Table of Contents

  • Anticoagulant rodenticides are the principal means of controlling pest rodents in the Nordic countries. Due to the intrinsic properties of second generation anticoagulants, i.e. extremely slow elimination from the body and high toxicity, they are prone to accumulate in the non-target species which consume poisoned rodents. Despite wide use there are no published studies on occurrence of residues of anticoagulant rodenticides in the non-target animals in the Nordic countries. This review of publicly available studies was aimed to find out which anticoagulant substances are found and in which species. The concentrations are reported as well as the proportion of exposed animals. We have further compiled a list of species that could potentially be exposed to anticoagulant rodenticides in the Nordic countries. The review shows that anticoagulant residues have been found everywhere they have been measured and secondary exposure to second generation anticoagulants is common among certain avian and mammalian predators. The results call for initiation of measurements of anticoagulant rodenticides also in the Nordic countries.

  • Residues of second generation anticoagulant rodenticides in wildlife have been found all around the world where the occurrence of residues has been investigated. Second generation anticoagulants, brodifacoum, bromadiolone, difenacoum, difethialone and flocoumafen, have been gradually introduced after the appearance of resistance to warfarin and other first generation substances. They are much more toxic and in particular more persistent in rodents and hence potent in accumulating in non-target species that feed on target and non-target rodents. Despite wide use of anticoagulants, there are relatively few studies where anticoagulant residues have been measured in non-target animals. More systematic incident studies have been done only in Britain and in some states of the US. In addition to Britain, published data on anticoagulant residues is available only in France in Europe. In Britain anticoagulant residues have been studied in particular in barn owl (Tyto alba) and in polecats (Mustela putorius). About one third of studied carcasses contained residues. In most cases, the residues were expected to be sublethal. No studies on the effects of the sublethal residues have been found. In the UK incident program other commonly exposed species were buzzard (Buteo buteo), red kite (Milvus milvus), and fox (Vulpes vulpes). In the US the commonly exposed species were great horned owls (Bubo virginianus), red-tailed hawks (Buteo jamaicensis), coyotes, foxes and raccoons. Anticoagulants are the dominant rodenticides in the Nordic countries, but no information is available on residues in non-target species. Information on residues is needed in order to judge whether the currently used risk mitigation measures are effective and whether further measures or restrictions should be considered.

  • Use of anticoagulant rodentcides is the dominating way to control undesired rodent species. The effectiveness of anticoagulants is due to the delayed mode of action which prevents rodents to connect the poisoning symptoms to the bait they have fed a few days ago. This is important in the control of rats which are cautious and avoid food which makes them ill. Anticoagulant rodenticides were introduced for the control of harmful rodents, but unfortunately non-target species are affected too, either directly through consumption of poisoned baits or indirectly through consumption of contaminated prey animals (secondary poisoning) (Lambert et al. 2007). The indirect poisoning, i.e. secondary poisoning threatens birds and mammals that feed on living or dead rodents. Secondary poisoning is most commonly associated with the second generation anticoagulants (Berny et al. 1997; Shore et al. 2003; Stone et al. 2003; Fournier-Chambrillon et al. 2004).

  • First generation anticoagulants were introduced as rodenticides in the late 1940's. The appearance of resistance to warfarin and other first generation substances led to the development of more potent, second generation anticoagulants (IPCS Environmental Health Criteria 175, Anticoagulant Rodenticides). The first generation anticoagulants, e.g. warfarin, sodium warfarin, chlorophacinone and coumatetralyl, are effective first after several repeated ingestions. The newer, second generation anticoagulants bromadiolone, brodifacoum, difenacoum, difethialone and flocoumafen may cause an effect already after a single ingestion and are toxic at a much lower dose than the first generation anticoagulants (Berny 2007).

  • Secondary poisoning of mustelids and raptors by anticoagulant residues in Britain has been demonstrated by e.g. Barnett et al. 2003, 2004; Newton et al. 1990; Shore et al. 1996, 2003; McDonald et al. 1998 and Walker et al. 2008a. Anticoagulant residues have been found in e.g. barn owl, buzzard, red kite, crow, fox, kestrel and polecat. More than 30% of studied barn owls and polecats contained difenacoum residues (Newton et al. 1997; Shore 2003). In general, the residue levels are low and are not considered to be the primary cause of death. We have not found studies where effects of sublethal residue levels have been examined. However, in some incidents the animals have been determined to having died of haemorrhages.

  • The existence of substantial incident data along with liver-residue analyses confirms that birds and non-target mammals are being exposed to anticoagulant rodenticides all around the world. The fact that numerous species of birds and mammals, including predators and scavengers have been exposed to these substances indicates that both primary and secondary exposure is occurring. Exposure of non-target animals is likely to be more widespread than the number of reported incidents suggests. Most surveys have been based on the activity of the general public who has sent carcasses to analysis when they have suspected poisoning. In many situations carcasses might not be detected, death may be attributed to natural mortality, or an incident may not be reported for a variety of reasons. In conclusion, studies made on secondary exposure reveal only the top of the iceberg. Residues found in predators and scavengers show that predominantly the second generation substances, i.e. brodifacoum, bromadiolone, difenacoum, flocoumafen and difethialone, cause secondary exposure. There were not many incidents where first generation anticoagulants were involved. The frequency of incidents is assumed to correlate to the use volumes of the substances. Unfortunately we have not found published data on sale volumes that could be compared to the number of incidents in the UK and US where most incident data are available. The statistics on sale volumes or use frequencies would enable the comparison of the likelihood or potency of the substances in causing the secondary poisoning. Due to lack of resources and/or ignorance of the potential risk measurements have been made only in few countries, and the incidents are likely to be much more widespread than reported here.