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© 2020 Campaign for Responsible Rodenticide Use UK
CONFIDENTIAL
REPORT SERIES:
VPU/20/002
VERTEBRATE PESTS UNIT
UNIVERSITY OF READING
Anticoagulant Resistance in Rats and Mice in the UK –
Summary Report with new data for 2019-20
Report from the Campaign for Responsible Rodenticide Use (CRRU) UK for the
Government Oversight Group
Test Facility
The Vertebrate Pests Unit
School of Biological Sciences
The University of Reading
Whiteknights
Reading RG6 6AJ, UK
Sponsor
Campaign for Responsible Rodenticide UK
c/o Killgerm Chemicals
Wakefield Road, Ossett
West Yorkshire
WF5 9AJ
Report prepared by:
Alan Buckle, Clare Jones, Montse Talavera and Colin Prescott
Dr C.V. Prescott ________________________ Date: 6th October 2020
Honorary Research Fellow
DISTRIBUTION:
University of Reading
Professor Philip Dash – Head of School
This report and the data contained in it are the property of CRRU UK Campaign for Responsible
Rodenticide Use UK) and may only be used to support the rodenticide authorisations held by the
CRRU UK Member Companies: Babolna Bioenvironmental Centre Ltd, BASF plc, Bayer
CropScience Ltd., Bell Laboratories Inc., Killgerm Group Ltd., LiphaTech S.A.S., LODI UK Ltd.,
Pelsis Ltd., PelGar International Ltd., Quimica de Munguia S.A., Rentokil Initial plc., Syngenta
Crop Protection AG, Unichem d.o.o., Zapi SpA.
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© 2020 Campaign for Responsible Rodenticide Use UK
Contents:
page
Contents
2
Summary
3
1. Introduction
4
2. Materials and Methods
5
2.1 Origins of samples
5
2.2 Methods of DNA analysis
5
2.3 The Rodenticide Resistance Action Committee (RRAC) interactive
global resistance map
6
3. Results
7
3.1 Norway rats
7
3.2 House mice
11
4. Discussion
15
5. References
18
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© 2020 Campaign for Responsible Rodenticide Use UK
SUMMARY
1. New resistance data are presented for tissue samples from Norway rats (Rattus
norvegicus) and house mice (Mus musculus) collected in the period September 2019 to
February 2020. Coronavirus restrictions at the University of Reading prevented
laboratory work after that date. Once again, efforts were made to obtain samples in
geographical areas in the UK from which none had been collected in the past.
2. A total of 54 Norway rat tissue samples were analysed, among which 14 were
anticoagulant-susceptible and 40 carried one or more of five different resistance
mutations (Y139S, Y139C, Y139F, L120Q, L128Q), in either homozygous or
heterozygous form. Therefore the prevalence of anticoagulant resistance in this Norway
rat sample was 74.1%.
3. For the first time more rats were found to carry the Y139C resistance mutation than the
widespread L120Q mutation. This may be because fewer samples were submitted and
sequenced from the large and well-known L120Q focus. The observation from previous
years was repeated in that resistant rats were again found in places which would not have
been expected from prior knowledge of resistance foci. For example, Y139C was found
for the first time on the coast of West Sussex. Rats carrying the Y139S mutation (i.e.
‘Welsh’ resistance) were again recorded from North Yorkshire, far outside the original
Welsh focus, at a greater frequency than previously, and the focus had apparently spread
into County Durham.
4. These ‘break-out’ foci, and the increasing geographical spread of existing foci, have
resulted in a phenomenon not previously reported for Norway rats in England, that of
‘hybrid resistance’. This is where a single individual carries more than one resistance
mutation. A surprising 20% of resistant rats carried two different mutations in this
limited sample. This is the result of previously distinct resistance foci meeting, merging
and interbreeding. The impact of this new phenomenon of hybrid resistance on our
ability to manage resistant rodents in the future is discussed.
5. Only six house mouse tissue samples were submitted for analysis. Among these five
(83.3%) carried one or more resistance mutations. Although the total number of records
for house mouse is small, both for the year reported here and for the accumulated total
for all years, these continue to show the wide extent of house mouse resistance to
anticoagulants across the UK. Therefore, attention is again drawn to the situation in
which permanent anticoagulant baiting is the predominant method for the control of the
house mouse among professional pest control practitioners. Yet only the widely resisted
difenacoum and bromadiolone active substances are permitted for use in permanent
baiting.
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© 2020 Campaign for Responsible Rodenticide Use UK
1. Introduction
Previous reports produced for the Campaign for Responsible Rodenticide Use (CRRU
UK) on the status of anticoagulant resistance among Norway rats (Rattus norvegicus) and house
mice (Mus musculus) in the UK have presented background information on resistance mutations,
explained resistance testing methodologies and provided information on the occurrence and
geographical distribution of resistance (see Prescott et al., 2017 and 2018; Jones et al. 2019).
This previously-presented information will not be reproduced in this report; rather a summary is
provided of new information that has been obtained since the last report was prepared as the
result of genomic resistance testing conducted at the University of Reading and funded by the
Rodenticide Resistance Action Committee of CropLife International (http://www.rrac.info/).
This report has been prepared for CRRU in response to a requirement of the Health and Safety
Executive (HSE) and the Government Oversight Group (GOG) to provide resistance monitoring
information on an annual basis to support their evaluation of the progress of the UK Rodenticide
Stewardship Regime (HSE, 2019) under the heading “Competent Workforce”.
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© 2020 Campaign for Responsible Rodenticide Use UK
2. Materials and Methods
2.1 Origins of samples
The tissue samples analysed for genetical mutations were either submitted by pest control
technicians or were collected after trapping by staff of the Vertebrate Pests Unit (VPU) at the
University of Reading. Thus, samples were generally received from areas in which technicians
had experienced difficulties in obtaining effective control with anticoagulants, possibly because
of resistance or, in the case of VPU sampling, were taken from the borders of known resistance
areas in an attempt to identify their boundaries.
During 2019 and 2020 additional effort was expended in obtaining samples from areas of the UK
from which samples had not previously been received. This was continued in the present
sampling period. The maps presented in previous reports had shown that samples have not been
obtained, for example, from a large area in the centre of the country, including many counties of
the Midlands. This area is of particular interest because, from the very few samples that have
been received, there appears to be a low incidence of anticoagulant resistance among Norway
rats. Consequently, calls were put out in the magazines serving the UK professional pest control
community asking for samples from these areas (see for example Jones and Talavera, 2019;
https://www.thinkwildlife.org/free-tests-and-new-guide-tackle-spread-of-resistant-rats/). These
efforts have been rewarded with more samples obtained from areas not previously studied.
2.2 Methods of DNA analysis
As in the previous studies described above, genetical material was obtained from the field
in the form of either tail tip samples or fresh droppings. Where possible, samples were placed in
tubes containing 80% alcohol and then stored at -20°C as quickly as possible. Some unfrozen
samples were shipped to the laboratory using a courier service, surface mail or by hand delivery,
and were frozen on receipt.
Genomic DNA was extracted using the Qiagen DNeasy tissue extraction kit following the
manufacturer’s recommendations (Qiagen Ltd., Crawley, West Sussex, UK). Briefly, a small
quantity of tissue (approximately 3mm x 2mm x 2mm) was shaved from each tail using a sterile
sharp razor blade, and then placed in a 1.5ml microtube. Pre-warmed extraction buffer ATL
(180 µl) was added, followed by 20 µl of proteinase K. The mixture was vortexed and incubated
at 55˚C on a rocking platform overnight (approx. 17 h). Genomic DNA was then purified and
eluted from spin-purification columns in 80 µl of elution buffer and the quality and yield were
assessed spectrophotometrically using a nano-drop instrument.
The three exons of the VKORC1 gene, designated 1, 2 and 3, were amplified by PCR following
the methodology of Rost et al. (2004). PCR products were purified using the QIAquick PCR
purification kit (Qiagen Ltd., Crawley, West Sussex, UK). Product samples (3.5µl) were then
sequenced with BigDye version 3.1 terminator chemistry (ABI) on a 9700 ABI thermal cycler,
and the terminated products were resolved on an ABI 3130XL capillary sequencer. The sequence
trace files were visually analysed and any ambiguous bases were edited using the DNASTAR
Lasergene software. The sequence alignments were compiled using ClustalW2.
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© 2020 Campaign for Responsible Rodenticide Use UK
A list of the VKORC1 mutations found in Norway rats and house mice in the UK is shown in
Table 1.
Table 1. VKORC1 mutations in Norway rats (NR) and House mouse (HM) in UK. From: Pelz et
al., 2005; Rost et al., 2009; Prescott et al., 2010; Pelz and Prescott, 2015;Clarke and Prescott,
2015 unpublished report. Major resistance mutations with known practical consequences shown
in bold.
Species
Mutation
Abbreviations
Where present
NR
Leucine128Glutamine
L128Q
†
Central Southern Scotland, Yorkshire,
Lancashire
NR
Tyrosine139Serine
Y139S†
Anglo-Welsh border
NR
Leucine120Glutamine
L120Q
†
Hampshire, Berkshire, Essex, Norfolk
and elsewhere
NR
Tyrosine139Cysteine
Y139C
†
Gloucestershire, Norfolk, Lincolnshire,
Yorkshire, SW Scotland and elsewhere
NR
Tyrosine139Phenylalanine
Y139F†
Kent, Sussex, Norfolk, Suffolk
NR
Argenine33Proline
R33P‡
Nottinghamshire
NR
Phenylalanin63Cysteine
F63C*
Cambridge/Essex
NR
Tyrosine39Asparagine
Y39N*
Cambridge/Essex
NR
Alanine26Threonine
A26T#
Cambridge/Essex
HM
Tyrosine139Cysteine
Y139C†
Reading
HM
Leucine128Serine
L128S†
Cambridge
† Known either from field experiments and/or field experience to have a significant practical effect on
anticoagulant efficacy
‡ Known from laboratory experiments to confer warfarin resistance
* Shown in laboratory experiments to have a significant impact on protein function
# Unlikely to confer any significant degree of resistance
2.3 The Rodenticide Resistance Action Committee (RRAC) interactive global resistance
map
The results from this study were provided to the funding body, the Brussels-based RRAC
of CropLife International (http://www.rrac.info/). The results are collated with those obtained
from other global studies and presented in an interactive form on the RRAC web-site. The maps
available (see example for the UK at: http://guide.rrac.info/resistance-maps/united-kingdom/) use
Google ‘heatmap’ technology to ascribe different weightings to records depending on the
numbers of positive samples and the frequencies of their closest neighbours. Users of the maps
are able to scroll in to find their own location, that of the nearest confirmed incidence of
anticoagulant resistance, the mutation of that record and to obtain advice about the correct use of
anticoagulants in the area. It is anticipated that this scheme will help pest control practitioners to
make informed choices about which anticoagulant active substance to use and will support a
‘competent workforce’.
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© 2020 Campaign for Responsible Rodenticide Use UK
3. Results
3.1 Norway rats
During the period September 2019 and February 2020 a total of 54 Norway rat tissue
samples was received that were capable of analysis using the gene sequencing technique. Six
samples were incapable of being sequenced. This number of samples was regrettably fewer than
in previous years because restrictions implemented by the University of Reading to protect staff
and students during the coronavirus outbreak prevented any laboratory work being done after
February 2020.
Among these 54 samples, 40 were found to possess one or more of the five known
Norway rat resistance single nucleotide polymorphisms (SNPs) and 14 were found to be
susceptible animals (Table 2). Hence, 74.1% of the samples received possessed one of the
resistance mutations, in either their homozygous or heterozygous form.
Table 2. The numbers of Norway rats tissue samples received and analysed and their status of
resistance or susceptibility. A total of six samples could not be sequenced. (See Table 1 for
further explanations of the different resistance mutations.)
Resistance Mutation
Homozygous
Heterozygous
Total
L120Q
2
5
7
L128Q
7
1
8
Y139C
2
10
12
Y139F
2
0
2
Y139S
1
2
3
L128Q and Y139C
0
4*
4
L128Q and Y139S
0
1*
1
L120Q and Y139C
0
3*
3
Susceptible
-
-
14
Total
28
26
54
*These eight animals were heterozygous for each of two the
resistance mutations.
The geographical origins of these new samples are shown in Figure 1. The discovery of several
new resistance foci and the further apparent spread of others are revealed when a comparison is
made of these findings and those published in the previous report (Jones et al., 2019). Of course,
as before, it is impossible to determine whether these are newly-developed resistance foci or have
been present undetected for some time.
The proliferation of foci of the Y139C focus continues with a new occurrence in West Sussex
near Shoreham. The nearest previous record of this mutation was in south-east Surrey. Once
again, a focus of the Y139C SNP has been discovered in association with a maritime/harbour
setting, indicating a possible link with shipping and transport from the continent where this
mutation predominates.
The spread of Welsh resistant Y139S rats from their original focus on the Anglo-Welsh border
was reported for the first time in the 2019 report (Jones et al., 2019). More Y139S rats were
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© 2020 Campaign for Responsible Rodenticide Use UK
again found in North Yorkshire in the 2019-20 sample and another heterozygous individual was
located over the border in County Durham. If the findings in North Yorkshire and County
Durham indicate a contiguous focus, it suggests a much larger area infested with Y139S rats than
previously thought and the likelihood that this resistance focus was present for some time before
it was discovered.
When the boundaries of previously isolated resistance foci expand and eventually merge, because
of the continued use of ineffective rodenticides, it is to be expected that rats carrying different
resistance SNPs will meet and interbreed. To date, this survey had identified only one individual
Norway rat which possessed two different resistance SNPs, an animal found near Edinburgh
carrying the L128Q and L120Q mutations. We use the new term “hybrid resistance” to describe
this apparently rare phenomenon. However, in the relatively small 2019-20 sample we have
found no fewer than eight rats with hybrid resistance. This is a surprising and troubling increase
over the period of just one year.
Three different hybrids were found (L120Q/Y139C, L128Q/Y139C, L128Q/Y139S) in many
widely separate locations, although all explicable by nearby documented foci of the separate
SNPs. L120Q and Y139C are perhaps the most severe of the resistance SNPs found anywhere
and animals carrying this hybrid combination were found in Greater Manchester, East Sussex and
Dorset. Individuals with both L128Q and Y139C were found widely across the north of England,
in Greater Manchester, East Yorkshire, West Yorkshire and County Durham. Finally, an animal
carrying the L128Q and Y139S mutations was found in Merseyside. This brings to four the
number of different resistance hybrids now found among Norway rats in the UK. The
consequences for rodent pest management of the widespread emergence of hybrid resistance will
be discussed later in this report.
Once again efforts were made better to delineate the contiguous area of putative anticoagulant
susceptibility that appears to exist in the Midlands. Consequently, susceptible animals were
found in Northamptonshire, Lincolnshire and Bedfordshire. These counties can now be added to
others in the Midlands and central southern England, namely West Midlands, Leicestershire,
Nottinghamshire and Hertfordshire where, to date, no resistant Norway rats have been found.
However, it must be emphasised that only very small sample sizes are involved (in some cases
only a single animal), and further confirmatory sampling will be conducted if possible.
The map shown in Figure 2 gives all accumulated data on the distribution of anticoagulant
resistance for Norway rats in the UK and includes the 2019-2020 data.
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© 2020 Campaign for Responsible Rodenticide Use UK
Figure 1. Map showing the geographical locations of Norway rat tissue samples submitted to the
Vertebrate Pests Unit in the period September 2019 to February 2020 and their resistance status.
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© 2020 Campaign for Responsible Rodenticide Use UK
Figure 2. Map showing the geographical locations of all Norway rat tissue samples submitted to
the Vertebrate Pests Unit to date and their resistance status.
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© 2020 Campaign for Responsible Rodenticide Use UK
3.2 House mice
The results from the analysis of a total of 6 house mouse tissue samples submitted in the
period September 2019 to September 2020 are shown in Table 3. Among six samples examined,
one carried the fully susceptible genotype. Table 1 shows that one or other of the two resistance
mutations commonly found among house mice in the UK were present in five out of the six animals.
L128S was found in homozygous form in 3 animals and Y139C in homozygous form in another.
One individual carried both mutations, each heterozygous.
Table 3. The numbers of house mouse tissue samples received and analysed and their status of
resistance or susceptibility. (See Table 1 for further explanations of the different resistance
mutations.)
Mutation
Homozygous
Heterozygous
Total
L128S
3
0
3
Y139C
1
0
1
L128S and Y139C
0
1*
1
Susceptible
1
0
1
Total samples
5
1
6
*This animals was heterozygous for each of two the resistance
mutations.
The geographical distribution of the samples analysed during September 2019 to February 2020
and reported here is shown in Figure 3. The combined data for all years is shown in Figure 4.
Resistance distribution data for house mice recorded in the previous reports (Prescott et al., 2017
and 2018; Jones et al., 2019) were mainly from Greater London and the south-east of England. The
samples now reported were again much more widely dispersed and demonstrate conclusively the
extent of anticoagulant resistance in UK house mice.
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© 2020 Campaign for Responsible Rodenticide Use UK
Figure 3. Map showing the geographical locations of house mouse tissue samples submitted to
the Vertebrate Pests Unit in the period September 2019 to February 2020 and their resistance
status.
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© 2020 Campaign for Responsible Rodenticide Use UK
Figure 4. Map showing all available data on the occurrence of resistance mutations among house
mice in the UK.
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© 2020 Campaign for Responsible Rodenticide Use UK
The relative few house mouse tissue samples submitted in the period covered by this report add
little information to that already obtained and shown in Figure 4. The L128S mutation appears to
be very widely distributed across much of England, from Tyneside in the north-east to the Channel
coast of East and West Sussex. New records in the 2019-20 samples for L128S were found in
Monmouthshire and Merseyside. The prevalence of resistance among house mouse in the London
area was further emphasised with a single Y139C record and another hybrid resistant mouse
carrying both L128S and Y139C. However, as before, we still lack data for the house mouse and
many of the records are for either single animals or very small samples.
Earlier reports provided information on a total of 88 house mouse samples and these are now
augmented by a further 6 samples. Among the previous 88 a total of 82 (93.2%) carried one or
more resistance mutations. With the addition of the six samples reported here, five of them
resistant, the prevalence of resistance in UK house mice is now 87 resistant individuals out of a
total of 93 (93.5%).
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© 2020 Campaign for Responsible Rodenticide Use UK
4. Discussion
This report is the fourth in a series compiled for CRRU UK by the Vertebrate Pests Unit
of the University of Reading to document the distribution and frequency of resistance to
anticoagulants among Norway rats (Rattus norvegicus) and house mice (Mus musculus) in the
UK. The sampling period, which comprised September 2019 to February 2020, was curtailed by
restrictions implemented by the University of Reading which prevented laboratory work during
the ‘coronavirus lockdown’.
Among the 54 tissue samples of Norway rats received in the sampling period, 40 carried one or
more of the known resistance SNPs (Table 1). This gives a frequency of Norway rat resistance in
this sample of 74.1%. As stated in previous reports, this is unlikely to be representative of the
UK Norway rat population as a whole, because samples are generally received from those who
conduct rodent pest management, are experiencing some difficulty in obtaining full control of an
infestation and suspect that resistance may be present. However, a further consideration that
affects the percentage of the sample that is found resistant is the fact that some samples were not
selected for DNA extraction and sequencing because they were taken from within 5 km of a
known resistance focus.
The small sample size has limited the new information that can be provided in this report.
However, some interesting observations are possible from these few records. The surprising
finding of Y139S (‘Welsh resistance’) in North Yorkshire in last year’s survey was repeated in
the 2019-20 data. Indeed, the known scope of the focus was extended over a wider area of the
county and across the border into County Durham (Figure 1). All the second-generation
anticoagulants are considered to be effective against this SNP, although some doubt exists about
the efficacy of bromadiolone (Buckle et al., 2007).
Two Norway rat samples were homozygous resistant for the severe L120Q mutation, one from
the Wiltshire-Somerset border and the other from central Gloucestershire (Figure 1). These
records add to our understanding of the western and northern spread of the large central-southern
England focus of this SNP (Figure 2). Only the most potent second-generation anticoagulants
brodifacoum, difethialone and flocoumafen are fully effective against Norway rats carrying this
mutation. Field trials of bromadiolone, difenacoum and brodifacoum against L120Q rats in
Hampshire and Berkshire have confirmed the partial or complete ineffectiveness of the two
former active substances. However, brodifacoum offered effective control of L120Q-resistant
rats, with considerably less active substance being emitted into the environment during treatments
using that compound (Buckle et al., 2020). The trials, much delayed by the ‘indoor only’
restriction on the use of brodifacoum, demonstrated the benefits for both the environment and
resistance management of the use of fully effective anticoagulant rodenticides against resistant
rodents.
As in previous reports, records of Norway rats carrying the Y139C mutation were widely
scattered. A single heterozygous resistant rat was found on the borders of Derbyshire and
Staffordshire, a first for both counties and far removed from the nearest known occurrence of this
SNP in Central Manchester. The mutation was also found on the border of Surrey and West
Sussex, a known focus, but also for the first time on the south coast of Sussex, near Shoreham.
This observation brings to three the number of different mutations known to be present in the
counties of Sussex, Surrey and Kent. These SNPs, L120Q, Y139C and Y139F, are the most
severe resistances in the Norway rat currently known. This may presage even greater difficulties
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© 2020 Campaign for Responsible Rodenticide Use UK
in conducting rat control in the south of England in the future than now exist. The occurrence of
the Y139F mutation among rats in central London was again confirmed.
A remarkable finding in the data for the period 2019-2020 is the apparently sudden emergence of
Norway rats possessing two different resistance mutations – i.e. ‘hybrid resistance’. Because
resistance SNPs may occur at several gene loci in rodents, in particular those at positions 120,
128 and 139 of exon 3 of chromosome 1 in Norway rats, it is possible for animals to carry more
than one resistance mutation. Up to this point, hybrid resistance was found in the UK only in a
single L120Q/L128Q hybrid from Scotland. However, in our sample of 54 rats we report no
fewer than eight that are hybrid-resistant. This is likely to have occurred as the result of
resistance foci, which were previously discrete, meeting, merging and interbreeding. Previous
reports in this series have documented the apparent spread of resistance across the UK. This is
the first time that evidence has been recorded of the widespread coalescence of previously
discrete resistance foci. At first, individuals that result from resistance interbreeding would be
expected to be heterozygous for the two SNPs concerned, the offspring inheriting one copy of
each mutant gene from each resistant parent; and that is what we see among all eight hybrid-
resistant rats in this sample. However, if hybrid-resistant rats become more common, they are
likely to breed both among themselves and with other resistant individuals. Some of these
offspring might then be expected to be homozygous for, perhaps, several resistance mutations.
This phenomenon is the predictable consequence of a regulatory policy, in place for 30 years in
the UK but nowhere else, in which the most efficacious resistance-breaking anticoagulants could
not be used to control Norway rats (Buckle, 2013). What is less predictable is the future impact
of hybrid resistance on rodent pest management and, consequently, public health in the UK.
Hybrid resistance might be most likely to occur in areas such as central southern England where
the majority of rats in our samples are already homozygous for one resistance SNP. However,
Pelz and Prescott (2015) summarised the pleiotropic costs of certain resistance mutations for the
animals that carry them. In some cases, VKORC1 mutations result in higher dietary requirements
for vitamin K, more so in homozygous animals than in those that are heterozygous, presumably
because they have a detrimental effect on the action of the vitamin K epoxide reductase enzyme.
It seems possible that hybrid resistance may not be viable with certain severe SNP combinations,
such as L120Q and Y139F, because they will prevent vitamin K epoxide reductase from
functioning properly. This may explain why we have not yet found hybrid resistance with these
SNPs in south east England, although Norway rats that are homozygous for them are common
and foci are in close proximity (Figure 1).
Our current knowledge of the practical impacts of the different resistance mutations depends on
two lines of research. Firstly, many different anticoagulants have been applied within known
resistance UK foci and their efficacy has been determined (see Buckle, 2013). In the second line
of research, resistance factors have been derived from laboratory blood clotting response tests,
both at the University of Reading and the Julius Kühn Institute at Münster in Germany, using
methods developed at the University of Reading (Prescott et al, 2007); with the work funded in
part by the Rodenticide Resistance Action Committee (RRAC) of CropLife International.
Information from these studies on resistance factors for L120Q, Y139C and Y139F Norway rats
and for L128S and Y139C house mice is provided at the RRAC website (see
https://guide.rrac.info/aim-and-authors.html) and permits understanding of the relative severity of
these resistance SNPs. However, both these lines of research provide information only on single
resistance mutations. It seems unlikely that any hybrid-resistant individuals that are heterozygous
for two mutations, such as all those reported here, would be more resistant to anticoagulants in
practice than an individual that is homozygous for the most severe L120Q mutation, although this
cannot be declared with certainty. However, the consequences for resistance management of a
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© 2020 Campaign for Responsible Rodenticide Use UK
Norway rat individual that is homozygous for more than one resistance SNPs is difficult to
predict.
A total of 14 rats were found to be susceptible individuals, many of these were once more
reported in the counties of the Midlands. In addition to these susceptible rats, and additional 34
rats were heterozygous for one or more resistance SNPs, thus giving 65% of resistant rats with
some susceptibility remaining in their genomes.
The very small sample of house mouse tissues that were submitted for testing in the period
September 2019 to February 2020 does not provide substantially improved understanding of
resistance in this species. As might be expected from previous data, susceptibility was not
common among the house mice studied; only one of the six individuals was susceptible.
Previously, we have reported that London is a ‘hotspot’ for anticoagulant resistance in house
mice; animals that are homozygous for both the L128S and Y139C mutations being prevalent in
the capital (Jones et al., 2019). Three of the six samples submitted were from London, one was
homozygous Y139C, one was susceptible and the third was hybrid-resistant L128S/Y139C.
Anecdotal reports of the failure of baits containing one of the most potent anticoagulants,
difethialone, to control mice in the centre of London could be attributable to these hybrid-
resistant animals. More resistance testing and laboratory evaluation of the hybrid-resistant mice
strain would be required to confirm this.
These few reports again draw attention to a regulatory anomaly. The predominant method for the
management of house mice in all commercial and (especially) in food storage/preparation/sale
premises is the use of permanent tamper-resistant mouse bait boxes containing anticoagulant
baits. However, we draw attention to rules on permanent baiting, embodied in current product
labels, which only permit the widely resisted bromadiolone and difenacoum to be used in
permanent baiting programmes (CRRU, 2019). It seems contrary that we have just emerged from
the virtual ‘ban’ on the use of effective resistance-breaking anticoagulants against Norway rats,
which has undoubtedly contributed to the massive spread of resistant Norway rats in the UK, and
now find ourselves in a similar contrary regulatory position with House mice. If this situation
continues it seems likely that the already severe situation of house mouse resistance in the UK
will further deteriorate.
It is with regret that we confirm the closure of the Vertebrate Pests Unit of the University of
Reading and the fact that this will be the last report of this kind written by its staff. Continuity of
resistance UK monitoring will be provided to CRRU from the laboratories of the Animal and
Plant Health Agency (APHA) at Weybridge in Surrey, under the direction of Dr Richard Ellis.
Those wishing to submit rodent tissue samples for DNA extraction and sequencing, for resistance
characterisation, should visit the CRRU website for further information and advice
(https://www.thinkwildlife.org/about-crru-uk/).
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5. References
Buckle, A. P. (2013) Anticoagulant resistance in the UK and a new guideline for the management
of resistant infestations of Norway rats (Rattus norvegicus Berk.) Pest Management Science
69(3):334-341.
Buckle, A.P., Endepols, S. and Prescott, C.V. (2007) Relationship between resistance factors and
treatment efficacy when bromadiolone was used against anticoagulant-resistant Norway rats
(Rattus norvegicus Berk.) in Wales. International Journal of Pest Management 53(4): 291 – 297.
Buckle, A.P., Jones, C.R., Rymer, D.J., Coan, E.E. and Prescott, C.V. (2020). The Hampshire-
Berkshire focus of L120Q anticoagulant resistance in the Norway rat (Rattus norvegicus) and
field trials of bromadiolone, difenacoum and brodifacoum. Crop Protection 137: 105301.
Clarke, D. and C. Prescott. (2015). Investigation of the current status of anticoagulant resistance in
UK Norway rats by VKORC1 genotyping. Summary of results – February 2015. University of
Huddersfield, University of Reading. Confidential report. 22 pp.
CRRU UK. (2019). CRRU UK Guidance Permanent Baiting. Revised August 2019. The
Campaign for Responsible Rodenticide Use (CRRU) UK. 11 pp.
HSE (2019). Report on the Rodenticides Stewardship Regime - Assessment of Implementation.
Rodenticides Stewardship Government Oversight Group. Health and Safety Directorate,
Redgrave Court, Merton Road, Bootle, Merseyside. January 2019. 9 pp. Available from:
http://www.hse.gov.uk/biocides/eu-bpr/rodenticides.htm. Date accessed: 30.09.19.
Jones, C. and Talavera, M. (2019). Rodenticide resistance: Will you help find the gaps? PEST
Magazine, Issue August/September 2019, pp 10-11, Available at:
http://www.pestmagazine.co.uk/. Date accessed: 01.10.19.
Jones, C., Talavera, M., Buckle, A. and Prescott, C. (2019). Anticoagulant Resistance in Rats and
Mice in the UK – Summary Report with new data for 2019. Report from the Campaign for
Responsible Rodenticide Use (CRRU) UK for the Government Oversight Group. Vertebrate
Pests Unit, University of Reading, UK. 30 pp. Available at:
http://www.thinkwildlife.org/downloads_resources/. Date accessed: 18.09.20.
Pelz, H.-J., S. Rost, M. Hűnerburg, A. Fregin, A-C. Heiberg, K. Baert, A. MacNicoll, C. Prescott,
A-S. Walker, J. Oldenberg and C. Muller. (2005). The genetic basis of resistance to
anticoagulants in rodents. Genetics 170 (4): 1839-1847.
Pelz, H.-J., and C. V. Prescott. (2015). Chapter 9. Resistance to Anticoagulants. Pp 187-208. In:
Rodent Pests and their Control. A. P. Buckle and R. H. Smith (eds.). CAB International,
Wallingford, Oxon, UK. 422 pp.
Prescott, C. V., A. P. Buckle, I. Hussain and S. Endepols. (2007). A standardised BCR resistance
test for all anticoagulant rodenticides. International Journal of Pest Management 53(4): 265-272.
Prescott, C. V., A. P. Buckle, J. G. Gibbings, E. N. W. Allan and A. M. Stewart. (2010).
Anticoagulant resistance in Norway rats (Rattus norvegicus Berk.) in Kent – a VKORC1 single
nucleotide polymorphism, tyrosine139phenylalanine, new to the UK. International Journal of
Pest Management 57(1): 61-65.
Prescott, C., Baxter, M., Coan, E., Jones, C., Rymer, D. and Buckle, A. (2017). Anticoagulant
Resistance in Rats and Mice in the UK – Current Status in 2017. Report from the Campaign for
Responsible Rodenticide Use (CRRU) UK for the Government Oversight Group. Vertebrate
Pests Unit, University of Reading, UK. 30 pp. Available at:
http://www.thinkwildlife.org/downloads_resources/. Date accessed: 30.09.19.
Prescott, C., Coan, E., Jones, C., Rymer, D. and Buckle, A. (2018). Anticoagulant Resistance in
Rats and Mice in the UK – Current Status in 2018. Report from the Campaign for Responsible
Rodenticide Use (CRRU) UK for the Government Oversight Group. Vertebrate Pests Unit,
University of Reading, UK. 35 pp. Available at:
http://www.thinkwildlife.org/downloads_resources/. Date accessed: 30.09.19.
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© 2020 Campaign for Responsible Rodenticide Use UK
Rost, S., H.-J. Pelz, S. Menzel, A. MacNicoll, V. León, K-J. Song. T. Jäkel, J. Oldenburg and C.
Műller. (2009). Novel mutations in the VKORC1 gene of wild rats and mice - a response to 50
years of selection pressure by warfarin. BMC Genetics 10(4): 9.
