Development of Resistance to Anticoagulant Rodenticides in Rodents

Unveiling the potency of ready-to-use brodifacoum blocks in lesser bandicoot rats: exploring toxicity, blood clotting factors, and organ histology

Background

The present study unveiled the effectiveness of ready-to-use brodifacoum blocks (0.005%) against the prevalent field rat species in southeast Asia, Bandicota bengalensis. Brodifacoum, a more potent second-generation anticoagulant, offers a solution for managing rodents resistant to other anticoagulants of its class.

Methodology

Male and female bandicoot rats caught wild were exposed to brodifacoum for 1, 2, and 3 days in both the no-choice and bi-choice tests. The observations included mortality rates, impact on body weight, food consumption, blood clotting factors, organ weights, and histological changes.

Results

Results indicated 100% mortality within 2-3 days in the no-choice tests, and 50.00%-83.33% mortality in the bi-choice tests within 5 to 8 days. The median lethal feeding periods were determined to be 2.10 and 2.33 days for male and female rats, respectively. Toxicity symptoms included bleeding from the nose, gums, and feet. While no significant effects were observed on body weight or organ weights, food consumption decreased notably in no-choice tests. Additionally, significant increases in prothrombin time and activated partial thromboplastin time were noted 24 h post-treatment in the no-choice tests, with post-treatment international normalized ratios of 9.45-14.20 and 1.52-3.03 in the no-choice and the bi-choice tests, respectively. Histological analysis revealed mild to severe necrotic changes in the liver and kidneys after brodifacoum treatment.

Conclusions

Overall, this study underscores the potential of ready-to-use brodifacoum blocks as an effective tool for rodent population control, offering a viable alternative to other second-generation anticoagulant rodenticides.

Impacts of climate change and human activities on three Glires pests of the Qinghai-Tibet Plateau

BACKGROUND

The range of Glires is influenced by human activities and climate change. However, the extent to which human activities and environmental changes have contributed to this relationship remains unclear. We examined alterations in the distribution changes and driving factors of the Himalayan marmot, plateau pika, and plateau zokor on the Qinghai-Tibet Plateau (QTP) using the maximum entropy (MaxEnt) model and a geographical detector (Geodetector).

RESULTS

The MaxEnt model showed that the contribution rates of the human footprint index (HFI) to the distribution patterns of the three types of Glires were 46.70%, 58.70%, and 59.50%, respectively. The Geodetector results showed that the distribution pattern of the Himalayan marmot on the QTP was influenced by altitude and the normalized difference vegetation index (NDVI). The distribution patterns for plateau pikas and plateau zokors were driven by HFI and NDVI. Climate has played a substantial role in shaping suitable habitats for these three Glires on the QTP. Their suitable area is expected to decrease over the next 30-50 years, along with their niche breadth and overlap. Future suitable habitats for the three Glires tended to shift toward higher latitudes on the QTP.

CONCLUSION

These findings underscore the impacts of environmental and human factors on the distribution of the three Glires on the QTP. They have enhanced our understanding of the intricate relationships between Glires niches and environments. This can aid in identifying necessary interventions for developing effective early warning systems and prevention strategies to mitigate Glires infestations and plague epidemics on the QTP. © 2024 Society of Chemical Industry.

Anticoagulant Rodenticide Toxicity in Terrestrial Raptors: Tools to Estimate the Impact on Populations in North America and Globally

Anticoagulant rodenticides (ARs) have caused widespread contamination and poisoning of predators and scavengers. The diagnosis of toxicity proceeds from evidence of hemorrhage, and subsequent detection of residues in liver. Many factors confound the assessment of AR poisoning, particularly exposure dose, timing and frequency of exposure, and individual and taxon-specific variables. There is a need, therefore, for better AR toxicity criteria. To respond, we compiled a database of second-generation anticoagulant rodenticide (SGAR) residues in liver and postmortem evaluations of 951 terrestrial raptor carcasses from Canada and the United States, 1989 to 2021. We developed mixed-effects logistic regression models to produce specific probability curves of the toxicity of ∑SGARs at the taxonomic level of the family, and separately for three SGARs registered in North America, brodifacoum, bromadiolone, and difethialone. The ∑SGAR threshold concentrations for diagnosis of coagulopathy at 0.20 probability of risk were highest for strigid owls (15 ng g-1 ) lower and relatively similar for accipitrid hawks and eagles (8.2 ng g-1 ) and falcons (7.9 ng g-1 ), and much lower for tytonid barn owls (0.32 ng g-1 ). These values are lower than those we found previously, due to compilation and use of a larger database with a mix of species and source locations, and also to refinements in the statistical methods. Our presentation of results on the family taxonomic level should aid in the global applicability of the numbers. We also collated a subset of 440 single-compound exposure events and determined the probability of SGAR-poisoning symptoms as a function of SGAR concentration, which we then used to estimate relative SGAR toxicity and toxic equivalence factors: difethialone, 1, brodifacoum, 0.8, and bromadiolone, 0.5. Environ Toxicol Chem 2024;00:1-11. © 2024 His Majesty the King in Right of Canada and The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC Reproduced with the permission of the Minister of Environment and Climate Change Canada.

Controlling Effects of Nanocomposite Sterilant ND-1 on the Growth of Wild Populations of Midday Gerbil (Meriones meridianus)

Grassland is not only an important part of the terrestrial ecosystem with multiple ecological functions, but also an important base for Chinese herdsmen to produce and live. However, the occurrence and spread of rodent infestation reduces the biodiversity and productivity of grassland ecosystems. It also severely threatens human life, health, and biosecurity through disease transmission. In this study, we explored the ability of the nanocomposite sterilant ND-1 to control grassland rodent populations. Semi-closed experimental and control plots were established in the desert area of Alashan, Inner Mongolia, China. In spring 2018, the nanocomposite sterile ND-1 (Nongda-1) was introduced once, and the control effect of ND-1 on the growth of the wild population of midday gerbils was measured for two years. We show that ND-1 significantly reduced the population of midday gerbils in the experimental area, with a negative population growth rate. In addition, in the second year, the ratio of female midday gerbils to sub-adults in the experimental area was significantly lower than that in the control area, which resulted in a significant difference in the sex ratio of midday gerbils. There were significantly fewer females than males, and the population growth of midday gerbils in the experimental area was significantly inhibited. ND-1 had no significant effect on the home range of midday gerbils, and sterile individuals continued to occupy the home range and consume resources. Therefore, ND-1 reduced the number of female midday gerbils during the breeding period and the sex ratio and population density and altered the age structure of the wild population. Additionally, competition between sterile and normal individuals had a significant control effect on the growth of wild populations. Our studies demonstrate the significance of ND-1 in the sustainable control of grassland rodent pests, with the potential for limiting grassland rodent damage in the future.

Investigation of anticoagulant rodenticide resistance induced by Vkorc1 mutations in rodents in Lebanon

Anticoagulant rodenticides (AR) remain the most effective chemical substances used to control rodents in order to limit their agricultural and public health damage in both rural and urban environments. The emergence of genetically based resistance to AR worldwide has threatened effective rodent control. This study gives a first overview of the distribution and frequency of single nucleotide polymorphism in the vitamin K epoxide reductase subcomponent 1 (Vkorc1) gene in rodents in Lebanon. In the Mus genus, we detected two missense mutations Leu128Ser and Tyr139Cys, that confer resistance to anticoagulant rodenticides in house mice and a new missense mutation Ala72Val in the Mus macedonicus species, not previously described. In the Rattus genus, we found one missense mutation Leu90Ile in the roof rat and one missense mutation Ser149Ile in the Norway rat. This is the first study to demonstrate potential resistance to AR in Lebanese rodents and therefore it provides data to pest control practitioners to choose the most suitable AR to control rodents in order to keep their efficacy.

The screening for anticoagulant rodenticide gene VKORC1 polymorphism in the rat Rattus norvegicus, Rattus tanezumi and Rattus losea in Hong Kong

Anticoagulants are a major component of rodenticides used worldwide, which function by effectively blocking the vitamin K cycle in rodents. The rat Vitamin K epoxide Reductase Complex (VKORC) subunit 1 is the enzyme responsible for recycling vitamin K, and five substitution mutations (Tyr139Cys, Tyr139Ser, Tyr139Phe and Leu128Gln and Leu120Gln) located in the VKORC1 could result in resistance to anticoagulant rodenticides. This study carried out a VKORC1-based survey to estimate the anticoagulant rodenticide resistance in three Rattus species (R. losea, R. norvegicus, and R. tanezumi) collected in Hong Kong. A total of 202 rats captured in Hong Kong between 2017 and 2021 were analysed. Sequencing of molecular marker cytochrome c oxidase subunit 1 (COX1) was carried out to assist the species identification, and the identities of 52 lesser ricefield rats (R. losea), 81 common rats (R. norvegicus) and 69 house rats (R. tanezumi) were confirmed. Three VKORC1 exons were amplified from individuals by PCR followed by Sanger sequencing. A total of 47 R. tanezumi (68.1%) contained Tyr139Cys mutation in VKORC1 gene, and half of them were homozygous. None of the collected R. losea and R. norvegicus were detected with the five known substitutions leading to anticoagulant rodenticides resistance, and previously undescribed missense mutations were revealed in each species. Whole genome sequencing was further carried out on some individuals, and single nucleotide polymorphisms (SNPs) were also identified in the introns. This is the first study investigating the situation of anticoagulant rodenticide resistance in the rats collected in Hong Kong. Given that the efficacy of rodenticides is crucial for effective rodent management, regular genetic testing as well as population genomic analyses will be required to both monitor the situation and understand the adaption of different rat haplotypes for integrated pest management. Susceptibility tests for individual rodenticides should also be conducted regularly to assess their effectiveness on local species.

Anticoagulant Rodenticide Contamination of Terrestrial Birds of Prey from Western Canada: Patterns and Trends, 1988-2018

As the dominant means for control of pest rodent populations globally, anticoagulant rodenticides (ARs), particularly the second-generation compounds (SGARs), have widely contaminated nontarget organisms. We present data on hepatic residues of ARs in 741 raptorial birds found dead or brought into rehabilitation centers in British Columbia, Canada, over a 30-year period from 1988 to 2018. Exposure varied by species, by proximity to residential areas, and over time, with at least one SGAR residue detected in 74% of individuals and multiple residues in 50% of individuals. By comparison, we detected first-generation compounds in <5% of the raptors. Highest rates of exposure were in barred owls (Strix varia), 96%, and great horned owls (Bubo virginianus), 81%, species with diverse diets, including rats (Rattus norvegicus and Rattus rattus), and inhabiting suburban and intensive agricultural habitats. Barn owls (Tyto alba), mainly a vole (Microtus) eater, had a lower incidence of exposure of 65%. Putatively, bird-eating raptors also had a relatively high incidence of exposure, with 75% of Cooper's hawks (Accipiter cooperii) and 60% of sharp-shinned hawks (Accipiter striatus) exposed. Concentrations of SGARs varied greatly, for example, in barred owls, the geometric mean ∑SGAR = 0.13, ranging from <0.005 to 1.81 μg/g wet weight (n = 208). Barred owls had significantly higher ∑SGAR concentrations than all other species, driven by significantly higher bromadiolone concentrations, which was predicted by the proportion of residential land within their home ranges. Preliminary indications that risk mitigation measures implemented in 2013 are having an influence on exposure include a decrease in mean concentrations of brodifacoum and difethialone in barred and great horned owls and an increase in bromodialone around that inflection point. Environ Toxicol Chem 2022;41:1903-1917. © 2022 Her Majesty the Queen in Right of Canada. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC. Reproduced with the permission of the Minister of Environment and Climate Change Canada.

Unexpected involvement of a second rodent species makes impacts of introduced rats more difficult to detect

Rodent predators are implicated in declines of seabird populations, and removing introduced rats, often, but not always, results in the expected conservation gains. Here we investigated the relationship between small mammal (Norway rat, wood mouse and pygmy shrew) abundance and Manx shearwater breeding success on the island of Rum, Scotland, and tested whether localised rodenticide treatments (to control introduced Norway rats) increased Manx shearwater breeding success. We found that Manx shearwater breeding success was negatively correlated with late summer indices of abundance for rats and mice, but not shrews. On its own, rat activity was a poor predictor of Manx shearwater breeding success. Rat activity increased during the shearwater breeding season in untreated areas but was supressed in areas treated with rodenticides. Levels of mouse (and shrew) activity increased in areas treated with rodenticides (likely in response to lower levels of rat activity) and Manx shearwater breeding success was unchanged in treated areas (p < 0.1). The results suggest that, unexpectedly, negative effects from wood mice can substitute those of Norway rats and that both species contributed to negative impacts on Manx shearwaters. Impacts were intermittent however, and further research is needed to characterise rodent population trends and assess the long-term risks to this seabird colony. The results have implications for conservation practitioners planning rat control programmes on islands where multiple rodent species are present.

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.

Challenges in the interpretation of anticoagulant rodenticide residues and toxicity in predatory and scavenging birds

Anticoagulant rodenticides (ARs) are part of the near billion-dollar rodenticide industry. Numerous studies have documented the presence of ARs in nontarget wildlife, with evidence of repeated exposure to second-generation ARs. While birds are generally less sensitive to ARs than target rodent species, in some locations predatory and scavenging birds are exposed by consumption of such poisoned prey and, depending on dose and frequency of exposure, exhibit effects of intoxication that can result in death. Evidence of hemorrhage in conjunction with summed hepatic AR residues >0.1-0.2 mg kg^-1 liver wet weight are often used as criteria to diagnose ARs as the likely cause of death. In this review focusing on birds of prey and scavengers, we discuss AR potency, coagulopathy, toxicokinetics and long-lasting effects of residues, and the role of nutrition and vitamin K status on toxicity, and identify some research needs. A more complete understanding of the factors affecting AR toxicity in nontarget wildlife could enable regulators and natural resource managers to better predict and even mitigate risk. Published 2020. This article is a U.S. Government work and is in the public domain in the USA.

Evaluating a Rapid Field Assessment System for Anticoagulant Rodenticide Exposure of Raptors

Anticoagulant rodenticides (ARs) are commonly used to control rodent pests. However, worldwide, their use is associated with secondary and tertiary poisoning of nontarget species, especially predatory and scavenging birds. No medical device can rapidly test for AR exposure of avian wildlife. Prothrombin time (PT) is a useful biomarker for AR exposure, and multiple commercially available point-of-care (POC) devices measure PT of humans, and domestic and companion mammals. We evaluated the potential of one commercially available POC device, the Coag-Sense^® PT/INR Monitoring System, to rapidly detect AR exposure of living birds of prey. The Coag-Sense device delivered repeatable PT measurements on avian blood samples collected from four species of raptors trapped during migration (Intraclass Correlation Coefficient > 0.9; overall intra-sample variation CV: 5.7%). However, PT measurements reported by the Coag-Sense system from 81 ferruginous hawk (Buteo regalis) nestlings were not correlated to those measured by a one-stage laboratory avian PT assay (r = - 0.017, p = 0.88). Although precise, the lack of agreement in PT estimates from the Coag-Sense device and the laboratory assay indicates that this device is not suitable for detecting potential AR exposure of birds of prey. The lack of suitability may be related to the use of a mammalian reagent in the clotting reaction, suggesting that the device may perform better in testing mammalian wildlife.