Some rodenticidal properties of coumatetralyl

The development and use of a test to identify resistance to the anticoagulant difenacoum in the Norway rat (Rattus norvegicus)

Feeding tests were carried out in the laboratory to obtain basic data on the susceptibility of wild Norway rats to difenacoum. The results were used to derive a standard test procedure for the identification of difenacoum resistance in warfarin-susceptible and resistant rats. Details are given of tests on rats from suspected difenacoum-resistant infestations on farms.

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.

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.

Anticoagulant-resistant rats: possible control by the use of the chloro analog of vitamin K 1

Strains of wild rats that are resistant to the anticoagulant action of coumarins and derivatives of indandione have been discovered in a number of geographic areas. These rats have now been shown to be more susceptible than normal rats are to the anticoagulant action of the vitamin K antagonist, 2-chloro-3-phytyl-1,4-naphthoquinone. This compound, either alone or in combination with warfarin, would appear to be an effective rodenticide in areas where resistance to the indirect anticoagulants is a problem.

Rodent resistance to the anticoagulant rodenticides, with particular reference to Denmark

Inherited resistance to anticoagulant rodenticides was discovered in populations of Rattus norvegicus about 14 years ago. Similar resistance has now been reported from several countries in north-western Europe and from the USA. In order to detect resistance and to control it effectively, basic data on the susceptibility of rat populations are required for each country, and trapping surveys should be made in any area where resistance is suspected. Acute poisons are needed to control resistant rats although the shift from anticoagulants to acute poisons is a retrograde step as far as efficiency is concerned, and increases the hazard of control operations to man and other animals. Resistance to anticoagulants in Mus musculus has been reported from England, and resistant mice are probably to be found in other countries also in view of the great individual variation in susceptibility of this species to these rodenticides.