In 1921, ranchers were dismayed at the sudden onset of lossage in their herds due to a strange condition: the animals bled to death. Small cuts failed to heal. The roughage cows eat will scratch their digestive systems, but unlike the normal case where such scratches are minor and readily heal, these scratches failed to heal and the animals died from internal hemorrhaging. What seemed odd was that the animals were being fed hay from fields that appeared not dissimilar from that of previous years. No sudden invasive plants of a poisonous nature had been found.
A researcher by the name of Karl Paul Link, working under the aegis of the Wisconsin Alumni Research Fund (WARF), did a careful analysis of the ensilage from ranches that suffered losses and those that did not. He discovered that a chemical, dicoumorin, found in the ensilage of sweetclover hay from those ranches suffering the losses, was a powerful anticoagulant. Dicoumarin is the result of a substance called coumarin, which is the chemical which gives new-mown hay its characteristic smell, being subjected to the heat and mold in a silo, and forming a double molecule. The year of the serious losses had been an unusually warm one after the ensilage was created. [This information is from Kingsley's book "Poisonous Plants", which is one of the first serious studies of the biology of plant toxins].
Link named this substance after the organization that supported his research. Hence the name warfarin.
In the mid-1940s, this substance was used as a rodenticide. Rats are quite intelligent, and when rats would see a number of dead rats around a poisoned bait (usually some fast-acting poison like arsenic or strychnine was used), they would say "This is a dangerous substance. Avoid it". This is known in the pest control trade as becoming "bait-shy".
However, rats are not all that intelligent. They are not, what is called in the perception trade, "time-binding". That is, a rat seeing another dead rat near some poison-laced bait concludes "This is trouble, avoid it". But a rat that dies three days later from internal hemorrhaging does not communicate "This is the consequence of that great meal I had on Tuesday". So as two rats are munching on warfarin-laced bait, they are talking about poor old Charley who died last week. "Stress, probably. That cat" or "These things just happen. And so young, too", but the association with the fact that he dined on the same bait they are eating is lost on them. So for decades, warfarin has been used as a rodenticide. One advantage of warfarin is that its effectiveness is based on proportion-to-body-weight. A 1kg rat or a 200mg mouse get a fatal dosage that might not be fatal to, and perhaps not even dangerous to, a 15kg dog or 30kg small child.
One of the problems of warfarin is the process that manufactures it tends to leave a number of deleterious impurities in the resulting product. When used as a rodenticide, this is not a major consideration. And these are only dangerous over a sustained period of time. But it was also determined that a good anticoagulant for human beings would be a Good Thing. The product trademarked Coumadin is a form of crystalline sodium warfarin that is created by a process that leaves no such impurities, and therefore is safe for long-term human consumption. Of course, since the patents on Coumadin have expired, many other firms now synthesize the medication, and call it by its generic name, "sodium warfarin". However, do not think that if you are on Coumadin/warfarin therapy that you can substitute rat poison; besides the impurities found in the warfarin used for that purpose, many other potentially fatal ingredients are used to enhance the effectiveness of the rodenticide, and none of these would be healthy to consume.
Warfarin was used in the 1950s as an anticoagulant for victims of heart attacks and strokes, but gained fame when it was used to treat President Dwight D. Eisenhower after his 1956 coronary (while in office).
Here is some more information on Warfarin.
http://ods.od.nih.gov/pubs/factsheets/coumadin1.pdfThere are newly approved alternatives to Warfarin that have no food, alcohol or drug interactions and do not require blood (INR) monitoring. These new drugs have also been shown to be more effective than Warfarin in preventing stroke and or systemic embolism.
