Effects of anticoagulant rodenticide poisoning on spatial behavior of farm dwelling Norway rats
Commensal rodent species cause damage to crops and stored products, they transmit pathogens to people, livestock and pets and threaten native flora and fauna. To minimize such adverse effects, commensal rodents are predominantly managed with anticoagulant rodenticides (AR) that can be transferred along the food chain. We tested the effect of the uptake of the AR brodifacoum (BR) by Norway rats (Rattus norvegicus) on spatial behavior because this helps to assess the availability of dead rats and residual BR to predators and scavengers. BR was delivered by oral gavage or free-fed bait presented in bait stations. Rats were radio-collared to monitor spatial behavior. BR residues in rat liver tissue were analyzed using liquid chromatography coupled with tandem mass spectrometry. Norway rats that had consumed BR decreased distances moved and had reduced home range size. Treatment effects on spatial behavior seemed to set in rapidly. However, there was no effect on habitat preference. Ninety-two percent of rats that succumbed to BR died in well-hidden locations, where removal by scavenging birds and large mammalian scavengers is unlikely. Rats that ingested bait from bait stations had 65% higher residue concentrations than rats that died from dosing with two-fold LD50. This suggests an overdosing in rats that are managed with 0.0025% BR. None of the 70 BR-loaded rats was caught/removed by wild predators/scavengers before collection of carcasses within 5–29 h. Therefore, and because almost all dead rats died in well-hidden locations, they do not seem to pose a significant risk of AR exposure to large predators/scavengers at livestock farms. Exposure of large predators may originate from AR-poisoned non-target small mammals. The few rats that died in the open are accessible and should be removed in routine searches during and after the application of AR bait to minimize transfer of AR into the wider environment.