VP) used in the control of ecto- and endoparasites in livestock. This active substance may reach the soil through the animals’ urine and feces, but the ecotoxicological data of this compound on non-target soil fauna is limited. This study aimed to evaluate the chronic ecotoxicity of a fenthion-based veterinary formulation on six species of soil invertebrates. Ecotoxicological tests were performed in a natural tropical soil (Entisol) with earthworms Eisenia andrei, enchytraeids Enchytraeus crypticus and Enchytraeus bigeminus and collembolans Folsomia candida, Sinella curviseta and Proisotoma minuta, following ISO protocols. The species sensitivity distribution (SSD) approach was employed to establish protective concentrations (PCs) for fenthion in soil. Collembolans were the most sensitive organisms; the EC50 for F. candida, S. curviseta and P. minuta were 0.72, 0.93 and 0.48 mg kg−1, respectively. Earthworms E. andrei presented intermediate sensitivity (EC50 = 28.40 mg kg−1). Enchytraeids were the least sensitive animals (EC50 for E. crypticus and E. bigeminus of 233.66 and 221.47 mg kg−1, respectively). SSD-derived PCs for 95%, 90%, 80% and 50% of soil species were estimated in 0.017, 0.054, 0.225 and 3.432 mg kg−1, respectively. The predicted environmental concentration (PEC) of fenthion in soil via veterinary use (0.032 mg kg−1) is almost twice the PC95 value (0.017 mg kg−1), indicating potential ecological risk of fenthion to soil species. This study brings new information regarding the ecotoxicological profile and ecological risk of a relevant VP to the edaphic community.