The sub-lethal effects of second generation anticoagulant rodenticides on birds

There are high economic, human health and environmental reasons for using Second Generation Anticoagulant Rodenticides (SGARs) but there is also widespread non-target exposure to a large number of bird of prey species, such as red kites and barn owls. My overall aim was to assess the potential biochemical and physiological impacts of sub-lethal exposure on birds. I determined environmentally realistic doses of two SGARs, brodifacoum (high acute toxicity) and difenacoum (most widely used and detected in wildlife), in a model species, Japanese quail (Coturnix coturnix japonica), and used these doses in subsequent studies. Anticoagulation profiles and liver residues associated with the doses were characterised. Unexpectedly, one dose (0.4 mg kg-1 body weight brodifacoum) induced persistent (> three weeks) residual anticoagulation. Half-life and repeat-exposure studies highlighted the risk to birds from multiple exposures of brodifacoum in particular. Liver half lives in quail were longer than in rats for brodifacoum but shorter for difenacoum. The magnitude and duration of anticoagulation was greatly increased by multiple exposures involving brodifacoum and was the result of a complex interplay (including inhibition of binding and replacement) between residues in the liver. Studies on quail chicks demonstrated that, while chicks were not especially sensitive to SGARs, exposure reduced growth by 5-10%. Overall, my results suggest that the most likely exposure scenario for UK wildlife, that of repeated exposures to difenacoum, is likely to pose relatively little increased risk compared to single exposures. This is not true for brodifacoum, but it is less widely used in the UK. Thus, exposure scenarios likely to be associated with greater risk, such as repeated exposures to brodifacoum or mixed exposure patterns, are probably less common in the UK. This may partially explain why the number of detected rodenticide-induced wildlife mortalities is low.