Assessing the Usefulness of Blood Samples to Monitor for Exposure to Anticoagulant Rodenticide in Red-Tailed Hawks

by Maureen Murray, DVM, Cummings School of Veterinary Medicine at Tufts University

Anticoagulant rodenticides (ARs) are rodent poisons that have been widely used globally for decades for the control of commensal rodents (those who live off what they obtain from human communities). Deaths due to exposure to these rodenticides have been documented in several bird of prey species, and an increasing number of studies from countries around the world have found residues of ARs in predatory wildlife. Due to the persistence of ARs in the tissues of animals who ingest them, ARs bioaccumulate, and their detection in numerous wildlife species indicates that they are likely pervasive in the food chain.

red-tailed hawk - photo by Ondrej Prosicky
photo by Ondrej Prosicky

ARs concentrate and persist to the highest extent in the liver, making it the tissue of choice for AR analysis. Therefore, most monitoring studies use liver tissue from deceased animals. It would be advantageous, however, to use blood samples to test for exposure to ARs, as blood can be collected in the field from live animals. However, the sensitivity of blood for detection of ARs has not been well examined.

This study, supported by a Christine Stevens Wildlife Award and published in the journal Environmental Toxicology and Chemistry,1 addressed whether blood samples can be used to detect exposure to ARs in red-tailed hawks. Birds in the study were admitted to Tufts Wildlife Clinic and either died or were humanely euthanized due to AR poisoning or injuries. No birds were euthanized to serve the study. 

Blood and liver samples collected from each bird were analyzed to determine if birds positive for ARs in the liver would have detectable residues in their blood. Forty-three red-tailed hawks were included in the study. Fourteen of the birds died due to AR toxicosis; in these 14, ARs were present in both blood and liver. In the remaining 29 birds—who died from causes other than toxicosis—ARs were found in the liver but not in the blood. 

The findings indicate that analysis of blood is not a reliable way to monitor for exposure to ARs in red-tailed hawks who do not have signs of AR toxicosis. Therefore, blood sampling within a select population would underestimate exposure. These data can inform future studies and risk assessments on AR exposure in birds. In addition, given that 100 percent of the hawks sampled for this study were positive for ARs in liver tissue, this further demonstrates that exposure to ARs in this species remains pervasive despite regulations enacted by the Environmental Protection Agency within the last decade intended to reduce the risk of ARs to wildlife. 

1. Editor’s note: Environmental Toxicology and Chemistry included this study ( in its annual list of exceptional papers for 2020.

This study was funded by the Christine Stevens Wildlife Awards program. To learn more about this program or to view additional studies, click here.