Rodenticides: Warfarin, still a good management tool
Rodents have been a menace to man for generations inflicting billions of dollars of crop and commodity damage each year. Rats and mice serve as reservoirs of numerous diseases transmitted to humans, such as plague, leptospirosis, Lyme disease, and rat bite fever. Millions of people died during the middle ages because of the spread of plague by rat in Europe. Rodent control products have been developed over the centuries including traps, glues, and chemical methods (rodenticides). Initial acute, or fast acting products were introduced and contained chemicals having no antidotes, such as arsenic, ANTU (α-naphthylthiourea), sodium monofluroacetate, strychnine, and norbormide. It was not until the late 1950s that rodent control was dramatically changed by the development and marketing of warfarin. The chemical is classified as an anticoagulant, or blood-thinner, and inhibits the production of vitamin K within the rodent, resulting in death over several days. After warfarin's initial success, other anticoagulants were added to the marketplace, including coumatetralyl, chlorophacinone, pindone, and diphacinone. The compounds came to be known as 'first generation anticoagulants'. These novel rodenticides quickly reduced the use of acute rodenticides which have no antidotes. Beginning in the early 1980s the more toxic 'second generation' chemicals were introduced into the marketplace, including brodifacoum, bromadiolone, and difethialone. The use of the chemicals soon began to diminish the use of the less toxic first generation group. This took place because of the perceived genetic resistance developed in US rodents. The lower dose baits were seen as the newest rodent management success story. As early as 1958 there were reports of warfarin resistance in Scotland in the Norway rat (Rattus norvegicus). After prolonged use of warfarin in the US, resistance was documented (based on WHO criteria) and published. Consequently, more toxic anticoagulants were synthesized to overcome the genetic resistance reports. The rodent control industry over a period of only a few years, moved from the first to second generation rodent baits. The marketing strategy was to: 1) implicate first generation rodenticides as ineffective against rats and mice and, 2) argue that the newer baits could kill rodents in a 'single feeding' (2nd generation rodenticides) compared to the 'multiple feeding' required by the 1st generation products. In the professional pest control industry, the goal was to convince the technician that more bait would be required with the less toxic products. It was economically cost effective to use less bait in a rodent control program. It was a good marketing idea, but in reality the story had its flaws.
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Document Type: Research Article
Publication date: June 1, 2012
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