It has now been shown over some fifty years that the use of anticoagulant rodenticides forms the most effective method of controlling commensal rodent populations.

The continued use of these anticoagulant rodenticides has, however, led to the development of resistance in commensal rodent species, the Norway rat Rattus norvegicus, the roof rat Rattus rattus, and the house mouse Mus musculus1. Resistant strains of the Norway rat may be restricted to certain geographical regions. Resistant mouse strains cannot be allocated geographically. Often, the occurrence of resistance is connected to certain conditions, such as the presence of livestock-feed with high content in vitamin K3, industrial infrastructure, and the continuous use of anticoagulant rodenticides with poor practice. However, in some cases the reasons for development of resistance cannot be attributed with certainty.

Remember, resistance is characterised by the ability of individuals within a rodent population in the field to continue feeding on the anticoagulant bait over many weeks

Continuous feeding from anticoagulant baits may not only be due to resistance, but may also be caused by under-baiting or immigration. However, once these alternatives have been eliminated, the probability that the cause of the continued feeding activity is anticoagulant resistance is high.

From the point of view of those undertaking practical rodent control, the term Practical Resistance is used to identify resistance that has led to the difficulty to control rodents in field situations.

"Anticoagulant resistance is a major loss of efficacy in practical conditions where the anticoagulant has been applied correctly, the loss of efficacy being due to the presence of a strain of rodent with a heritable and commensurately reduced sensitivity to the anticoagulant." Greaves, 1994

There are other scientific definitions of the term resistance.

This document on rodenticide resistance provides understanding of the nature of anticoagulant resistance, introduces data on known resistant strains and provides help to those wishing to recognize resistance and manage it.

Figure 1: Example of progress of an anticoagulant treatment in terms of bait consumption during a 35 day baiting. Within a population, susceptibility varies individually, and therefore some individuals may survive a few days longer. This natural range is indicated by red and green ranges. However, individual feeding behaviour can lead to the same effect. With resistant animals, bait consumption will stay at a constant level, and no sufficient control will be possible.

1 Several sub-species and variations exist of Rattus rattus and Mus musculus. There is no evidence that subspecies differ in their susceptibility to anticoagulants.
2 Greaves, J.H. (1994). In: Pelz, H-J. and Prescott, C.V. Chapter 9. Resistance to anticoagulant rodenticides. Rodent Pests and their Control (Buckle, A.P. and Smith, R.H. eds). 2nd Edition, CAB International, Wallingford, UK. pp 187-208.