Prevalence of anticoagulant rodenticides in dogs and red foxes

Anticoagulant rodenticides (AR) are used worldwide in urban and agricultural rodent pest control. Ingestion of ARs is a major cause of poisoning in dogs and secondary exposure (ingestion of poisoned prey) in wildlife is a global problem. Despite this, few studies have examined the excretion of ARs in dogs, and elimination time is largely unknown in this species. In addition, no studies of AR prevalence in mammals have been conducted in Norway. Furthermore, no previous studies have determined the prevalence of ARs in faeces from presumed healthy animals across a country. Previous estimations of AR exposure in wildlife may be overestimated, as prevalence is assessed by analyses of livers from opportunistically sampled dead animals, not including possibly unexposed healthy living animals. Increased knowledge of exposure in living wildlife and the healthy dog population is an important part of the assessment of the impact of ARs. Furthermore, a better understanding of the elimination of ARs in dogs could generate ideas for new treatment options and optimize individual therapy in poisoned dogs. This study was initiated to determine the occurrence of ARs in red foxes in Norway by reversed phase ultra-high performance liquid chromatography–tandem mass spectrometry (UPLC-MS/MS) analyses of faeces. Faecal samples from 163 presumed healthy wild red foxes from most counties in Norway were collected and analysed for six different ARs, and residues were detected in 54% of the animals. Brodifacoum was most frequently detected, followed by coumatetralyl, bromadiolone, difenacoum, difethialone and flocoumafen. More than one substance was detected in 40% of the positive foxes with several exposed to up to four different ARs. There were no statistically significant seasonal, age or sex differences in foxes exposed to ARs. These results indicate a high unintended exposure from ARs in healthy wild foxes throughout Norway. This study also compared AR levels between faeces and liver from 40 of the same wild red foxes to determine the value of assessing AR exposure by faecal analysis. Residues of ARs were detected in 53% of the faecal samples and 83% of the liver samples. We found good concordance between AR residues in faeces and liver for coumatetralyl, difenacoum, and difethialone. Bromadiolone occurred in significantly greater frequency in livers compared to faecal samples, but no significant difference in concentration between faeces and liver could be detected. However, brodifacoum displayed a significant difference in concentration and occurrence of positive samples between liver and faeces. The AR concentrations were analysed in accidentally exposed dogs, displaying biphasic elimination of ARs in faeces. Long terminal half-lives in faeces of 81, 190, 200-330 days were detected for coumatetralyl, difenacoum and brodifacoum, respectively. Comparatively shorter terminal half-life of 30 days was detected for bromadiolone. One of the poisoned dogs gave birth to four healthy puppies several months after exposure, and low concentrations of brodifacoum were detectable in the puppies’ faeces for at least one month after parturition. When analysing blood and faeces from 110 healthy domestic dogs, we detected low prevalence of ARs in the healthy dog population. This suggests low exposure of ARs in healthy dogs in Norway. Together, the present work provides important findings of AR prevalence in the red fox in Norway and the healthy dog population previously not described. Our research of prevalence and elimination in both blood and faeces contribute to broader knowledge of the long excretion of ARs in Canidae. This will improve the background for decision making regarding use of these rodenticides in Norway and internationally.