Mice do not become “immune” to poisons in the traditional sense. Instead, they develop “resistance,” which is the ability of a population to withstand a harmful substance through evolutionary processes. This differs from individual immunity and explains how some mouse populations survive rodenticide exposure, affecting pest management.
How Rodenticides Work
Rodenticides are chemicals used to control rodent populations. They are categorized as anticoagulants or non-anticoagulants. Anticoagulant rodenticides, the most common type, interfere with blood clotting. They block Vitamin K epoxide reductase (VKOR), an enzyme vital for recycling Vitamin K, which is needed for blood clotting factors. This disruption causes internal bleeding and death, typically occurring several days after ingestion.
First-generation anticoagulants like warfarin require multiple feedings for a lethal dose. Second-generation anticoagulants, such as brodifacoum, are more potent, often lethal after a single feeding. Non-anticoagulant rodenticides work through different mechanisms, for instance, bromethalin affects the central nervous system.
The Science of Mouse Resistance
Mice develop resistance to rodenticides through natural selection acting on genetic mutations. A few mice in a population may have a genetic variation that reduces their susceptibility. When rodenticides are used, susceptible mice die, while those with the protective mutation survive and reproduce. These resistant individuals pass their genes to offspring, increasing the proportion of resistant mice over generations.
A common genetic basis for resistance to anticoagulant rodenticides involves mutations in the VKORC1 gene. This gene codes for the VKOR enzyme, which is the target of anticoagulant poisons. Mutations in VKORC1 can alter the enzyme, preventing the anticoagulant from binding effectively and allowing the enzyme to continue its role in blood coagulation, even in the presence of the poison. Multiple mutations in this gene have been identified in resistant mouse populations across various regions.
Challenges of Resistant Mouse Populations
The spread of rodenticide resistance challenges effective pest control. When mice become resistant, traditional rodenticides are less effective, leading to persistent infestations. This increases pest management costs, as higher doses or different chemicals may be needed. Unchecked mouse populations cause property damage, contaminate food, and transmit diseases.
Resistant mice also carry more rodenticide residues, increasing secondary poisoning risk for predators and scavengers. Resistance genes continue to expand geographically, with resistant individuals spreading. This complicates control and can lead to a cycle where continued rodenticide use selects for even more resistant strains.
Effective Management Approaches
Managing mouse populations, especially resistant ones, requires Integrated Pest Management (IPM). IPM focuses on long-term prevention using a combination of methods, not just chemicals.
Sanitation is a primary component. This involves removing food, water, and shelter that attract mice, such as storing food in sealed containers and properly disposing of garbage.
Exclusion, or pest-proofing, is another key step. Seal all potential entry points into buildings. Mice can squeeze through tiny openings, so seal holes larger than a quarter-inch with durable materials like steel wool, caulk, or cement.
Mechanical controls, such as snap traps and electronic traps, effectively reduce mouse numbers without chemicals. If rodenticides are necessary, rotate active ingredients or use non-anticoagulant options, especially where resistance is known. Consulting pest management professionals ensures appropriate strategies for resistant infestations.