Fuzzy: a genetic marker for warfarin resistance in the Norway rat

No recombination was observed between fuzzy (fz) and warfarin resistance (Rww) in 245 rats from a backcross test for linkage. This locates fz very close to Rw in linkage group I. The map position established for fz makes this allele particularly suitable for use in combination with p (pink-eyed dilution) for linkage tests in group I and as a genetic marker for the Rw locus in studies of warfarin resistance. The map position of fz in the rat corresponds to that of fr (frizzy) in the mouse, suggesting homology between these loci in the 2 species.

Field trials of second-generation anticoagulants against difenacoum-resistant Norway rat populations

Trials of rodenticidal baits containing 50 p.p.m. difenacoum, 50 p.p.m. bromadiolone or 20 p.p.m. brodifacoum were carried out on farmsteads against populations of Rattus norvegicus containing difenacoum-resistant individuals. Six difenacoum treatments failed in 14--42 days of baiting. Two treatments with bromadiolone succeeded in 23 and 33 days, but four further treatments lasting 35--56 days failed to eradicate the populations. Brodifacoum gave virtually complete control of six populations in 21--73 days and of the ten residual populations left behind by the other two compounds, after baiting for a further 11--85 days. The performance of both bromadiolone and brodifacoum was well below that reported by previous investigators, indicating the possibility of low-grade resistance to these compounds in the difenacoum-resistant strain.

Multiple allelism at the locus controlling warfarin resistance in the Norway rat

Two warfarin-resistant strains of the Norway rat, Rattus norvegicus, derived independently from wild populations in Wales and Scotland and both homozygous for a major gene at the warfarin-resistance locus, Rw, were found to differ in their hypoprothrombinaemic response to simultaneous dosage with warfarin and vitamin K. The Welsh strain gave a small response and the Scottish strain a large response. These two response levels segregated in Mendelian fashion in various crosses involving the two resistant strains and a third, non-resistant strain. This indicates that the Rw locus has a series of three multiple alleles, denoted Rww, Rws and Rw+. The results are discussed briefly in relation to biochemical, ecological and evolutionary aspects of warfarin resistance.

Wavy: a new recessive rexoid mutant in the Norway rat

A new rexoid mutant is reported for the Norway rat, Rattus norvegicus, designated as wavy (symbol wv) and inherited as an autosomal recessive. Breeding data indicate that wavy is not linked with the known rexoid variant, rex (Re) or with genes in linkage groups I and IV. Additional data indicate that rex is not linked with the linkage group IV gene, nonagouti (a) and confirm that it shows dominant autosomal inheritance. Since both wavy and rex are of full penetrance and viability and may tag new linkage groups, they should be useful as markers in linkage studies.

Warfarin resistance: a balanced polymorphism in the Norway rat

The frequency of monogenic resistance to anticoagulant rodenticides in Rattus norvegicus in an area straddling the England–Wales border was monitored from 1967 until 1975. Rats were trapped on farms and tested in the laboratory by administering a dose of warfarin lethal to susceptibles. The mean incidence of resistance was 44% and did not change significantly, despite the extensive use of anticoagulants by farmers during the 9-year period. In 1975 more refined techniques showed that the frequencies of susceptible (SS) and resistant (RR) homozygotes were significantly below the Hardy–Weinberg expectations and simple estimates of the relative fitness ratios for the RR, RS and SS phenotypes were 0·37, 1·0 and 0·68 respectively. In two relatively isolated valleys, where selection with anticoagulants was minimal, the frequency of resistance decreased significantly from 57% to 39% during 1973–5. The results are consistent with the hypothesis that a balanced polymorphism is being maintained. Selection against susceptible homozygotes by the use of anticoagulant rodenticides, and against the resistant homozygote due to its high susceptibility to a primary deficiency of vitamin K gives the heterozygotes a selective advantage. A number of ecological factors that influence the incidence of the resistance are discussed briefly.

Unifactorial inheritance of warfarin resistance in Rattus norvegicus from Denmark

Wild populations of the Norway rat, Rattus norvegicus, in Jutland have been known to be resistant to the anticoagulant rodenticide warfarin since 1962. The inheritance of the resistance was investigated in the F1, backcross and intercross. The results are consistent with the resistance being due to a major gene at the Rw locus. Resistant homozygotes, heterozygotes and susceptible homozygotes appeared to be distinguishable experimentally on the basis of differences in their susceptibility to vitamin K deficiency. The results are discussed in relation to previous studies of the inheritance of warfarin resistance in rats.

The susceptibility of Tatera indica, Nesokia indica and Bandicota bengalensis to three anticoagulant rodenticides

Three South-Asian rodent past species were tested for susceptibility to anticoagulant rodenticides. Wheat fluor containing 0-025% warfarin 0-0375% coumatetralyl or 0-005% difenacoum was fed to 260 Tatera indica, 140 Nesokia indica and 81 Bandicota bengalensis for 1-56 days. Tatera was about as susceptible to anticoagulants as Rattus has been reported to be. Nesokia and Bandicota were extremely variable: though the majority were highly susceptible, the slopes of the dose-mortality curves were close to zero. The difenacoum diet appeared to be more toxic than the warfarin diet to all three species, but less toxic than the coumatetralyl diet to Tatera and Nesokia. All of the anticoagulants were eventually lethal to all of the animals tested.

Inheritance of Scottish-type resistance to warfarin in the Norway rat

The inheritance of resistance to the rodenticide, warfarin, in the Norway rat, Rattus norvegicus, derived from a wild rat population in Scotland was studied in the backcross, intercross and testcross. The resistance was found to be due to a major gene with about the same map position in Linkage Group I as the warfarin-resistance gene, Rw2, which occurs in the wild rat population in Wales. In heterozygotes, the Scottish resistance gene, unlike the Welsh gene, is incompletely penetrant in expression, though the penetrance was found to increase markedly in response to selection. Differences between the Scottish and Welsh types of resistance suggest that the two resistance genes are allelic.

Laboratory evaluation of gophacide as a rodenticide for use against Rattus norvegicus and Mus musculus

Laboratory tests were carried out to assess the efficacy of gophacide as a rodenticide against the Norway rat (Rattus norvegicus) and the house mouse (Mus musculus). Results of feeding tests with wild animals suggest that the compound would be more useful against mice than rats, and that 0.3% would be a near optimal concentration for field trials for both species. The hazards of using gophacide as a rodenticide are discussed.

Some properties of calciferol as a rodenticide

The potentiality of calciferol (alone and combined with warfarin) for the control of commensal rats and mice has been examined in the laboratory. Nearly all animals fed on 0.1% calciferol for 2 days died. Though illness usually reduced food intake after the first 24 hr. there was no sign of aversion to the poison at 0.1% - which is considered to be the lowest concentration suitable for use against Rattus norvegicus, R. rattus and Mus musculus in the field. There was some indication that resistance to warfarin in R. norvegicus may be correlated with susceptibility to calciferol. Toxicity tests with calciferol combined with warfarin indicated an additive effect between the compounds. No evidence for synergism was found however, although elsewhere there is some evidence for this.

Laboratory tests of 5-p-chlorophenyl silatrane as a rodenticide

The properties of 5-p-chlorophenyl silatrane as a rodenticide against Rattus norvegicus and Mus musculus were investigated in the laboratory. The high oral toxicity of the compound was confirmed. When the compound was given to laboratory rats and mice by stomach tube at lethal dosages, signs of poisoning were observed within a minute. When caged wild rats and mice were given a choice between plain and poisoned baits the optimum rodenticidal concentration in the bait was about 0.5% for both species, producing 50% mortality in wild rats and 95% mortality in wild mice. The results are discussed in relation to safety in use and the probable effectiveness of the compound as a rodenticide in field conditions.

Warfarin resistance and vitamin K requirement in the rat

The development of vitamin K deficiency, vitamin K requirement and warfarin susceptibility were studied in several types of warfarin-resistant and non-resistant rat. Domesticated HW strain rats needed about 13 times as much vitamin K1 as did Wistar rats to maintain normal blood-clotting function, while HS strain rats were intermediate in this and in their susceptibility to warfarin. The vitamin K requirement of Wistar hybrids resembled that of the Wistar parent or was intermediate. Hypoprothrom-binaemia was induced at least as readily in non-resistant wild rats as in HS rats. A new diet suitable for inducing vitamin K deficiency in conventionally maintained Wistar rats is described, and it is suggested that the HW strain could be used for vitamin K bioassay. The genetical basis of warfarin resistance is discussed.

Some rodenticidal properties of coumatetralyl

The toxicity and palatability of coumatetralyl (3-(α-tetralyl)-4-hydroxycoumarin) to rats (Rattus norvegicus Berk.) were investigated in the laboratory by means of feeding tests. Animals resistant to warfarin (3-(α-acetonylbenzyl)-4-hydroxy-coumarin) and warfarin-resistant rats from infestations refractory to coumatetralyl, as well as non-resistant animals, were employed in the tests.

Medium oatmeal containing a concentration of 0·1% coumatetralyl was not markedly less palatable than the same food unpoisoned. In comparison warfarin at 0·05% but not at 0·025% was significantly less readily eaten than the plain food. Coumatetralyl at 0·05% and 0·005% was about as toxic as 0·005% warfarin is reported to be to non-resistant rats. Warfarin-resistant rats were significantly less susceptible to coumatetralyl than were non-resistant rats. Warfarin-resistant rats from an infestation refractory to coumatetralyl were significantly less susceptible to coumatetralyl than were animals from other sources.

It is considered that coumatetralyl at concentrations of the order of 0·05% in bait would be a good alternative to warfarin against non-resistant rats. While it would be expected that, at this concentration, coumatetralyl would often give good results against warfarin-resistant infestations, this use might eventually produce an increase in the incidence of resistance to both anticoagulants.

We thank Baywood Chemicals Ltd., for supplying the coumatetralyl used in this work. We are also indebted to several colleagues for supplying wild rats and information on field investigations and to Messrs G. Snell and P. Romer for technical assistance.

Some laboratory observations on the toxicity and acceptability of norbormide to wild Rattus norvegicus and on feeding behaviour associated with sublethal dosing

The median lethal dose of orally administered norbormide for wild Rattus norvegicus found to be 9·0 mg./kg. of body weight and the LD 95 about 17·0 mg./kg.

In tests in which various concentrations of norbormide or zine phosphide were added to the food of individually caged wild rats, mortality increased with concentration of poison, though more slowly with norbormide than with zine phosphide. The mortality that occurred among rats offered a choice between unpoisoned food and the same food with added norbormide or zinc phosphide indicates that in control treatments in the field the optimum concentration of norbormide in bait would be about 0·;5% and that this might be expected to give results comparable with those obtainable with 2·;5% or 5·0% zine phosphide. Other methods of estimating suitable field strengths indicate that concentrations of norbormide higher than 0·;5% may be preferable.

Some animals that survived exposure to a choice of plain food and the same Food poisoned with norbormide or zine phosphide at field concentrations avoided mating lethal amounts by reacting to the taste of the poison. Others learned to use the taste of the food, not that of the poison as a cue and later avoided eating the food when it contained no poison.

When either poison was presented to rats in the more palatable of two foods in the choice situation mortality was relatively high. Some of the surviving animals subsequently rejected the more palatable food in preference to the normally less palatable alternative.