Investigation of safety and efficacy of febantel and fenbendazole in fish and exposure assessment

Fish are susceptible to blood-sucking parasite infections, which cause severe anemia, dyspnea, and ultimately death. However, veterinary drugs available for fish to treat such infectious diseases are lacking; thus, livestock drugs have been repurposed as aquatic animal drugs. Febantel (FBT) and fenbendazole (FBZ) are representative antiparasitic agents for livestock such as cattle, swine, and poultry, and are considered suitable as aquatic animal drugs. Therefore, we investigated the safety and efficacy of FBT and FBZ in fish and performed a risk assessment to determine the maximum residue limit in fish. Most studies indicate that FBT is rapidly converted to FBZ, which is metabolized to oxfendazole and oxfendazole sulfone. FBZ was frequently detectable in the plasma and tissues (e.g., muscle, skin, and the liver) in significant quantities than other metabolites. We regarded the liver as the target organ because reversible hepatocytic changes were observed in fish after administration of 100 mg/kg FBT for 9 days. No toxicological effects, such as increased mortality or decreased appetite, were observed when the fish were administered 50 mg/kg FBT for 3 days. The efficacy of the drugs was verified in various parasites, including H. heterocerca, H. okamotoi or Z. japonica, and M. seriolae, as causative agents of beko disease through laboratory and field trials. Although toxicity studies on FBZ in fish are limited, its safety has been demonstrated from toxicity studies in a wide range of animal models. The risk from using FBT and FBZ was negligible for human health because the ratio of the estimates of dietary exposure and acceptable daily intake was 78.4%.