In the United States, new regulatory restrictions have been placed on the use of some second-generation anticoagulant rodenticides. This action may be offset by expanded use of first-generation compounds (e.g., diphacinone; DPN). Single-day acute oral exposure of adult Eastern screech-owls (Megascops asio) to DPN evoked overt signs of intoxication, coagulopathy, histopathological lesions (e.g., hemorrhage, hepatocellular vacuolation), and/or lethality at doses as low as 130 mg/kg body weight, although there was no dose-response relation. However, this single-day exposure protocol does not mimic the multiple-day field exposures required to cause mortality in rodent pest species and non-target birds and mammals. In 7-day feeding trials, similar toxic effects were observed in owls fed diets containing 2.15, 9.55 or 22.6 ppm DPN, but at a small fraction (<5%) of the acute oral dose. In the dietary trial, the average lowest-observed-adverse-effect-level for prolonged clotting time was 1.68 mg DPN/kg owl/week (0.24 mg/kg owl/day; 0.049 mg/owl/day) and the lowest lethal dose was 5.75 mg DPN/kg owl/week (0.82 mg/kg owl/day). In this feeding trial, DPN concentration in liver ranged from 0.473 to 2.21 μg/g wet weight, and was directly related to the daily and cumulative dose consumed by each owl. A probabilistic risk assessment indicated that daily exposure to as little as 3-5 g of liver from DPN-poisoned rodents for 7 days could result in prolonged clotting time in the endangered Hawaiian short-eared owl (Asio flammeus sandwichensis) and Hawaiian hawk (Buteo solitarius), and daily exposure to greater quantities (9-13 g of liver) could result in low-level mortality. These findings can assist natural resource managers in weighing the costs and benefits of anticoagulant rodenticide use in pest control and eradication programs.
New regulatory restrictions have been placed on the use of some second-generation anticoagulant rodenticides in the United States, and in some situations this action may be offset by expanded use of first-generation compounds. We have recently conducted several studies with captive adult American kestrels and eastern screech-owls examining the toxicity of diphacinone (DPN) using both acute oral and short-term dietary exposure regimens. Diphacinone evoked overt signs of intoxication and lethality in these raptors at exposure doses that were 20 to 30 times lower than reported for traditionally used wildlife test species (mallard and northern bobwhite). Sublethal exposure of kestrels and owls resulted in prolonged clotting time, reduced hematocrit, and/or gross and histological evidence of hemorrhage at daily doses as low as 0.16 mg DPN/kg body weight. Findings also demonstrated that DPN was far more potent in short-term 7-day dietary studies than in single-day acute oral exposure studies. Incorporating these kestrel and owl data into deterministic and probabilistic risk assessments indicated that the risks associated with DPN exposure for raptors are far greater than predicted in analyses using data from mallards and bobwhite. These findings can assist natural resource managers in weighing the costs and benefits of anticoagulant rodenticide use in pest control and eradication programs.
Rodents have been noteworthy pests in agricultural areas for decades. Because rodents impact diverse ecosystems, anticoagulant rodenticides have been heavily used throughout the world to control rodent populations. This continued use has led to the development of resistance to anticoagulant rodenticides in some populations of targeted rodents. Although many studies have investigated the genetic and molecular basis of anticoagulant resistance, few have focused on potential changes in metabolic function of resistant animals. In this study, vole (Microtus californicus, Peale) liver microsome preparations were made from unexposed animals living in areas that had never used anticoagulant rodenticides for either crop protection or for the control of commensal rodents and exposed voles living in artichoke fields that have used anticoagulant rodenticides since the mid-1990s. Using these microsome preparations, the metabolism of diphacinone and chlorophacinone was tested. Microsomes from both male and female voles from exposed areas metabolized significantly more anticoagulant than unexposed animals. Also, both exposed and unexposed animals metabolized more diphacinone than chlorophacinone. These findings suggest that alterations in metabolic function may play a role in anticoagulant resistance.
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