Second-generation anticoagulant rodenticides were developed in the 1970s after rodents began evolving resistance to first-generation compounds such as warfarin. These newer compounds, including bromadiolone and difethialone, work by interfering with the vitamin K cycle, specifically the VKORC1 enzyme complex, which is essential for recycling vitamin K1 and producing functional clotting factors. A poisoned animal ultimately dies from internal hemorrhaging, but the process is slow, often taking several days. During that time, the animal can accumulate a lethal body burden of the compound and continue moving through the environment. Because SGARs are extremely persistent in liver tissue, with half-lives that can stretch from weeks to months, they do not simply vanish when the target rodent dies. Instead, they linger in the carcass and in the tissues of any animal that consumed the poisoned prey, creating a pathway for secondary poisoning up the food chain.

The research team, led by Luigi F. Richardson and Albrecht I. Schulte-Hostedde of Laurentian University, set out to answer a deceptively simple question: are the bait stations that pest management professionals deploy around buildings in urban and suburban settings leaking poison into the surrounding wild rodent community? To find out, they used snap traps to capture white-footed mice in urban wildland areas of Toronto and Vaughan, deliberately choosing trapping locations near ongoing professional baiting programs that used products containing bromadiolone and difethialone. White-footed mice are an ideal sentinel species for this kind of work. They are abundant, territorial, and have relatively small home ranges, which means that any residue found in their tissues likely reflects local exposure rather than long-distance movement from a contaminated area far away.

The results were unambiguous, if modest in absolute numbers. Out of 111 mice tested, nine carried detectable residues of bromadiolone and two carried difethialone. That is roughly one in ten animals carrying a potent anticoagulant poison in a population that was never the intended target. Concentrations in liver tissue ranged from 0.008 to 0.03 micrograms per gram for bromadiolone and from 0.008 to 0.023 micrograms per gram for difethialone. While these figures may sound vanishingly small, the authors note that concentrations in this range may be sublethal for raptors, meaning that a bird of prey feeding regularly on contaminated mice could accumulate a toxic burden over time even if no single mouse delivers a lethal dose.

Perhaps the most striking spatial finding is how far from the bait stations the contaminated mice were caught. SGAR residues were detected in individuals trapped at distances of 0 meters, 5 meters, 20 meters, 40 meters, 70 meters, and 100 meters from active rodenticide stations. This pattern suggests that the compounds are not simply leaking a few centimeters into the surrounding soil or being picked up only by animals that wander directly into a bait box. Instead, the exposure appears to be distributed across the landscape, likely through a combination of mechanisms. Poisoned commensal rodents may disperse into wildland areas before dying, contaminated carcasses may be scavenged or partially consumed by mice, and bait itself may be carried out of stations by target rodents or other animals. Invertebrates that feed on bait have also been proposed in the broader literature as a potential vector, though the present study did not directly test that pathway.

When the researchers compared the SGAR-positive mice with the SGAR-negative ones, they found no significant differences in sex, age, body condition, distance to the rodenticide source, capture date, or capture site. In other words, there was no obvious profile of the contaminated mouse. Exposure did not concentrate in juveniles, in animals in poor condition, or in individuals trapped closest to bait stations. This lack of pattern is itself informative. It suggests that exposure risk is broadly distributed across the local mouse population rather than confined to a particular demographic subset or a narrow zone around bait deployment. It also complicates any attempt to predict which individuals or areas are most at risk based on simple proximity measures.

That said, the study did note that four of the exposed individuals were classified as being in poor body condition. While the sample size is too small to draw firm conclusions about a causal relationship between rodenticide burden and physical condition, the observation is consistent with the possibility that sublethal exposure imposes physiological costs. Anticoagulant poisoning, even at doses that do not kill outright, can cause chronic internal bleeding, anemia, and reduced fitness. For a wild mouse, those costs could translate into reduced survival or reproductive output, and for the predators that depend on mice as prey, a population of weakened or poisoned rodents represents a hidden hazard embedded in a normal food source.

The broader context of this work is a growing body of evidence that anticoagulant rodenticides have become pervasive contaminants in urban and suburban food webs. Studies in Chicago have documented widespread exposure among common urban mesopredators, while research in British Columbia has found these compounds in endangered American badgers and fishers. Raptors in Ontario have already been shown to carry SGAR residues, and the compounds have been detected in unexpected places, from urban reptiles in Australia to frogs and freshwater fish in Germany. What has been missing, particularly in Ontario, is a clear understanding of the exposure pathways, the specific mechanisms by which the poison moves from a locked bait station to a hawk’s bloodstream. By demonstrating that nontarget wild rodents near professional baiting programs carry SGAR residues, this study helps fill that gap and points to a plausible link between routine pest control and the contamination of predators.

The authors also raise the possibility that some of the exposure they detected may have come from residential SGAR use rather than from the professional baiting programs they focused on. In Ontario, as in many jurisdictions, consumers can purchase rodenticide products for home use, and these products are often deployed with less oversight than professional applications. Distinguishing between professional and residential sources of exposure is methodologically difficult, but the finding underscores that the problem is not confined to any single sector of the pest control industry. It is a landscape-scale contamination issue driven by the sheer volume of persistent anticoagulants being deployed across urban environments, whether by licensed technicians or by homeowners dealing with a mouse in the garage.

The implications for policy and practice are significant. Several jurisdictions, including parts of the United States and the European Union, have already moved to restrict public access to SGARs or to limit their use in certain settings, and Health Canada has imposed restrictions on commercial-class rodenticides in agricultural contexts. This study adds weight to the argument that urban and suburban baiting practices deserve similar scrutiny, particularly near wildland fragments that support native rodent populations and the predators that feed on them. The authors suggest that the pest management industry’s use of rodenticides in these settings is likely causing some degree of nontarget spillover in Toronto and the Greater Toronto Area. As cities continue to densify and the boundary between built and natural habitat grows ever more porous, the question is no longer whether rodenticides reach nontarget wildlife, but how much exposure ecosystems can absorb before the cumulative toll on predators and other wildlife becomes unsustainable. For now, the white-footed mice of Toronto’s ravines are carrying the answer in their livers, one contaminated individual at a time.

Subject of Research: Nontarget exposure of white-footed mice to second-generation anticoagulant rodenticides in urban Ontario

Article Title: Exposure of nontarget white-footed mice (Peromyscus leucopus) to second-generation anticoagulant rodenticides in an urban context

Article References: Richardson, L. F., & Schulte-Hostedde, A. I. (2026). Exposure of nontarget white-footed mice (Peromyscus leucopus) to second-generation anticoagulant rodenticides in an urban context. Environmental Monitoring and Assessment, 198(11), Article 1195. https://doi.org/10.1007/s10661-026-16038-x

Image Credits: AI Generated

DOI: 10.1007/s10661-026-16038-x

Keywords: anticoagulant rodenticides, SGARs, white-footed mouse, Peromyscus leucopus, bromadiolone, difethialone, urban wildlife, ecotoxicology, secondary poisoning, Toronto, pest control, nontarget exposure

The logic behind the approach rests on the peculiar ecology of Lyme disease in the northeastern United States. The blacklegged tick, Ixodes scapularis, passes the bacterium Borrelia burgdorferi sensu stricto through its life stages, but the pathogen’s persistence in a given landscape depends heavily on one particular host: the white-footed mouse, Peromyscus leucopus. These abundant rodents serve as the principal natural reservoir of the spirochete, and larval ticks that feed on infected mice are far more likely to acquire the bacterium than larvae feeding on deer, birds, or other mammals. When those larvae molt into nymphs the following year, the infected nymphs become the primary threat to humans, delivering the bite that transmits Lyme disease in the vast majority of cases.

Interrupting the cycle at the mouse-tick interface has therefore long been an attractive target for researchers. Earlier laboratory and simulated field studies had already demonstrated that a bait containing just 0.005 percent fipronil could systemically control Ixodes scapularis ticks parasitizing treated white-footed mice. Fipronil, the same active ingredient found in some companion-animal flea and tick products, works systemically: after a mouse ingests the bait, the compound and its metabolites circulate in the animal’s blood, and ticks that attach and begin feeding are killed before they can complete their blood meal. The question the Connecticut study set out to answer was whether that laboratory promise would survive contact with real backyards, real weather, and real wildlife.

Between 2022 and 2024, the research team deployed reusable rodent bait stations on residential properties during the period when blacklegged ticks are active in their nymphal and larval stages, roughly May through August. The stations dispensed the fipronil-treated bait to free-ranging mice, and the team monitored consumption, replenishing bait as needed to keep the system running through each season. On average, each property received 4.9 kilograms of bait per year, which translates to approximately 247 milligrams of fipronil—a remarkably small total quantity of active ingredient distributed across an entire property over an entire season.

To confirm that the mice were actually being exposed to the compound, the researchers captured white-footed mice during the larval season each year on both treated and untreated control properties and analyzed their blood plasma. The results were striking: fipronil sulfone, a metabolite that serves as a marker of exposure, was detectable in 100 percent of plasma samples analyzed in both 2022 and 2023, and in 83.3 percent of samples in 2024. In other words, the bait stations were doing their job, delivering the active ingredient to the target animals with high consistency even under fully natural field conditions.

The effect on the ticks themselves was even more impressive. On treated properties, the number of larvae parasitizing captured mice fell by up to 84.2 percent compared with control properties, and the number of larvae that successfully engorged—those that managed to complete a blood meal, which is the critical step for both tick survival and pathogen acquisition—dropped by up to 92.5 percent. A larval tick that fails to engorge on a mouse not only fails to acquire Borrelia from that mouse; it also faces greatly reduced odds of surviving to molt into the next life stage. The bait, in effect, turned the most important host in the Lyme disease system into a lethal trap for its parasites.

The benefits extended to the pathogen as well. Molecular assays used to detect Borrelia burgdorferi infection in the captured mice showed that infection prevalence in white-footed mice on treated properties was significantly reduced by up to 34.1 percent relative to controls. That reduction matters because fewer infected mice means fewer larval ticks acquiring the spirochete, which in principle should mean fewer infected nymphs questing for human blood the following spring. The study thus demonstrated a measurable dent in the reservoir competence of the local mouse population, achieved not by reducing mouse numbers but by breaking the tick-mouse transmission loop.

The picture was more nuanced, however, when the researchers turned to the stage of the tick life cycle that matters most for human health. Drag sampling and flagging during the nymphal seasons were used to estimate the abundance of host-seeking nymphs on the properties, and molecular testing assessed how many of those nymphs carried Borrelia. While both nymph density and nymphal infection showed reductions on treated properties, neither difference reached statistical significance compared with the control sites. The authors suggest that the bait application rate during the larval season may need to be increased to push these downstream effects over the threshold of detectability—a reminder that intervening at one link of a complex transmission chain does not automatically translate into reduced risk at every other link.

That caveat aside, the study represents one of the most thorough field evaluations to date of a systemic acaricide bait aimed at the wildlife reservoir of Lyme disease. Its design—multi-year, property-level, with matched controls, bait-consumption monitoring, pharmacokinetic confirmation, and molecular pathogen testing on both rodents and questing ticks—addresses many of the weaknesses that have plagued earlier tick-control trials. The extremely low dose of active ingredient involved also raises the prospect of an environmentally targeted intervention: rather than broadcasting insecticides across vegetation, the treatment is delivered only to the small mammals that seek out the bait stations, and only in quantities measured in hundreds of milligrams per property per year.

The researchers, based at Genesis Laboratories in Colorado and the Connecticut Agricultural Experiment Station, were supported by a contract with the United States Centers for Disease Control and Prevention, and the work benefited from molecular detection assistance from CDC scientists. They emphasize that additional field studies are needed to confirm the findings and to explore what large-scale treatment across neighborhoods or landscapes might achieve, since tick populations on untreated properties can continually seed treated areas. Still, the core result stands: a bait box, a few hundred milligrams of fipronil, and the natural foraging behavior of white-footed mice were enough to strip the vast majority of larval ticks from the animals that sustain Lyme disease in the northeastern United States. As integrated tick management strategies evolve, the humble mouse bait station may prove to be one of the most quietly powerful tools in the fight against America’s most common vector-borne disease.

Subject of Research: Field efficacy of low-dose fipronil bait for controlling blacklegged ticks on white-footed mice and reducing Lyme disease transmission

Article Title: Efficacy of a low-dose fipronil bait in controlling blacklegged tick (Ixodes scapularis) larvae parasitizing white-footed mice (Peromyscus leucopus) under field conditions in the northeastern United States

Article References: Poché, D. M., Williams, S. C., Linske, M. A., & Poché, R. M. (2026). Efficacy of a low-dose fipronil bait in controlling blacklegged tick (Ixodes scapularis) larvae parasitizing white-footed mice (Peromyscus leucopus) under field conditions in the northeastern United States. Parasites & Vectors, 19(1), Article 423. https://doi.org/10.1186/s13071-026-07676-8

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07676-8

Keywords: Lyme disease, Ixodes scapularis, Peromyscus leucopus, fipronil, tick control, Borrelia burgdorferi, vector-borne disease, systemic acaricide, bait stations, integrated tick management, Connecticut, public health