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To determine the susceptibility of the wild roof rat, Rattus rattus to warfarin, a no-choice feeding test with 0.025% warfarin bait was carried out using roof rats collected from seven buildings of Tokyo in 1978 and 1979,where the rat population could not be controlled with this bait. All colonies from the buildings showed lower susceptibility than our susceptible laboratory colony. For example, 15 to 25% of the rats of two colonies survived for 90 days. It was suggested that the resistance to warfarin might have developed in some rat populations in Tokyo.
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Naganuma et al. (1981) studied this question.
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