Acute toxicity to freshwater organisms of antiparasitic drugs for veterinary use

The acute toxicity of five antiparasitic drugs used in the veterinary field—amprolium hydrochloride (APH), bithionol (BT), levamisole hydrochloride (LVH), pyrimethamine (PYM) and trichlorfon (TRC)—to the aquatic organisms Oryzias latipes, Daphnia magna, and Brachionus calyciflorus was examined. The toxicity test with O. latipes was conducted in accordance with the OECD Guidelines for the Testing of Chemicals (1993) to determine the 24-, 48-, 72-, and 96-h LC50 values. In addition, 24- and 48-h EC50 values for D. magna and a 24-h EC50 for B. calyciflorus were determined with the DAPHTOXKIT F™ magna (Creasel, Belgium) and the ROTOXKIT F™ (Creasel, Belgium), respectively. High-performance liquid chromatographic analysis revealed that APH, LVH, and PYM were stable in water, but BT was unstable, decreasing by 84% on average at 24 h. TRC rapidly decomposed, with only 0.7% of the initial concentration remaining after 96 h, forming dichlorvos. The toxicity of TRC to O. latipes was determined in two ways: exposure to the same medicated water for 96 h (static test) and exposure to medicated water replaced every 24 h (semistatic test). AMP, LVM, and PYM were tested in the static condition, and BT was tested in the semistatic condition. BT was most toxic to O. latipes, with a 96-h LC50 of 0.24 mg L−1, followed by PYM, with a 96-h LC50 of 5.6 mg L−1. The 24-, 48-, 72-, and 96-h LC50 values of TRC in the static test were 92.0, 45.2, 29.5, and 17.6 mg L−1, respectively, which tended to be lower than those in the semistatic test, especially late in the observation period. D. magna was the most susceptible to TRC, with a 48-h EC50 as low as 0.00026 mg L−1. The 48-h EC50 values of BT, PYM, and LVH for D. magna were 0.3, 5.2, and 64.0 mg L−1, respectively. B. calyciflorus was the most susceptible to BT, with an EC50 of 0.063 mg L−1, followed by PYM, with an EC50 of 15.0 mg L−1. Among the test compounds, APH was the least toxic to all the freshwater organisms tested, with a 96-h LC50 of >600 mg L−1 for O. latipes, a 48-h EC50 of 227 mg L−1 for D. magna, and an EC50 of 403 mg L−1 for B. calyciflorus. © 2005 Wiley Periodicals, Inc. Environ Toxicol 20: 60–66, 2005.