Published April, 2010

Many horse owners do not realize that baits used to kill mice, rats, and small rodents are also poisonous to horses and other mammals. Several types of rodenticide baits are available, but the most commonly used products are the anticoagulant rodenticides, which prevent clotting of blood. The more common ones include brodifacoum, diphacinone, and warfarin.

Anticoagulant rodenticides cause a depletion of blood clotting factors leading to uncontrolled bleeding and death. Bleeding can occur anywhere in the body and may not be evident externally. Clinical signs usually start two to five days after exposure and are dependent upon where bleeding occurs in the body. Signs can include depression, weakness, pale membranes, bleeding from gums or nostrils, blood in urine or feces, trouble breathing, lameness, abdominal distension, and external swellings.

Products come in a variety of formulations such as treated grains or grain-based products, blocks, pellets, granules, place-packs, powders, and bars. They also come in many different colors, including green, blue, red, and tan. Products are often flavored to tempt rodents, usually with grains or flavors such as peanut butter--all of which are also very attractive to horses and other animals. Products can be purchased at any farm and ranch supply, home-and-garden retailer, or grocery store.

The newer anticoagulant rodenticides, such as brodifacoum, are much more potent than older products such as warfarin, and ingestion of much smaller amounts can cause poisoning. The minimum toxic dosage for each of the rodenticides has not been determined in horses. Most clinical cases of anticoagulant rodenticide poisoning in horses occur when the animals gain access to storage areas that contain baits.

Treatment of horses exposed to anticoagulant rodenticides can be very successful, especially if the exposure was witnessed and treatment is initiated before clinical signs occur. Contact your veterinarian immediately if you suspect your animal has had access to a rodenticide bait. Keep all rodenticide products securely out of reach of horses and other domestic animals or children. Be aware that rodents can move baits to nests or other areas. Also be aware that dogs and cats are at much greater risk of poisoning from rodenticide products than are horses, due to their much smaller size. Lastly, consider using a good barn cat to provide rodent control rather than using poisonous rodenticide products.

Cynthia Gaskill, DVM, PhD, clinical veterinary toxicologist at the University of Kentucky Livestock Disease Diagnostic Center, provided this information.