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.