Challenges of Anticoagulant Rodenticides: Resistance and Ecotoxicology

Control of rodent pests worldwide relies heavily on the use of rodenticides. In the 40s’, O’Connor (1948) isolated and first suggested the use of dicoumarol (a naturally occurring substance responsible for the “sweet clover disease” in cattle) as a rodenticide. In the early 50s’, anticoagulant rodenticides (warfarin and later indane-dione derivatives) replaced the acute poisons with great success. It is well known that Norway rats (Rattus norvegicus) are very suspicious to new, unfamiliar items in their environment (neophobia) and may eat only a small, non-lethal dose of a new bait. If they survive, they learn to avoid the bait. This phenomenon is also known as food aversion (Lund, 1972). Neophobia may also extend to bait boxes and traps when first introduced in the rat environment. Unlike rats, house mice do not exhibit neophobia (Bonnefoy et al, 2008). As Anticoagulant (or anti-vitamine K AVK) compounds are cumulative, small amounts ingested daily will eventually lead to the consumption of a lethal dose. In the early 60s’, massive use of these first generation AVK was considered a great opportunity to reduce or even eradicate rat populations from many areas at that time, despite their behavioral traits. First-generation AVK include warfarin, coumafuryl, coumachlor, coumatetralyl, diphacinone, pindone and chlorophacinone (Bentley, 1972). Unfortunately, a first detection of a resistant strain of rats was reported in Scotland in 1958, followed by similar reports in other areas in Europe: Wales, Denmark, the Netherlands, and Germany (Lund, 1972). At the same time, Brooks and Bowerman (1973) tested several strains of Norway rats from New York City and confirmed that warfarin resistance was also common among rat populations heavily treated with warfarin in the US. The World Health Organization (WHO) rapidly recognized this event and suggested some guidelines for the rapid detection of resistant rodent, based on feeding tests (Drummond and Rennison, 1973). Pelz et al. (2005) published a survey of countries reporting resistance in commensal rodents (2005). The major findings are described in Table 1 below. Recently, warfarin resistance has also been reported in another rat species: the Lesser Ricefield rat (Rattus losea) (Wang et al., 2008). Rodent species can develop complex behavioral patterns and one of this is considered to be “behavioral resistance”. There is evidence, for instance in the roof rat, that animals can learn to avoid toxic baits (Leung and Clark, 2005). This complex approach has not been given full attention and will not be developed in this review.