Bromadiolone resistance does not respond to absence of anticoagulants in experimental populations of Norway rats.

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Resistance to anticoagulant rodenticides in Norway rats (Rattus norvegicus) is documented to be associated

with pleiotropic effects, notably with an increased dietary vitamin K requirement. The aim of this study was to quantify

these effects in small populations of Norway rat in Denmark and to see how bromadiolone-resistant phenotypes are

manifested when bromadiolone selection is absent. Experimental populations were established under semi-natural

conditions with wild rats trapped at two Danish farms. The individuals caught on each of the two farms were divided

into two experimental groups. One group was regularly exposed to bromadiolone whereas the other group was

untreated. The level of bromadiolone resistance in the experimental populations was followed for two years. The

results presented here are those results obtained in the absence of bromadiolone selection.

The pleiotropic selection against resistance in the two non-treatment populations was found to be insignificant.

Thus, absence of anticoagulant, under the environmental conditions provided, did not lead to a selection favouring

anticoagulant-sensitive rats. However, we found some evidence of selection against presumed homozygous resistant

rats under non-anticoagulant conditions. Haemorrhagic symptoms are not only observed in sensitive rats exposed to

anticoagulants, but are also a symptom for severe vitamin K deficiency in resistant rats. This suggests that bromadiolone

resistance leads to loss of fitness, albeit that the cost is not strong enough to reduce the phenotypic resistance

level or minimise the effect of random genetic drift.

Original languageEnglish
Title of host publicationRats, Mice and People: Rodent Biology and Management
EditorsG.R. Singleton, L.A. Hinds, C.J. Krebs, D.M. Spratt
Place of PublicationCanberra
Publication date2003
ISBN (Electronic)1 86320 357 5
Publication statusPublished - 2003

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