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Mouse Poisons Kill Pets and Wildlife

By Lee Pickett- Q: I need to put some mouse poison in the basement, but I’m worried my cats may find it. I understand that poisoned mice bleed to death. Can mouse poison have the same effect on cats?

A: Yes, and worse.

photo by Jamie Street

The first rodenticides were anticoagulants, which caused death from internal bleeding. Short-acting chemicals like warfarin became less effective as rodents developed resistance, so companies developed longer-acting, more toxic anticoagulants such as brodifacoum, bromadiolone, difenacoum and difethialone.

These products are still available, but their use has been somewhat restricted, and even more effective — that is, toxic — rodenticides have been developed: bromethalin, cholecalciferol and zinc phosphide.

Bromethalin targets the central nervous system, causing tremors, loss of coordination, seizures, paralysis and death. Cats are 10 times more sensitive to bromethalin than dogs.

Cholecalciferol is actually vitamin D3, which increases calcium levels in the blood and therefore affects all body systems. High levels cause heart problems, kidney failure and death.

Zinc phosphide is converted in the body to highly toxic phosphine gas, which smells like rotten garlic or dead fish. If your cat is exposed, rush to the veterinary hospital with the car windows open, because if you can smell the gas, you are already being exposed to potentially toxic levels. Fortunately, zinc phosphide is the least popular of these rodenticides.

Anticoagulants, bromethalin, cholecalciferol and zinc phosphide kill rodents as well as cats, dogs, raptors, other wildlife and humans. Nontarget species are exposed by eating the bait or through relay toxicosis, which occurs when mice that ingested the toxin are eaten by predators such as cats.

It would be safer to use other forms of mouse control in your home. Cut off the rodents’ food supply by, for example, storing bird seed and pet food in tightly closed metal containers. Plug gaps in your foundation so mice can’t enter.

Place humane traps in the basement, and release the captured mice outdoors. Consider snap traps or electronic traps, or invite your cats to take on the job of rodent control.

Q: I started taking my dog, Stetson, to a new vet, who recommended I give him his monthly heartworm medicine throughout the year instead of only during the warm months as I’d been doing. What do you recommend?
A: I have always given my dogs their monthly dewormer throughout the year, for two reasons. First, most chewable heartworm pills also kill intestinal worms, some of which can infect humans. Second, the mosquitos that transmit heartworms can still bite dogs during the winter.

Let’s look first at the many intestinal worms that can infect the dogs we snuggle with. The most common are roundworms and hookworms, which can cause diarrhea and vomiting in dogs. They also can infect humans, causing blindness, seizures, organ damage and skin problems. Fortunately, most monthly heartworm chewables kill roundworms and hookworms.

Dogs and humans also share some species of tapeworms, which can cause liver and lung problems in humans. Some monthly heartworm preventives kill tapeworms, too.

Whipworms also infect dogs, and they’re hard to identify during fecal testing, so monthly prevention is a good idea. Check to be sure Stetson’s monthly dewormer kills whipworms.

Another reason to give Stetson his monthly dewormer throughout the year is to better protect him from heartworms, which are carried by mosquitos. Just one bite will infect him.

Many people think mosquitos die off during the winter, but parasitologists warn that some survive in “microclimates,” small areas that offer comfortable conditions. In cold states, these could be warm sheds and roof soffits, while in dry states, bird baths and ponds promote mosquito survival.

So take your new veterinarian’s advice. Restart Stetson’s monthly heartworm preventive now, and commit to continuing it year-round.

Lee Pickett, VMD, practices companion animal medicine in North Carolina. Contact her at

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