Exposure of non-target small mammals to anticoagulant rodenticide during chemical rodent control operations

Rodent control strategies and Lassa virus: some unexpected effects in Guinea, West Africa

The Natal multimammate mouse (Mastomys natalensis) is the host of Lassa mammarenavirus, causing Lassa haemorrhagic fever in West Africa. As there is currently no operational vaccine and therapeutic drugs are limited, we explored rodent control as an alternative to prevent Lassa virus spillover in Upper Guinea, where the disease is highly endemic in rural areas. In a seven-year experiment, we distributed rodenticides for 10-30 days once a year and, in the last year, added intensive snap trapping for three months in all the houses of one village. We also captured rodents both before and after the intervention period to assess their effectiveness by examining alterations in trapping success and infection rates (Lassa virus RNA and IgG antibodies). We found that both interventions reduced the rodent population by 74-92% but swiftly rebounded to pre-treatment levels, even already six months after the last snap-trapping control. Furthermore, while we observed that chemical control modestly decreased Lassa virus infection rates annually (a reduction of 5% in seroprevalence per year), the intensive trapping unexpectedly led to a significantly higher infection rate (from a seroprevalence of 28% before to 67% after snap trapping control). After seven years, we conclude that annual chemical control, alone or with intensive trapping, is ineffective and sometimes counterproductive in preventing Lassa virus spillover in rural villages. These unexpected findings may result from density-dependent breeding compensation following culling and the survival of a small percentage of chronically infected rodents that may spread the virus to a new susceptible generation of mice.

First evidence of widespread positivity to anticoagulant rodenticides in grey wolves (Canis lupus)

Second-generation Anticoagulant Rodenticides (ARs) can be critical for carnivores, due to their widespread use and impacts. However, although many studies explored the impacts of ARs on small and mesocarnivores, none assessed the extent to which they could contaminate large carnivores in anthropized landscapes. We filled this gap by exploring spatiotemporal trends in grey wolf (Canis lupus) exposure to ARs in central and northern Italy, by subjecting a large sample of dead wolves (n = 186) to the LC-MS/MS method. Most wolves (n = 115/186, 61.8 %) tested positive for ARs (1 compound, n = 36; 2 compounds, n = 47; 3 compounds, n = 16; 4 or more compounds, n = 16). Bromadiolone, brodifacoum and difenacoum, were the most common compounds, with brodifacoum and bromadiolone being the ARs that co-occurred the most (n = 61). Both the probability of testing positive for multiple ARs and the concentration of brodifacoum, and bromadiolone in the liver, systematically increased in wolves that were found at more anthropized sites. Moreover, wolves became more likely to test positive for ARs through time, particularly after 2020. Our results underline that rodent control, based on ARs, increases the risks of unintentional poisoning of non-target wildlife. However, this risk does not only involve small and mesocarnivores, but also large carnivores at the top of the food chain, such as wolves. Therefore, rodent control is adding one further conservation threat to endangered large carnivores in anthropized landscapes of Europe, whose severity could increase over time and be far higher than previously thought. Large-scale monitoring schemes for ARs in European large carnivores should be devised as soon as possible.

Toxicology of chemical biocides: Anticoagulant rodenticides - Beyond hemostasis disturbance

The use of anticoagulant rodenticides (ARs) is one of the most commonly employed management methods for pest rodents. ARs compete with vitamin K (VK) required for the synthesis of blood clotting factors in the liver, resulting in inhibition of blood coagulation and often animal death due to hemorrhage. Besides rodents (target species), ARs may affect non-target animal species and humans. Out of hemostasis disturbance, the effects of ARs may be related to the inhibition of proteins that require VK for their synthesis but are not involved in the coagulation process, to their direct cytotoxicity, and their pro-oxidant/proinflammatory activity. A survey of the cellular and molecular mechanisms of these sublethal/asymptomatic AR effects is given in this review. Data from field, clinical, and experimental studies are presented. Knowledge of these mechanisms might improve hazard characterization and identification of potential ecotoxicological risks associated with ARs, contributing to a safer use of these chemicals.

Consumption of Rodenticide Baits by Invertebrates as a Potential Route into the Diet of Insectivores

Non-target species are commonly exposed to anticoagulant rodenticides worldwide, which may pose a key threat to declining species. However, the main pathway of exposure is usually unknown, potentially hindering conservation efforts. This study aimed to examine whether baits mixed with the biomarker rhodamine B can be used to track invertebrate consumption of rodenticides in a field environment, using this to observe whether invertebrate prey are a potential vector for anticoagulant rodenticides in the diet of insectivores such as the European hedgehog (Erinaceus europaeus). Rhodamine B baits were found to create an observable response. Uptake was negligible in captured insects; however, 20.7% of slugs and 18.4% of snails captured showed uptake of bait. Maximum temperature, distance from bait, proximity to buildings, and the addition of copper tape to bait boxes all influenced the rate of bait uptake in molluscs. Based on these data, it seems likely that molluscs could be a source of rodenticide poisoning in insectivores. This research demonstrates which prey may pose exposure risks to insectivores and likely environmental factors, knowledge of which can guide effective mitigation measures. We suggest that further investigation into using mollusc repellents around bait boxes should be considered.

Large-scale identification of rodenticide resistance in Rattus norvegicus and Mus musculus in the Netherlands based on Vkorc1 codon 139 mutations

BACKGROUND

Resistance to rodenticides has been reported globally and poses a considerable problem for efficacy in pest control. The most-documented resistance to rodenticides in commensal rodents is associated with mutations in the Vkorc1 gene, in particular in codon 139. Resistance to anticoagulant rodenticides has been reported in the Netherlands since 1989. A study from 2013 showed that 25% of 169 Norway rats (Rattus norvegicus) had a mutation at codon 139 of the Vkorc1 gene. To gain insight in the current status of rodenticide resistance amongst R. norvegicus and house mice Mus musculus in the Netherlands, we tested these rodents for mutations in codon 139 of the Vkorc1 gene. In addition, we collected data from pest controllers on their use of rodenticides and experience with rodenticide resistance.

RESULTS

A total of 1801 rodent samples were collected throughout the country consisting of 1404 R. norvegicus and 397 M. musculus. In total, 15% of R. norvegicus [95% confidence interval (CI): 13-17%] and 38% of M. musculus (95% CI: 33-43%) carried a genetic mutation at codon 139 of the Vkorc1 gene.

CONCLUSION

This study demonstrates genetic mutations at codon 139 of the Vkorc1 gene in M. musculus in the Netherlands. Resistance to anticoagulant rodenticides is present in R. norvegicus and M. musculus in multiple regions in the Netherlands. The results of this comprehensive study provide a baseline and facilitate trend analyses of Vkorc1 codon 139 mutations and evaluation of integrated pest management (IPM) strategies as these are enrolled in the Netherlands. © 2022 The Dutch Pest and Wildlife. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Anticoagulant rodenticide use in oil palm plantations in Southeast Asia and hazard assessment to non-target animals

Anticoagulant rodenticides (ARs) are used worldwide for the control of rodent pests and are the main method of control of rat pest populations in agricultural areas. The main aim of this review is to discuss the risk of ARs to non-target wildlife in oil palm areas in Southeast Asia, mainly Indonesia and Malaysia. We discussed AR use in oil palm areas and toxicities of ARs on target and non-target animals. We also reviewed published literature on wildlife species reported in oil palm areas in Southeast Asia and utilizing this information, we assessed the hazard risk of ARs to non-target wildlife in oil palm plantations. ARs are a secondary exposure hazard to rodent-consuming mammalian carnivores, such as leopard cats and civets, and rodent-consuming raptors, such as barn owls. Consumption of dead poisoned prey puts scavengers, such as water monitors, at high risk for AR exposure. Domestic livestock and granivorous birds are at high risk for AR exposure via primary exposure to toxic bait, while omnivores such as macaques and wild pigs are at moderate risk for both primary and secondary exposure to ARs. The effects of ARs on barn owls have been well studied in the field and in laboratory secondary toxicity studies. Thus, the nest-box occupancy and reproductive parameters of local barn owl populations can be monitored as an indicator of the AR exposure level in the area. CLINICAL TRIALS REGISTRATION: No clinical trials were involved in this study.

Prevalence of anticoagulant rodenticide exposure in red-tailed hawks (Buteo jamaicensis) and utility of clotting time assays to detect coagulopathy

Anticoagulant rodenticides (ARs) continue to be used across the United States as a method for controlling pest rodent species. As a consequence, wild birds of prey are exposed to these toxicants by eating poisoned prey items. ARs prevent the hepatic recycling of vitamin K and thereby impede the post-translational processing of coagulation factors II, VII, IX, and X that are required for procoagulant complex assembly. Through this mechanism of action, ARs cause hemorrhage and death in their target species. Various studies have documented the persistence of these contaminants in birds of prey but few have attempted to use affordable and accessible diagnostic tests to diagnose coagulopathy in free-ranging birds of prey. In our study free-ranging red-tailed hawks were found to be exposed to difethialone and brodifacoum. Eleven of sixteen (68%) livers tested for AR exposure had detectable residues. Difethialone was found in 1/16 (6%), and brodifacoum was detected in 10/16 (62%) liver samples that were tested for rodenticide residues. Difethialone was found at a concentration of 0.18 ug/g wet weight and brodifacoum concentrations ranged from 0.003-0.234 ug/g wet weight. Two out of 34 (6%) RTHA assessed for blood rodenticide had brodifacoum in serum with measured concentrations of 0.003 and 0.006 ug/g. The range of clotting times in the prothrombin time (PT) and Russell's viper venom time assays for control RTHA were 16.7 to 39.7 s and 11.5 to 91.8 s, respectively. One study bird was diagnosed with clinical AR intoxication with a brodifacoum levels in blood of 0.006 and 0.234 ug/g wet weight in blood and liver respectively, a packed cell volume (PCV) of 19%, and PT and RVVT times of >180 s. No correlation was found between PT and RVVT in the control or free-range RTHA, and there was no relationship found between the presence of liver anticoagulant residues and clotting times in the PT and RVVT.

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.

Use of rodenticide bait stations by commensal rodents at the urban-wildland interface: Insights for management to reduce nontarget exposure

BACKGROUND

Pest management professionals use anticoagulant rodenticides, usually placed in tamper-resistant bait stations, to control commensal rodents, but significant concerns remain about exposure of nontarget species, especially at the urban-wildland interface. We deployed digital cameras to monitor use of bait stations placed in 90 residential yards across Orange County, California, USA. Two bait stations, supplied with nontoxic bait, were monitored in each yard for approximately 30 consecutive days during two camera-trapping sessions between December 2017 and March 2019. One station was placed on the ground, while the other was elevated 1-1.5 m to determine if elevating stations could reduce nontarget exposure.

RESULTS

Black rats (Rattus rattus L.) were present at 80% of sites, with mean activity ranging from 0 to 9.6 h each night. There were no significant differences between elevated and ground stations in the time to discovery, time to bait station entry, or nightly activity of rats. Rats discovered bait stations more quickly, and mean nightly activity was greater, in yards where rats were detected more frequently. Although native rodents visited and entered bait stations occasionally, they were relatively rare among our sites (13.3%), and were detected five times less often at elevated stations compared to those on the ground. Yards visited by these rodents were significantly nearer to areas of green open space and natural vegetation, and tended to have no significant barriers to entry, e.g. solid fences or walls.

CONCLUSIONS

By elevating bait stations and avoiding placing rodenticides in yards that are likely to be visited by wildlife, pest management professionals may be able to reduce the risk of nontarget exposure, including secondary poisoning of predators and scavengers, while still providing effective control of commensal pests. © 2021 Society of Chemical Industry.

Impact of contraceptive hormones on the reproductive potential of male and female commensal black rats (Rattus rattus)

The black rat is considered one of the world's top pests. With increased restrictions on rodenticides, new alternatives to manage rats are urgently needed. Research on the use of contraceptive hormones, levonorgestrel (LE), and quinestrol (QU), have been evaluated against some rodent species, and this research is the first study to assess these on black rats. Hormones were incorporated into rodent bait at 10 and 50 ppm concentrations singly and in combination (EP-1). Groups of 10 animals of each sex were fed the baits over 7 days. Lower bait consumption was observed with slight body mass reductions. On dissection, it was observed that the uterus was in a state of edema and male reproductive organs weighed less with reduced sperm counts/motility. The 2 most promising baits, 50 ppm QU and EP-1, were used to assess impact on pregnancy and litter size. Pregnancy was reduced from 70% success when both males and females consumed untreated bait, down to 30% when males had consumed contraceptive bait but females had not, and down to 0% when females had consumed contraceptive bait, regardless of whether they had paired with a treated or untreated male. Litter size in the untreated pairs was 8 pups, but only 4 pups in those cases where the male only had consumed the contraceptive. Further studies should investigate how long the effect lasts and its reversibility. Field studies at the population level may also shed light on the practicality of using contraceptive baits for black rats in different habitats.

Exposure of passerine birds to brodifacoum during management of Norway rats on farms

The exposure of non-target wildlife to anticoagulant compounds used for rodent control is a well-known phenomenon. Exposure can be primary when non-target species consume bait or secondary via uptake of poisoned animals by mammalian and avian predators. However, nothing is known about the exposure patterns in passerine birds that are commonly present on farms where rodent control is conducted. We used liquid chromatography coupled with tandem mass spectrometry to screen for residues of anticoagulant rodenticides (ARs) in liver tissue of passerine birds that were present during rodent control with a product containing brodifacoum (BR). The 222 birds of 13 species were bycatch of rodent snap trapping in 2011-2013 on 11 livestock farms run synchronously with baiting. During baiting, ARs were detected in about 30% of birds; 28% carried BR. In liver tissue of 54 birds that carried BR, concentrations ranged from 4 to 7809 ng/g (mean 490 ± 169 ng/g). Among common bird species with AR residues, BR was most prevalent in robins (Erithacus rubecula) (44%) and dunnocks (Prunella modularis) (41%). Mean BR concentration was highest in great tits (Parus major) (902 ± 405 ng/g). The occurrence and concentrations of BR residues were about 30% higher in birds collected close to bait stations compared to birds collected further away. The results demonstrate that several ground feeding songbird species are exposed to ARs used on farms. If BR was present in liver tissue, concentrations were variable, which may imply a combination of primary and secondary exposure of songbirds. Exposure was mostly restricted to the immediate surroundings of farms where bait was used, which might limit the transfer to the wider environment. Efforts should be made to reduce the access for birds to AR bait to prevent high exposure.

Efficacy of a combined insecticide-rodenticide product on ectoparasite and commensal rodent mortality

BACKGROUND

Ectoparasites may transfer zoonotic pathogens from rodents to humans or livestock when rodents are managed with rodenticides. This could be minimized using a product combining a rodenticide with a delayed action and a systemic insecticide/acaricide that rapidly kills ectoparasites. Such a combination was tested in commensal pest rodent species to assess efficacy and timing of responses in rodents, and fleas and ticks feeding on them. Ticks or fleas attached to rats (Rattus norvegicus) and house mice (Mus musculus domesticus) were exposed to a product containing brodifacoum (50 ppm) and fipronil (40 ppm) for three days.

RESULTS

98-100% of fleas on treated rodents died within one to two days after first exposure, whereas >90% fleas survived on control rodents. The effect persisted for four or more days after bait uptake. Ticks started to succumb to the effect of the combination product within one day (mice) and within four days (rats) of first exposure, with all ticks dying by Day (D)8. Tick survival in control rodents was 90-100%. Rodent mortality began at D3 (rats) and D4 (mice) after first consumption of product and all were dead by D9 (rats) and D7 (mice).

CONCLUSION

This product effectively killed ectoparasites and rodents. Flea mortality was swift and complete, generally within one day of exposure, whereas it took ticks up to four days to die, but before the rats and house mice died. The combination product might help to prevent ectoparasites migrating from dying rodents to another host. Field trials are warranted. © 2020 Society of Chemical Industry.

Baiting location affects anticoagulant rodenticide exposure of non-target small mammals on farms

BACKGROUND

Commensal rodents such as Norway rats (Rattus norvegicus Berk.), black rats (R. rattus L.) and house mice (Mus musculus L.) damage stored produce and infrastructure, cause hygienic problems and transmit zoonotic pathogens to humans. The management of commensal rodents relies mainly on the use of anticoagulant rodenticides (ARs). ARs are persistent and bio-accumulative, which can cause exposure of non-target species. We compared the baiting strategies to use brodifacoum (BR) in bait boxes indoors only versus in and around buildings in replicated field trials at livestock farms to assess resulting BR residues in non-target small mammals.

RESULTS

When bait was used indoors only, the percentage of trapped non-target small mammals with BR residues as well as BR concentration in liver tissue was about 50% lower in comparison to bait application in and around buildings. These effects occurred in murid rodents and shrews but not in voles that were generally only mildly exposed. During the baiting period, BR concentration in murids was stable but decreased by about 50% in shrews.

CONCLUSION

Restricting the application of BR bait to indoors only can reduce exposure of non-target species. The positive effect of this baiting strategy on non-target species needs to be balanced with the need for an effective pest rodent management within a reasonable time. More research is needed to clarify which management approaches strike this balance best.

Biomarkers Potency to Monitor Non-target Fauna Poisoning by Anticoagulant Rodenticides

The widespread use of pesticides to control agricultural pests is a hot topic on the public scene of environmental health. Selective pest control for minimum environmental impact is a major goal of the environmental toxicology field, notably to avoid unintended poisoning in different organisms. Anticoagulant rodenticides cause abnormal blood coagulation process; they have been widely used to control rodents, allowing inadvertent primary and secondary exposure in domestic animals and non-target predatory wildlife species through direct ingestion of rodenticide-containing bait or by consumption of poisoned prey. To report toxic effect, the most common approach is the measurement of liver or plasma residues of anticoagulant rodenticides in dead or intoxicated animals showing clinical symptoms. However, one major challenge is that literature currently lacks a hepatic or plasma concentration threshold value for the differentiation of exposure from toxicity. Regarding the variation in pharmacology properties of anticoagulant rodenticides inter- and intra-species, the dose-response relationship must be defined for each species to prejudge the relative risk of poisoning. Beyond that, biomarkers are a key solution widely used for ecological risk assessment of contaminants. Since anticoagulant rodenticides (AR) have toxic effects at the biochemical level, biomarkers can serve as indicators of toxic exposure. In this sense, toxicological knowledge of anticoagulant rodenticides within organisms is an important tool for defining sensitive, specific, and suitable biomarkers. In this review, we provide an overview of the toxicodynamic and toxicokinetic parameters of anticoagulant rodenticides in different animal species. We examine different types of biomarkers used to characterize and differentiate the exposure and toxic effects of anticoagulant rodenticide, showing the strengths and weaknesses of the assays. Finally, we describe possible new biomarkers and highlight their capabilities.

Efficacy of Management and Monitoring Methods to Prevent Post-Harvest Losses Caused by Rodents

The presence of pest rodents around food production and storage sites is one of many underlying problems contributing to food contamination and loss, particularly influencing food and nutrition security in low-income countries. By reducing both pre- and post-harvest losses by rodents, millions of food-insecure people would benefit. As there are limited quantitative data on post-harvest rice losses due to rodents, our objectives were to assess stored rice losses in local households from eight rural communities and two rice milling factories in Bangladesh and to monitor the effect of different rodent control strategies to limit potential losses. Four treatments were applied in 2016 and 2017, (i) untreated control, (ii) use of domestic cats, (iii) use of rodenticides, (iv) use of snap-traps. In total, over a two-year period, 210 rodents were captured from inside people's homes, with Rattus rattus trapped most often (n = 91), followed by Mus musculus (n = 75) and Bandicota bengalensis (n = 26). In the milling stations, 68 rodents were trapped, of which 21 were M. musculus, 19 R. rattus, 17 B. bengalensis, 8 Rattus exulans, and 3 Mus terricolor. In 2016, losses from standardised baskets of rice within households were between 13.6% and 16.7%. In 2017, the losses were lower, ranging from 0.6% to 2.2%. Daily rodent removal by trapping proved to be most effective to diminish stored produce loss. The effectiveness of domestic cats was limited.