Environmental risk assessment of veterinary parasiticides used in cattle

Abstract

All veterinary medicines have to undergo a rigorous environmental risk assessment before gaining a marketing authorisation; post-marketing pharmacovigilance ensures constant surveillance for any environmental impacts that had not been anticipated from earlier research. Products are labelled to reflect any potential risk points. Despite these procedures, some compounds, notably parasiticides of the macrocyclic lactone class that are used in large animal practice, have gained a reputation among some scientists, the general public and the media for having adverse effects on dung insect fauna and for causing delays in the degradation of faeces on pasture. While the toxicity of faecal residues of insecticides to non-target insect fauna, when measured under controlled conditions in short-term experiments is indisputable, the consequences of such activity at ecological scales are much less well defined. Dung provides an ephemeral habitat for an enormous array of macro- and micro-invertebrates, fungi and bacteria, whose composition changes as pats age and disintegrate. Perturbations to such complex systems inevitably have a variety of outcomes and this is reflected in field trials, which can produce conflicting results. This article summarises pertinent aspects of the underlying biology and ecology of the bovine faecal pat and the evidence for significant impact of veterinary medication.