Assessing anticoagulant resistance in rats and coagulation effects in birds using small-volume blood samples

The second-generation anticoagulant brodifacoum is currently used for possum and rodent control on the New Zealand mainland, and for rodent eradication on offshore islands. To investigate whether these uses have produced undesirable effects in localised wild rat populations (anticoagulant resistance) or in birds (effects on blood coagulation times), blood clotting response tests utilising small-volume sampling were established using albino laboratory rats (Rattus norvegicus, Wistar) and domestic chickens (Gallus domesticus). These tests were then used on blood samples taken from wild rats (R. norvegicus) and kiwi (Apteryx australis mantelli). During the development of the blood-clotting test in laboratory rats we observed a reduced anticoagulant response after a second dose of 0.1 mg/kg of brodifacoum, compared with the initial exposure. There was no evidence of anticoagulant resistance in rats from areas where there was a history of 2.5 years' use of brodifacoum. Statistical differences were found in the clotting times of kiwi from areas with a history of anticoagulant use and those from areas where brodifacoum has not been used, but the clotting times were the opposite of what would be expected if they were interpreted on the basis of exposure history alone. These results highlight uncertainties regarding resistance, sub-lethal effects, threshold concentrations in the liver, and the toxicodynamics of anticoagulants in mammals and birds.