Environmental risks associated with equine veterinary drugs and manure management

This project uses a mixed approach - assessment of ecotoxicity and quantification of drug residues - to evaluate more accurately the environmental risk of soil amendment by horse manure compost including these veterinary drug residues. Use of veterinary drugs was assessed over 4 years and we defined an experimental protocol with two events involving intensive use of veterinary substances: the castration of young horses coupled with annual antiparasite treatment. The results showed that of the six substances used for these events (one antiparasite, two antibiotics, one anesthetic, one anti-inflammatory, and one diuretic), only ivermectin (a pesticide) and penicillin G (one of the two antibiotics given after castration) could be quantified before manure composting. Bioassays were carried out on two aquatic organisms (daphnia and green microalgae) and three terrestrial plants. The highest concentrations tested for which no effect on the organism was observed (NOEC) at 72 h were determined for the green microalgae and for the daphnia. Values were all above 1 mg/L and therefore well above the observed environmental concentrations, except for ivermectin with an NOEC of 100 ng/l. Procaine did not affect the reproductive capacity of daphnia but 10 ng/l of ivermectin led to a 52% decline in offspring. However, when the six molecules were used in "combination", aquatic toxicity was observed. Ivermectin, dihydrostreptomycin, and hydrochlorothiazide were all found to have phytotoxic effects, causing inhibition of root growth in all three plants. These results may explain the toxicity of some manure samples from stalls after veterinary treatment of horses. Overall, ecotoxicity tests on composts were similar for manure that received any or all of these treatments and control manure. No significant effect is observed on soil amended by horse manure compost following veterinary treatments.