In the 1940’s, with the development of warfarin, a new class of rodenticides became available which substantially improved chemical control of rodents while being less hazardous than some older acute rodenticides. These new compounds are anticoagulants and their mode of action involves reducing the ability of blood to clot so that exposed animals bleed internally and die.
Anticoagulants act relatively slowly compared to most acute rodenticides; rodents typically die several days after initial ingestion if anticoagulant consumption has been steady. The usually slow onset of undramatic toxic effects allows anticoagulant baits to be formulated with very low concentrations of active ingredient, which avoids their being repellent. Typically, rodents feed repeatedly on the rodenticide bait without becoming “bait shy”. In the case of warfarin and other so-called first-generation anticoagulant baits, multiple feeding over several days is usually necessary before a lethal dose accumulates in the rodent.
If the poisoning is identified or diagnosed early, the slow action of the first-generation anticoagulants allows more time for treatment of poisoned non-target species than with most non-anticoagulant materials. Most important, vitamin K1 is an effective antidote for anticoagulant poisoning. For these reasons, and because of their effectiveness, anticoagulants have become the most widely-used type of rodenticide. An estimated 95% of all chemical control of commensal rodents in the United States is now conducted with anticoagulants.