Rodenticides are toxic chemicals used for the control of rats, mice and other pest species of rodents. Poisoned baits are the most generally effective and widely used means of formulating rodenticides, but some are used as 'contact' poisons (such as dusts, foams and gels), where the toxicant adheres to the fur of the animal and is ingested during subsequent grooming, while a few are applied as fumigants to burrows or infested premises. To respond to the development of resistance to acute toxic poisons, these have been replaced by slower acting chemicals, such as anticoagulants, that cause rodents to bleed to death. These second-generation rodenticides have helped control pest numbers since their introduction in the 1980s, but their potential for bioaccumulation means that they have become concentrated in the bodies of rodent predators. Serious declines in populations of predators would curb the rate of natural control of rodents, requiring even more use of rodenticides.
Although toxicity levels of rodenticides may vary between target and non-target species, all poisons must be presumed to be lethal to humans. The hazard is by accidental poisoning because there is no evidence that rat poisons could enter the human food chain. Acute poisons are potentially more dangerous than chronic ones because they are rapid in action, non-specific, and generally lack effective antidotes. Anticoagulants, on the other hand, are slow and cumulative, allowing adequate time for the administration of the reliable antidote, vitamin K. The concentrations of active ingredients in contact formulations of a given poison are higher than those in bait preparations, thus making operator hazard considerably greater. Fumigants present a special danger when used to treat infested premises, holds of ships, etc, and should only be used by trained technicians. The gassing of rodent burrows, although less hazardous, must also be carried out with extreme caution.
In 2000 it was reported that about 30 percent of UK barn owls found on roadside verges ("road-kills") contained second-generation rodenticides. (In 1983 this had been only five percent.) In two percent of cases, the poisons had been directly responsible for the birds' deaths. A similar study of polecats and foxes found that about 45 percent contained rat poisons and that 10 of the 100 foxes had died from the poison. About one quarter of weasels and stoats were similarly affected.