Rodenticide Resistance in Rats
Updated: Sep 3
Causes, Risks and Prevention
Quick overview
Rodenticide resistance is an increasingly important issue in the control of rats and mice in the UK. Some rodent populations carry inherited genetic changes that can reduce their susceptibility to anticoagulant rodenticides, making infestations more difficult to control.
Since this article was originally published in 2019, understanding of rodenticide resistance has developed considerably. UK surveillance has identified different resistance-associated mutations in Norway rats, including evidence of multiple mutations occurring within the same populations. This updated guide explains what rodenticide resistance is, how inherited resistance can develop within rat populations, the difference between genetic resistance and behavioural bait avoidance, and what current UK surveillance tells us about the distribution of resistance.
It also looks at the signs of rat activity, the potential health risks associated with rats, and practical measures that can help prevent infestations by reducing access to food, water, shelter and buildings.
The available surveillance data should be interpreted carefully. Resistance testing is not based on a random sample of all rats in the UK, so reported figures should not be treated as an estimate of the percentage of the entire UK rat population that is resistant.
Rodenticide Resistant Rats!
A recent study carried out by the University of Reading has identified new hotspots of rats that are resistant to anticoagulant rodenticides. A statement from the Campaign for Responsible Rodenticide Use (CRRU), who commissioned the study said:
"In East Anglia and West Yorkshire, it identifies for the first time the L120Q gene, responsible for the most severe form of resistance. This gene renders first generation anticoagulant rodenticides and two of the second-generation groups ineffective. It is widespread across the whole of central southern England and also found increasingly outside that area."
They continue:
"Another serious concern is that three different types of resistant rats are now found in West Yorkshire and on the Anglo-Welsh border. Also, a worry is the almost complete lack of data from central England."
The British Pest Control Association (BPCA) commented that:
"The study highlights the fact that resistance is growing in rat species across a swathe of the country."
It also reports that rats without the genetic mutation are being killed off by poison, so the resistant species are taking their place, leaving a growing population of resistant pure-breds.
"With their numbers expanding there could be a significant risk to public health if their population is left unchecked, in both urban and rural environments."
Many pest controllers consider that the most likely cause of the spread of these resistant rats has been accelerated by the application of rodenticides, by amateurs such as home and business owners doing it themselves or employing unqualified individuals to try to resolve the problem.
What Is Rodenticide Resistance?
Rodenticide resistance is the ability of some rodents to survive exposure to a rodenticide that would normally be expected to control susceptible animals.
In the UK, the main concern is resistance to anticoagulant rodenticides, which work by interfering with the vitamin K cycle needed for normal blood clotting. Genetic changes can reduce the effectiveness of these rodenticides in some rats and mice.
Resistance is an inherited characteristic. An individual rat does not become genetically resistant simply because it has been exposed to rodenticide. Instead, where resistant rodents are present, the use of rodenticides can place selection pressure on the population: susceptible animals may be controlled while resistant animals survive and reproduce, potentially increasing the proportion of resistant rodents over time.
It is also important to distinguish genetic resistance from other reasons why rodent control may appear to have failed. Poor bait placement, insufficient monitoring, competing food sources, bait avoidance and failure to follow product-label instructions can all contribute to ineffective control.
For this reason, continued rat activity following a rodenticide treatment should not automatically be taken as proof of genetic resistance. The cause of treatment failure needs to be assessed as part of a wider rodent-control programme.
How Do Rats Become Resistant to Anticoagulant Rodenticides?
Anticoagulant resistance in rats is associated with inherited genetic changes that reduce the susceptibility of some individuals to anticoagulant rodenticides.
Within a rat population, genetic variation can occur naturally. Where some rats carry a resistance-associated genetic variant, those animals may be better able to survive exposure to an anticoagulant than susceptible rats.
When anticoagulant rodenticides are used, this can create selection pressure. Susceptible rats are more likely to be controlled, while rats carrying resistance-associated variants may be more likely to survive. The surviving rats can reproduce and pass these variants to their offspring. Over successive generations, this can increase the proportion of resistant rats within a local population.
This process does not mean that an individual rat becomes genetically resistant during its lifetime because it has been exposed to rodenticide. Rather, resistance is an inherited characteristic that can become more common within a population when resistant animals survive and reproduce.
The development and spread of resistance can be influenced by how rodenticides are used. Repeated or inappropriate use of anticoagulant rodenticides can increase selection pressure, particularly where the active ingredient being used is less effective against the resistance present in the local rat population.
However, continued rat activity after treatment does not, by itself, demonstrate genetic resistance. Other factors, including inadequate bait uptake, competing food sources, bait avoidance, reinvasion or problems with the treatment programme, can also result in control failure.
Understanding whether resistance is present, and which resistance mechanism may be involved, can therefore be important when investigating persistent infestations.
Genetic Resistance vs Behavioural Resistance
Rodenticide resistance can involve different mechanisms, and it is important to distinguish genetic resistance from behavioural responses that can make rodent control more difficult.
Genetic resistance
Genetic resistance is an inherited characteristic. In rats, resistance to anticoagulant rodenticides is associated with specific genetic changes that can reduce their susceptibility to some anticoagulant active substances.
Where resistant rats survive treatment, they can reproduce and pass resistance-associated genetic variants to their offspring. Over generations, this can result in resistant animals becoming more common within a local population.
The presence and type of genetic resistance can vary between rat populations. This is why resistance surveillance and an understanding of local resistance patterns are important when investigating persistent control problems.
Behavioural resistance and bait avoidance
Rats can also avoid rodenticide bait for reasons that are not related to genetic resistance. For example, rats may reject or avoid a bait because of its taste, smell, formulation or placement. They may also have access to other food sources that are more attractive.
Previous experience can also influence feeding behaviour. This can make a rodenticide treatment appear unsuccessful even when the rats are susceptible to the active substance.
Behavioural avoidance and genetic resistance are therefore different issues. A rat that does not consume enough bait cannot be assumed to be genetically resistant, and continued rodent activity following treatment is not, by itself, evidence of genetic resistance.
When control is unsuccessful, the possible causes should be assessed systematically. This can include checking bait uptake and placement, the availability of alternative food sources, evidence of bait avoidance, reinvasion and the suitability of the treatment programme, as well as considering whether genetic resistance may be involved.
Taking this broader approach can help ensure that rodenticide is used appropriately and that other effective control measures are not overlooked.
VKORC1 and Rodenticide Resistance Mutations
Many cases of anticoagulant resistance in Norway rats (Rattus norvegicus) are associated with changes in a gene called VKORC1.
VKORC1 provides the instructions for making vitamin K epoxide reductase, an enzyme involved in the recycling of vitamin K. Vitamin K is needed for the normal production of several blood-clotting factors. Anticoagulant rodenticides interfere with this process, ultimately affecting the rodent's ability to clot blood normally.
Some genetic changes, known as resistance-associated mutations, can alter the VKOR protein and reduce the susceptibility of rats to anticoagulant rodenticides.
Several VKORC1 mutations with known practical consequences have been identified in UK Norway rats. The principal mutations are:
L120Q (Leu120Gln)
L128Q (Leu128Gln)
Y139C (Tyr139Cys)
Y139F (Tyr139Phe)
Y139S (Tyr139Ser)
These mutations are not interchangeable. They can have different effects on susceptibility to different anticoagulant active substances. For this reason, the presence of a resistance-associated mutation does not necessarily mean that a rat will be resistant to every anticoagulant rodenticide.
Resistance-associated mutations can occur in either one copy of the VKORC1 gene (heterozygous) or both copies (homozygous). The effect on susceptibility can depend on the particular mutation and genetic combination involved.
Understanding which resistance-associated mutations are present in a rat population can therefore help inform resistance surveillance and the management of persistent rodent infestations.
Resistance testing and surveillance are important because the distribution of these mutations is not uniform across the UK and can change over time. The absence of a recorded resistance mutation in an area should not necessarily be interpreted as evidence that resistance is absent, particularly where surveillance data are limited.
What Is Hybrid Resistance?
Hybrid resistance is a term used to describe a rat that carries more than one resistance-associated genetic mutation.
In Norway rats, different resistance mutations have developed in separate geographical areas. As these resistance areas expand and overlap, rats carrying different mutations can come into contact and reproduce. Their offspring can inherit resistance-associated mutations from both parental lines.
For example, UK resistance surveillance has identified rats carrying combinations including:
L120Q and Y139C
L128Q and Y139C
L128Q and Y139S
Hybrid resistance has been identified in different parts of the UK. The occurrence of these combinations provides evidence that previously separate resistance populations can overlap and interbreed.
Why is hybrid resistance important?
The practical consequences of hybrid resistance are still being investigated. There is not enough evidence to assume that every rat carrying two resistance mutations will be more resistant than a rat carrying a single mutation.
However, the presence of multiple resistance-associated mutations is important because each mutation can affect susceptibility to anticoagulant rodenticides in different ways. As resistance populations spread and overlap, hybrid resistance could make resistance management more complex.
For this reason, continued surveillance is important. Identifying the resistance mutations present in rat populations helps scientists and pest-management professionals understand how resistance is changing and where particular resistance profiles occur.
Hybrid resistance should therefore be regarded as an important development in the evolution and spread of anticoagulant resistance, rather than as evidence that rats are immune to all rodenticides.
Current UK Rodenticide-Resistance Statistics 2026
Rodenticide resistance has been detected in rat populations in several parts of the UK. However, available surveillance data should be interpreted with care. The samples submitted for testing are not a random sample of the UK's rat population and may be disproportionately collected from infestations where rodent control has been difficult. The results therefore cannot be used to estimate the proportion of all UK rats that are resistant.
The latest CRRU Anticoagulant Resistance in Rats Report covers samples collected between August 2024 and July 2025. During this period, 140 rodent tissue samples were received for DNA sequencing, including 127 Norway rat (Rattus norvegicus) samples and 13 mouse samples. Five of the rat samples could not be successfully sequenced, leaving 122 Norway rat samples suitable for analysis.
Among the 122 Norway rat samples that could be analysed:
93 carried the L128Q resistance mutation, including rats that also carried Y139C.
15 carried Y139C, including seven rats that also carried L128Q.
Three carried L120Q.
Two carried Y139F.
Seven rats carried both L128Q and Y139C, demonstrating hybrid resistance.
16 rats had the susceptible, or wild-type, sequence for the five principal resistance mutations examined.
The high number of L128Q results in this survey reflects the large number of samples obtained from Scotland, where this resistance mutation is well established. The geographical composition of samples is therefore important when comparing resistance results between different years.
Earlier UK surveillance provides a longer-term picture. Between 2009 and July 2024, 631 Norway rat tissue samples were examined, of which 474 (75.2%) carried one or more resistance mutations known to have a significant effect on anticoagulant rodenticide efficacy.
However, CRRU advises that these figures should not be interpreted as the prevalence of resistance across the entire UK Norway rat population because of the way samples are collected.
Sources and further information
The figures above are taken from the Campaign for Responsible Rodenticide Use (CRRU) UK Resistance Report 2025
Signs of Rat Activity
Rats are mainly active at night, so they may not always be seen. However, there are several signs that can indicate the presence of rats in or around a property.
Common signs include:
Droppings – Rat droppings are usually dark, relatively large and spindle-shaped, although their appearance can vary depending on the species and age of the droppings.
Gnaw marks – Rats have continuously growing incisors and may gnaw wood, plastic, packaging, electrical cables and other materials.
Greasy marks and smears – Rats can leave dark or greasy marks along walls, skirting boards and other surfaces where they regularly travel.
Burrows – Outside, rats may create burrows in soil, compost areas, vegetation, banks and around buildings.
Nesting material – Shredded paper, insulation, fabric and other soft materials may be used to construct nests.
Food damage or contamination – Gnaw marks on food packaging, missing food and contamination with droppings or urine can indicate rodent activity.
Unusual smells – A persistent musky or ammonia-like odour may occur where rats are present in enclosed areas.
Noises – Scratching, gnawing or movement sounds may sometimes be heard, particularly at night.
Tracks and runways – Rats may repeatedly use the same routes between feeding and nesting areas, leaving visible tracks or disturbed surfaces.
Finding one sign does not necessarily indicate a large infestation. The number, location and freshness of signs can help determine whether there is current rodent activity.
It’s important that individuals and businesses do not attempt to solve problems by treating rodent infestations themselves.
A qualified, professional exterminator will be able to purchase and apply the appropriate products to tackle resistance issues.
Professional pest controllers, like PGM & Son Pest Control - who serve all postcodes across Herefordshire and Worcestershire - have a greater understanding of the behavioural traits and gestation periods of rodents so can advise better on preventative measures instead of focussing solely on extermination which is rarely a long-term solution.
With Britain in for a lengthy cold snap and forecasters predicting more freezing conditions to come, the rat season is firmly upon us.
Pest controllers report a significant increase in call-outs for rats and mice during January, February and March as rats look for warm buildings to set up home – signalling we’ve entered the peak season for rodent infestations.
As temperatures plummet, house-holders must be vigilant in spotting the signs of rodents’ arrival and to take steps to try and stop rats and mice setting up home in their property.
There are thought to be around 10 million rats in the UK, which is why people often say you’re never far away from one!
What to look out for:-
Greasy marks on skirting boards and walls
Droppings and urine (which carries a very strong, distinctive smell)
Gnaw marks, chewed surfaces
Possible entry points, for example, gaps in brick work, pipes and cables which extend to outside your property
Pieces of cardboard and soft materials which they may be using to build nests
Scratching noises
Rat holes and burrows
Rat nests
Footprints / tracks
Common places for rodents to nest are: lofts and attics, kitchens, laundries, behind kitchen appliances particularly where pipes and cables extend outside the property.
Health Risks from Rats
Rodents pose a serious health risk to humans and so it’s vital that people are aware of the risks. Rats carry nasty diseases which can spread to humans such as Leptospirosis or Weil's disease, Salmonella, Listeria, Toxoplasma gondii and Hantavirus.
The HSS report around 40 cases of Weil’s disease every year. Two types of rat tapeworm spread to humans though eggs found in rat faeces.
If you think you have rats, or mice, don’t panic. Call PGM & Son Pest Control Herefordshire on 01981 540088 today for advice and solutions. We cover Herefordshire and Worcestershire for our rodent control services, so give us a call, or drop us an email to: contact@pgmpestcontrol.co.uk
Don’t let a rodent problem get worse – rats and mice breed rapidly so what may start as a small problem can quickly escalate – call in the professionals today to sort out a rodent infestation.
