Problems with Mice and Rats; Use Caution with Baits

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Editor's Note: Summer brings in flux of mice and rats into our farms, barns and stables. There are many methods used to control these disease-carrying pests that constantly seem to leave their manure in places we don't want to find it. Unfortunately, some of the "baits" used to kill mice and rats can have deadly effects on other animals, including dogs, cats, humans and horses. There are newer baits that claim to be non-toxic to animals such as cats and dogs that consume the dead mouse or rat. However, these baits are deadly and should be treated as such around your farm. Understand that one female mouse can produce six to 12 babies per litter, and multiple litters per year, perhaps totaling 40 or more babies from that one female. Also understand that a female mouse can start reproducing when she is two months old, so all of those female babies can also have multiple litters in that first year. Now you know why the population of mice in your barn seems to explode!

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 the 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 rodent's body, and bleeding might not be evident externally. Clinical signs usually start two to five days after the rodent eats the bait 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 because of their smaller body mass.

Lastly, consider using a good barn cat to provide rodent control rather than using poisonous rodenticide products.

This information from the University of Kentucky College of Agriculture was written by Cynthia Gaskill, DVM, PhD, a clinical veterinary toxicologist at UK's Livestock Disease Diagnostic Center.