Exposure of non-target white-footed mice (Peromyscus leucopus) to Second-generation Anticoagulant Rodenticides in an urban context

A study in urban Ontario found that professional second-generation anticoagulant rodenticide baiting causes sublethal spillover exposure in non-target white-footed mice, suggesting these rodents may act as vectors for predators and highlighting the need to avoid such treatments near naturalized landscapes.

The Gist

The Invisible Poison Trail: A Story of Mice, Traps, and Predators

Imagine a city like Toronto or Vaughan as a giant, bustling neighborhood. In the basements and back alleys of commercial buildings, there are unwanted guests: rats and mice that humans call "commensals" (the ones who live with us). To get rid of them, pest control professionals use a very potent, invisible weapon: anticoagulant rodenticides. Think of these as "slow-acting poison bait." They don't kill the rat immediately; the rat eats the bait, feels fine for a few days, and then slowly succumbs.

But here's the problem: nature doesn't just stop at the building's back door.

The "Spillover" Effect

This study is like a detective story investigating what happens when that poison leaks out of the city buildings and into the nearby "wild" patches of forest (urban parks).

The researchers were worried about non-target mice—specifically the white-footed mouse. These are the "good guys" of the forest. They aren't rats living in your basement; they are wild creatures that live in the trees and grass. The fear was that these wild mice might accidentally eat the poison meant for the rats, or eat a poisoned rat, and then become a walking, talking (or rather, scurrying) package of poison for the animals that eat them, like hawks, owls, and foxes.

The Analogy: Imagine a farmer spraying a field to kill weeds. If the wind blows the spray onto the neighbor's garden, the neighbor's flowers get sick. In this case, the "spray" is poison bait, the "weeds" are rats, and the "flowers" are the wild mice.

The Investigation

The researchers set up a trap line in seven different urban parks, right next to buildings where pest control was actively using these poisons. They set up "speed traps" (snap traps) at various distances: right next to the building, 20 meters away, 40 meters away, all the way out to 100 meters.

They caught 111 adult white-footed mice and tested their livers to see if they had any poison in them.

The Findings: A Quiet Danger

The results were a mix of relief and concern:

1. The "Spillover" is Real, but Small: About 10% of the mice (11 out of 111) had poison in their systems. It wasn't a massive outbreak, but it proved that the poison is escaping the buildings and reaching the wild mice.
2. The Distance Doesn't Matter Much: You might think a mouse caught 100 meters away is safe, and one caught right next to the bait station is doomed. But the study found that poisoned mice were caught at every single distance, from 0 meters (right next to the trap) to 100 meters away. The poison didn't care about the distance; the mice just wandered into it.
3. No "Super-Mice" or "Super-Females": The researchers wondered if bigger, older, or male mice were more likely to get poisoned because they roam further. The answer? No. A small mouse was just as likely to get poisoned as a big one. A female was just as likely as a male. The poison was a random lottery.
4. The Specific Poisons: The main poison found was bromadiolone (a common second-generation poison). Interestingly, they didn't find brodifacoum, which is a very strong poison often found in birds of prey in the same province. This suggests that while the poison is getting to the mice, it might not be the same type of poison that is killing the hawks, or perhaps the hawks are eating something else entirely.

The "Sublethal" Puzzle

Here is the tricky part: The amount of poison found in the mice was low. It wasn't enough to kill the mouse immediately. The mouse was still alive and healthy when caught.

The Metaphor: Think of it like a person drinking a tiny bit of alcohol. They aren't drunk enough to pass out, but they aren't 100% sober either. They are "buzzed."

For the mouse, this "buzz" might not kill it, but it makes it a poisoned delivery truck. If a hawk or owl eats this "buzzed" mouse, the poison transfers to the bird. Because birds are higher up the food chain, that tiny bit of poison can add up and become deadly for them.

Why This Matters

The study concludes that even though the pest control companies are trying to keep the poison inside the buildings, it is leaking out. The wild mice are acting as vectors (or delivery drivers), picking up the poison and carrying it deep into the forest, where predators live.

The researchers suggest that we need to be more careful. Just because a mouse isn't dying right next to the bait station doesn't mean the ecosystem is safe. The poison is traveling further than we thought, hitching a ride on the backs of innocent wild mice, and potentially threatening the birds of prey that keep our skies clean.

The Bottom Line: The poison isn't staying in the city; it's seeping into the wild, and we need to change how we use it to protect the whole food chain, not just the rats.

Technical Summary

1. Problem Statement

Anticoagulant rodenticides (ARs), particularly Second-generation Anticoagulant Rodenticides (SGARs), are the global standard for rodent control due to their delayed lethality, which prevents bait shyness. However, their use poses significant risks of non-target exposure and secondary poisoning to predators (e.g., raptors) that consume poisoned rodents.

• Knowledge Gap: While spillover has been extensively studied in agricultural settings, there is a lack of data regarding AR exposure in urban wildland areas in Canada, specifically Ontario.
• Specific Concern: Pest Management Professionals (PMPs) in Canada primarily use SGARs (like bromadiolone) for commensal rodents (e.g., rats, mice) in commercial and residential settings. It is unclear if these practices cause significant spillover to native non-target rodents (NTR) like the white-footed mouse (Peromyscus leucopus), which inhabit adjacent urban forests and serve as a primary prey base for local predators.
• Hypothesis: The authors hypothesized that non-target mice would be exposed to SGARs as a function of sex (males having larger home ranges), age (mature individuals), and proximity to bait stations. They predicted bromadiolone would be the most prevalent chemical.

2. Methodology

Study Design & Location:

• Sites: Seven urban wildland sites (municipal parks with mixed deciduous forests) in Toronto and Vaughan, Ontario, adjacent to commercial buildings with ongoing, permanent exterior SGAR baiting programs.
• Duration: June 28, 2024, to August 2024 (Summer season).
• Sampling Strategy:
  • Trapping: Snap traps (lethal sampling) were placed in secured bait stations along transects perpendicular to the treated buildings.
  • Distances: Traps were set at specific intervals from the rodenticide source: 0m (edge of wildland), 20m, 40m, 70m, and 100m.
  • Effort: 334 trap nights across three trapping events per site (traps set on weekends, checked within 24 hours).
• Sample Processing:
  • Only mature adult P. leucopus (identified by testes or perforated vaginas) were selected for analysis.
  • Analysis: Liver samples were analyzed via Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) at the University of Guelph Animal Health Lab.
  • Target Analytes: 13 compounds including brodifacoum, bromadiolone, difethialone, and various first-generation ARs.
• Statistical Analysis:
  • Fisher's exact tests for frequency comparisons (sex, age, site).
  • Mann-Whitney U tests for body morphometry (length, weight, condition) and environmental factors (urban land %).
  • Logistic regression and Principal Component Analysis (PCA) to identify predictors of exposure and concentration.

3. Key Results

• Exposure Prevalence:
  • Out of 111 mature mice trapped, 11 (9.9%) tested positive for anticoagulant rodenticides.
  • Exposure occurred at 5 out of 7 study sites.
• Chemical Profile:
  • All detected compounds were SGARs.
  • Bromadiolone: Detected in 9 individuals (concentrations: 0.008–0.03 µg/g liver).
  • Difethialone: Detected in 2 individuals (concentrations: 0.008 and 0.023 µg/g).
  • Not Detected: Brodifacoum (despite its prevalence in Ontario raptor studies), warfarin, and other first-generation ARs.
• Spatial Distribution:
  • Exposed mice were caught at distances ranging from 0m to 100m from active bait stations.
  • One mouse was caught directly adjacent to a bait station (0m); another at 100m.
  • No significant correlation was found between distance from the source and AR concentration.
• Demographics & Morphology:
  • Sex/Age: 64% of exposed mice were adult males, 18% adult females, and 18% immature females. However, statistical tests showed no significant difference in exposure rates based on sex or age compared to unexposed mice.
  • Body Condition: Exposed mice had slightly higher median body lengths, weights, and better body condition residuals, but these differences were not statistically significant (p > 0.05).
  • Predictors: No single variable (sex, age, distance, urban land %, Julian date) significantly predicted exposure status or concentration.
• Temporal Factors:
  • 45% of exposed mice were caught within two weeks of a scheduled PMP site visit (bait refill), suggesting exposure peaks shortly after baiting.

4. Key Contributions

• Urban Context Data: Provides one of the first in-situ examinations of SGAR spillover specifically in urban wildland environments in Canada, contrasting with the abundance of agricultural studies.
• Chemical Specificity: Identifies bromadiolone as the dominant SGAR in this urban setting, while noting the absence of brodifacoum in non-target mice despite its high detection in Ontario raptors. This suggests different exposure pathways for predators versus direct non-target ingestion.
• Methodological Validation: Confirms that lethal snap trapping is a viable method for assessing AR contamination in non-target populations, though it may underestimate total exposure if moribund animals are missed.
• Risk Assessment: Demonstrates that while exposure prevalence is relatively low (9.9%), it is ubiquitous across distances and sites, confirming that urban pest control programs act as a source of ARs for native wildlife.

5. Significance and Implications

• Ecological Risk: Although the detected concentrations (0.008–0.03 µg/g) are below reported lethal thresholds for raptors (0.1–0.2 µg/g), the presence of ARs in a primary prey species (P. leucopus) confirms a pathway for secondary poisoning. The authors note that these concentrations may still be sublethal or lethal depending on the specific susceptibility of the predator species.
• Management Recommendations: The study concludes that SGAR usage should be avoided near naturalized landscapes in urban and suburban settings. The lack of correlation with distance implies that even "safe" buffer zones may not prevent spillover due to rodent movement and caching behaviors.
• Regulatory Context: Highlights a gap in Canadian regulations where PMPs are authorized to use SGARs outdoors, potentially exposing native species not intended as targets. The absence of brodifacoum in mice but presence in raptors suggests that predators may be selectively consuming poisoned commensal rodents or that exposure occurs via different landscape dynamics (e.g., residential vs. commercial).
• Future Research: The authors suggest that seasonal variations (winter vs. summer) and the potential for rodents to cache baits (delaying exposure) require further investigation to fully understand the risk to predator populations.

Conclusion: The study confirms that urban rodenticide programs cause measurable non-target spillover to white-footed mice, creating a vector for secondary poisoning of predators, even if the immediate exposure rate is lower than in agricultural settings.