24 October 1997

Rat resistance found by CSL unexpected

WARFARIN-RESISTANT rats have been found on two-thirds of pig and poultry units sampled by the Central Science Laboratory since 1995.

Researcher and team leader, Alan MacNicoll, cautions that once warfarin resistance occurs, resistance to second generation anticoagulants, all of which have the same mode of action, could follow.

The discovery of resistance on two-thirds of farms with sizeable rat populations was unexpected.

From 1995, sampling has been concentrated on pig and poultry units outside the known hot spots of resistance. Earlier work showedhigh levels of vitamin K supplemented feed might enhance selection of anticoagulant resistant rats.

The development at CSL of a Blood Clotting Response test, which enabled sequential testing of rodenticides, has identified the problem of cross-resistance.

Since the introduction of the test, resistance to bromadioline has been found to be more widespread than that to difenacoum, although difenacoum resistance has been found in central southern England – an area where a small number of rats displayed high resistance to several of the anticoagulants.

A first case of resistance to the highly potent flocoumafen has been recorded on a farm in Berkshire in 1996. Information obtained from these laboratory studies will enable annual monitoring of the extent of rodenticide resistance across the country.

Advice can then be given to farmers and pest controllers on the management of anticoagulant resistant populations. Alternative treatment with the different anticoagulants can be beneficial in preventing the widespread development of resistance.

New rodenticides are needed – but they are not there. Anticoagulant resistance is a problem which needs to be addressed. Farmers do not complain enough when their efforts to control rat populations are not working. &#42