Potentiation of warfarin toxicity to roof rats (Rattus rattus) by L-histidine and by vitamin K adsorbers

The efficacy of warfarin alone or with various additives was tested against roof rats (Rattus rattus). Warfarin alone at 50 mg kg^?1 of bait killed 37% of the roof rats where-as the additives, L -histidine (40 mg kg^?1 of bait), activated-clay, charcoal and carbon (10 g kg^?1 of bait), tested individually, produced no mortality. However, when warfarin wascombined with each of these additives in turn, only the L -histidine combination resulted in 100% mortality while the other combinations killed 88, 75 and 63% of the test rats respectively. Further trials of paired comparison tests were carried out in order to evaluate the efficacy of the warfarin/L -histidine combination under different environmental conditions. All the rats succumbed to this combination in both cage and rattery conditions when there was no alternative food. Mortality decreased to 60% in cage and 70% in rattery when alternative food was provided alongside the poison bait. Rats tended to die more quickly with the L -histidine combination than with warfarin alone. Field trials also confirmed that this combination was effective in reducing roof rat infestations as shown by the yield of corpses, the post-poison bait intake and post-operational surveillance.