When the poison is part of the problem
Anticoagulant rodenticides are the best-known form of rat and mouse control, but some UK rodents carry genes that reduce their effect. This article summarises the Campaign for Responsible Rodenticide Use (CRRU) UK “Resistance Report 2025”, posted on its Think Wildlife website on 18 February 2026 and covering samples from August 2024 to July 2025, together with the Health and Safety Executive (HSE) page on rodenticides. The report is UK-wide and describes findings in Scotland, England and Northern Ireland. The HSE page covers Great Britain and Northern Ireland, which are regulated under different biocidal rules.
Who produces the data and why
CRRU coordinates the UK Rodenticide Stewardship Regime. The report says HSE and the Government Oversight Group require CRRU to provide information on anticoagulant resistance in Norway rats and house mice each year. The data come from DNA sequencing of tissue samples, mainly submitted by professional pest control technicians and by people controlling rodents on agricultural holdings. In 2024 to 2025, sequencing was done for the first time by Science and Advice for Scottish Agriculture (SASA). The results are shared with the Rodenticide Resistance Action Committee, which publishes interactive online maps.
What the 2024 to 2025 samples showed
The report records 140 rodent samples: 127 Norway rat and 13 mouse samples. After quality checks, 122 rat samples were viable, and only four house mouse samples could be sequenced. Because of a separate SASA collection, the sample was heavily weighted towards Scotland, which earlier reports had under-sampled.
- The most frequent mutation was L128Q, found in 93 rats, including hybrid-resistant animals. Of rats carrying only that mutation, 75 were homozygous and 11 heterozygous, which the report describes as indicative of a long-established and embedded resistance focus.
- Seven rats carried L128Q together with Y139C, described as hybrid resistance, all from the south of Scotland and Northumberland.
- Other mutations were Y139C in 15 rats (including those seven), L120Q in three and Y139F in two. Two of the L120Q rats came from the known focus in Hampshire, and the Y139F rats from Kent.
- Sixteen rats carried the susceptible genome: 12 in Scotland, three in England and one in Northern Ireland.
- All four viable house mouse samples were resistant.
The longer record, and its limits
Across all sequencing since 2009, the report says 753 Norway rat and 144 house mouse samples were successfully sequenced, and 75.1% of the rats and 94.4% of the mice carried one or more mutations known to significantly affect anticoagulant efficacy. The report cautions that these figures may not reflect true frequency, because samples are often sent by people who are having difficulty controlling infestations with anticoagulants. It also states that sample numbers fell short of a target of 100 in the period and that the new data added little to understanding in England and Wales, where few samples were received. The percentages therefore describe the sample, not every rat or mouse in the UK.
Why the type of mutation matters
According to the report, L128Q confers a relatively low level of resistance, and the UK Rodenticide Resistance Action Group states that all five authorised second-generation anticoagulants may be expected to be effective against it. By contrast, the report describes Y139C as a more severe mutation against which bromadiolone and difenacoum are not recommended. For hybrid-resistant rats the authors say there is no definitive information from laboratory or field on how well anticoagulants work, and suggest it may be prudent to use only the most potent anticoagulants and non-anticoagulant substances against them.
The history also carries a Scottish dimension: the report records that the first anticoagulant resistance was found in a rat on a west of Scotland farm in 1958, and that a resistance focus was later shown across much of the Central Belt.
Why continued use against resistant rodents is discouraged
The report lists three adverse consequences of continuing to use anticoagulants against resistant populations. Treated infestations are removed more slowly, with consequent risks to human and animal health. Resistance spreads and increases in severity when susceptible rodents are removed but resistant ones remain. And resistant rodents survive for long periods after unsuccessful treatments carrying persistent anticoagulant residues, so predators and scavengers that eat them may be exposed.
What HSE says about limiting resistance
HSE lists good hygiene, physical barriers, traps and biocidal products as options, and says that anticoagulants present a higher risk to people and non-target animals than is normally acceptable for authorisation, so other methods should be considered before reaching for them. Its two main strategies against resistance are minimising the use of biocidal products where possible and keeping a variety of products with different modes of action available. It adds that products for the general public may be restricted by the amount of active substance, where they can be used and the maximum pack size. The wider rules on public and professional access are covered in Rodenticide Rules: Why Professional Rat Poison Access Differs From What You Can Buy.
What a householder can reasonably ask
The report says pest controllers can check how close a site is to a known resistance focus using the published maps, and that knowing the type of resistance shows which active substances are most effective. A householder cannot assess that, but can ask a contractor about the approach: which methods are proposed, whether proofing and non-chemical options come first, and how the contractor decides on the product. The steps in Integrated Pest Management: What the BPCA’s Recommended Approach Actually Involves cover the same ground.
The bottom line
The CRRU 2025 report documents resistance to anticoagulant rodenticides as a UK-wide feature of rat and mouse samples, prevalent in Scotland and parts of England, and HSE’s guidance stresses limiting anticoagulant use and keeping other methods available. The percentages in the report come from samples often sent in because control was difficult, and few samples came from England and Wales in the latest year, so they should not be read as national prevalence. Sensible control combines hygiene, proofing, traps and carefully chosen products applied by someone who knows the local resistance picture.
Sources
- Campaign for Responsible Rodenticide Use UK, “CRRU Resistance Report 2025” (Think Wildlife, posted 18 February 2026) (thinkwildlife.org)
- Health and Safety Executive, “Rodenticides” (hse.gov.uk)