Breeding experiments determined a dominant autosomal warfarin-resistance gene on chromosome 1 in Norway rats. According to their origin and resistance properties several geographically distinct resistant strains were described in the UK, Denmark, Germany and the USA. These were originally designated Scottish-, Welsh-, Hampshire-, Muensterland/Westphalia-, Jutland- and Chicago-type resistance, thus identifying different resistance alleles in geographically distinct Norway rat populations. This was later confirmed by the detection of the gene vitamin K epoxide reductase complex subunit 1 (VKORC1), a gene encoding an anticoagulant-sensitive component of the VKOR. Sequence variants leading to amino acid substitutions were found in this gene in rats as well as in house mice. A number of region-specific sequence variants developed independently in this gene, each conferring a certain level of resistance to anticoagulants. Among the variants with confirmed impact on the resistance status, mutations at position 139 of the gene are the most frequent.
The most widespread variants in Norway rats, with confirmed impact on warfarin and at least some other anticoagulants, are:
Tyr139Cys (Y139C): prevailing in Denmark and Germany and found so far in parts of the Azores, France, Hungary, The Netherlands and several parts of the UK.
Tyr139Phe (Y139F): prevailing in France and Belgium and also found in The Netherlands, in UK and outside Europe in South Korea.
Tyr139Ser (Y139S): conferring ‘Welsh-type resistance’, is known only from Wales.
Leu120Gln (L120Q): known in UK from Hampshire and Berkshire and now more widely across southern England, was also found in some places in France and in Belgium.
Leu128Gln (L128Q): the mutation conferring ’Scottish-type resistance’ was found in Scotland, northern England and in a few locations in central France.
Arg35Pro (R35P): marking Chicago-type resistance, was found in rats from the Chicago/USA-area and in Europe in one location in central France only. The biological role of this polymorphism remains un-clear.
Results of laboratory and field studies indicate that most of the genetic resistance variants confer practical resistance to first generation anticoagulants. Single nucleotide polymorphisms (SNP) at VKORC1 position 120 and 139 also impair the efficacy of bromadiolone and difenacoum (except for Tyr139Ser where this impairment is insufficient to cause practical treatment failure). In Rattus norvegicus there is currently no evidence that the highly potent compounds brodifacoum, difethialone or flocoumafen may be resisted.
The biochemical mechanism of anticoagulant resistance has been studied in several geographic strains/VKORC1-variants of the Norway rat. Amino acid substitutions in the VKOR seem to alter its structure and function, resulting in decreased sensitivity to anticoagulant inhibition, depending on strain characteristics. Studies showed that sequence variants at Tyr139 decrease sensitivity against warfarin to varying degrees while at other positions they dramatically reduce VKOR activity. It was hypothesized that these sequence variants, in addition to generating structural changes in the VKOR protein, may also induce compensatory mechanisms to maintain blood clotting.
Anticoagulant resistance may be accompanied by disadvantageous effects like an increased dietary requirement for vitamin K, or the promotion of arterial calcification. Such physiological fitness costs that would usually decrease the incidence of resistance in rodent populations in the absence of anticoagulant selection can be compensated for by a diet rich in vitamin K, as found in animal feeds that are frequently supplemented with vitamin K3.
Vitamin K levels that occur naturally in the diet are too low to act. as an antidote to anticoagulants.
Table 2: Three of the most important polymorphisms of the VKOR proven to induce resistance to anticoagulants in Norway rats (Rattus norvegicus), and resistance factors in male and female resistant rats, based on BCR data. Also given are the ED50 values for males and females in mg/kg bodyweight of the susceptible baseline strain. The data in this table are the result of work funded by RRAC and conducted by Dr C Prescott and Mr D Rymer (the University of Reading, UK) and Dr A Esther, (Julius Kuehn Institute, Germany).
|VKOR||Resistance factors in male/female homozygous rats|
|Susceptible strain ED50: (males/females)||Bromadiolone||Difenacoum||Brodifacoum||Flocoumafen||Difethialone|
|0.47 / 0.62||0.65 / 0.79||0.22 / 0.23||0.29 / 0.34||0.43 / 0.49|
|L120Q||10 / 14||4.8 / 12||2.8 / 6.7||2.5 / 3.2||2.2 / 2.3|
|Y139C||17 / 15||1.6 / 2.9||1.2 / 1.8||0.8 / 1.0||0.5 / 0.8|
|Y139F||7 / 9||1.4 / 1.9||1.3 / 1.3||1.0 / 1.0||0.9 / 0.8|
Table 4: Polymorphisms of the VKOR, and compounds recommended (+) to control these strains of the Norway rat (Rattus norvegicus), based on BCR data and field trials. Products containing rodenticides marked with (-) shall not be used to control respective strains.
|VKOR||Compounds recommended (+) and not recommended (-) for control|