Feature pest control | PPC90 March 2018

Move with it ...or miss out! Adrian Meyer is an author and editor of the British Pest Management Manual (BPM). He talks us through the issue of pesticide resistance and the research being conducted.

Speed read

• Bed bugs, cockroaches, flies, fleas, stored product insects, Norway rats and house mice are evolving avoidance mechanisms to pesticides 
• University of Reading research unit  reveals growing and worrying data on Norway rat resistance
• Mechanism of house mouse resistance is thought to be the same as in Norway rats
• Mice carrying Y139C mutation possess a degree of resistance to difenacoum
• 88% of samples analysed had at least one resistant gene
• Advice is to not use bromadiolone in the UK as it may not result in adequate control and will increase resistance

Public health pest control is very much an evolving subject. As I sit down, as an author and editor of the British Pest Management Manual, to undertake the regular review and edit of the vertebrate sections of the manual, I am deluged with sets of new data on anticoagulant resistance that will (or should) change the way in which we work.

The manual is unrecognisable now when compared with the Local Government Training Board Manual published in the early 1980s. The ‘look’ has changed and it’s now more than twice the length of the original, with new sections and certainly much more to say for itself! Nothing stays the same for very long.

When reviewing the contents over the years it’s clear that many changes have occurred, not only in practice but in the safety aspects (thank goodness) of the work that we undertake. As for the legislation, well, it just gets more complicated and increasingly impacts on our operations.

On reflection though, the single area that has probably evolved (I use the term advisedly) the most is the susceptibility of pests to the pesticides we use against them. These pests, including bed bugs, cockroaches, flies, fleas, stored product insects as well as Norway rats and house mice, are evolving avoidance mechanisms to pesticides faster than we can develop new ones to control them.

This fact has been emphatically highlighted in only the last few weeks with the release of a new paper on the distribution and abundance of resistant commensal rodents from the UK’s leading research unit, headed by Colin Prescott, on anticoagulant resistance at the University of Reading. The paper reviews the growing and worrying data set on Norway rat resistance, but also for the first time gives us an insight into what the problems may be as house mice are concerned and this new data should concern us all. 

There has always been a greater emphasis on Norway rat resistance than on house mouse resistance, probably because the impact of rats on farming has been considered more significant. However, we have known, since the mid 1950s, that there was an issue with house mouse resistance to first generation anticoagulants (FGARs) – warfarin, chlorophacinone and coumatetralyl. After the introduction of second generation anticoagulants (SGARs) in the early 1970s, the use of the FGARs was avoided. However, we have very little data on resistance in house mice to the SGARs.

The mechanism of house mouse resistance is now thought to be the same as that in Norway rats, with mutations of the vitamin K epoxide reductase complex unit 1 (VKORC1) governing anticoagulant resistance. Most of the work relating to house mouse resistance has been done on two strains of mice identified in about the 1980s. The first, found in Cambridge, was called the Cambridge Cream strain and this carries the ‘L128S Mouse’ mutation. The second strain was identified in Reading and carries the ‘Y139C Mouse’ mutation. There has therefore been evidence for some time that we have at least two anticoagulant resistant strains of house mice in the UK. But what does this mean?

The mechanism of house mouse resistance is now thought to be the same as that in Norway rats, with mutations of the vitamin K epoxide reductase complex unit 1 (VKORC1) governing anticoagulant resistance.

We do know from work undertaken at Reading, that the Y139C strain shows a significant degree of resistance to bromadiolone and mice carrying this Y139C mutation are known to possess a degree of resistance to difenacoum.

The situation with the L128S strain is less certain, but there are past records that house mice could not be controlled using difenacoum.

However, the problem has always been that we have no idea how widespread these two mutations are in house mice. This has now started to change.

As a result, largely of significant efforts by London Boroughs who are members of the Greater London Pest Liaison Group, some 44 tissue samples (house mouse tails) have recently been analysed from London by the team at Reading, the results are a cause for significant concern. Of the 44 samples analysed only five (11%) were from susceptible mice.

Of the remainder, 88% of the samples analysed possessed at least one of the resistant genes!

19 (43%) of the samples had the L128S and 17 (39%) of the samples had the Y139C mutation. Three (7%) of the samples were from house mice carrying both mutations.

An additional concern is that of the 39 mice carrying the mutations, 38% were homozygous for the mutation concerned. This means that both the parents of these house mice were carrying the mutation, confirming the very high prevalence of the mutations in house mice in London. These homozygous resistant mice will have a selective advantage over other house mice.

Well, what does this actually mean in practice, particularly in London?

As far as the Y139C mutation is concerned, we know, from work undertaken at Reading, that the mice carrying this mutation possess a significant degree of resistance to bromadiolone. So, the advice from the Rodenticide Resistance Action Group (RRAG) chaired by Dr Alan Buckle, is not to use bromadiolone in the UK as it may not result in an adequate level of control and will exacerbate resistance problems.

The situation with difenacoum is more equivocal. This anticoagulant is widely used in successful mouse control treatments. However, mice carrying the Y139C mutation possess a degree of resistance to difenacoum. The situation with L128S is more uncertain. However, it is certain that 30 years ago some individuals within mouse infestations could not be controlled with difenacoum baits, and it is unlikely that this situation has improved during the intervening period. It would therefore be prudent, in areas where resistance in house mice is suspected, not to use products that contain difenacoum.

It would therefore be prudent, in areas where resistance in house mice is suspected, not to use products that contain difenacoum.

While these recommendations may seem dramatic and will clearly have an impact on many users of rodenticides, there are alternative SGARs available, to which there is no evidence of resistance and which should be effective if consumed by resistant house mice.

Importantly, we need more information on the prevalence of the resistance mutations. The samples collected in London are simply a start to what should be a more extensive sampling programme undertaken throughout the UK. It is a pity that it has taken so long to start collecting these first samples.

Reading matters!

For those who want to know more about the history of resistance and the recent research, there are a number of publications available that are worth reading. These include two sets of guidelines on resistance from the Rodenticide Resistance Action Group:

• House Mouse Resistance Guidelines
• Anticoagulant resistance in the Norway rat and Guidelines for the management of resistant rat infestations in the UK

A publication from the Rodenticide Resistance Action Committee (RRAC) in Europe is also worth reading:

• RRAC Guidelines on Anticoagulant Rodenticide Resistance Management (Crop Life International)
• The most up-to-date report has been released by Reading University and is from the Campaign for Responsible Rodenticide Use (CRRU), available on its website:
• Anticoagulant Resistance in Rats and Mice in the UK – Current Status in 2017

Providing a sample for Reading’s resistance research

The RRAC has an interactive checklist for any rodenticide users that think they might be encountering resistance issues in rodents. Answer the questions carefully and you’ll get tailored advice on your particular rodent problem.

checklist.rrac.info

As you go through the checklist you might be invited to send your rodent tail to Reading for resistance testing.

The sampling process is very easy and simply involves collecting very fresh tails from recently trapped house mice and sending them to Reading. However, before this happens you should go through the RRAG checklist and get full details of the procedure by contacting Emily Coan.

e.e.coan@reading.ac.uk

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