ANTICOAGULANT RESISTANCE INTENSIFIED IN MICE AND RATS

[SURVEILLANCE BY the Campaign for Responsible Rodenticide Use (CRRU) UK has found genes for resistance to anticoagulant rodenticides in 78% of rats and 95% of house mice. According to CRRU chairman Dr Alan Buckle, they include ‘small but troubling numbers’ with two or more such genes, labelled ‘hybrid resistance’. In both rats and mice, the geographical distribution of both single-gene and hybrid resistance continues to spread.

In UK house mice for the first time, the latest annual study, published in December, also identifies a new four-component ‘spretus’ resistance strain in Hertfordshire.

“This was acquired by house mice in Spain through interbreeding with the Iberian mouse species, mus spretus ,” says Dr Buckle. “The Hertfordshire spretus mice have almost certainly come in from the continent. More generally, London is now a clear hotspot for mice with both single-gene and hybrid resistance.”

In rats during the two sampling years 2020-22, the surveillance also found a proliferation of one resistance gene in particular, Y139C, to 30 widely-spread new sites in England. It is one of the three most severe rat resistance genes, each of which is now being found in previously low incidence or resistance-free areas.

Dr Buckle warns that these latest findings suggest there are now few places in England with pest rodent populations wholly susceptible to anticoagulants.

“Continued use of anticoagulant rodenticides against resistant rats or mice has serious downsides: incomplete control with consequent ongoing threats to human and animal health; faster geographical spread of surviving resistant individuals; and lengthy survival of resistant pests carrying persistent anticoagulant residues that could be taken as prey by predators.”

CRRU’s urgent guidance to farmers, gamekeepers and pest controllers is to monitor the resistance status of their own locations.

Under contract to CRRU, annual DNA testing for rodenticide resistance genes is carried out by the government’s Animal and Plant Health Agency, with data analysis and reporting by the University of Reading. The report is available at thinkwildlife.org/downloads q