Fate and Effects of Parasiticides in the Pasture Environment

Abstract

Parasiticides are used in the treatment of livestock animals. Following use, they may be excreted to the environment where they can impact non-target organisms. European regulations require an environmental risk assessment (ERA) for parasiticides before they are authorised for use, and guidance exists on how risks should be assessed. The methods employed are simple and conservative so it would be beneficial if approaches could be developed that more accurately assess the risks. The aim of this study was therefore to develop, through a combination of field and modelling investigations, an improved understanding of those factors and processes determining the risks of parasiticides in the pasture environment. The study focused on the avermectin compound, ivermectin (IVM). Following administration to cattle, IVM was found in manure at levels up to 1.3 mg/kg (dry weight); this is an order of magnitude greater than the No Observed Effect Concentration (NOEC) for IVM to dung flies. Once released into the field, residues persisted in dung. Small amounts of IVM were transported into soil, probably as a result of the activity of soil and dung fauna. Mesocosm studies showed that in surface waters IVM will rapidly dissipate from the water column through photodegradation and partitioning to the sediment. In sediment, the IVM is highly persistent. Matrix population modelling was used to extrapolate the results of excretion, persistence and ecotoxicity data to the wider environment. The modelling predicted only a small impact of IVM on the abundance of a fast-breeding fly, a finding supported by the results of published monitoring studies. The study has demonstrated that population modelling approaches provide a valuable tool for use in ERAs for parasiticides. The study has also highlighted a number of areas, including a rigorous consideration of analytical method performance that should be considered when assessing the fate and effects of parasiticides in the environment.