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When should I give my dog heartworm preventatives?

Veterinarians recommend giving dogs heartworm preventative year-round. Dr. Lee discusses the reason for heartworm awareness beyond warm months with concerned dog parents.

When should I give my dog heartworm preventatives?

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.

Editor’s Note: Each year, heartworm affects millions of pets across the country, resulting in often debilitating illness and significant veterinary costs. The American Heartworm Society has designated April as Heartworm Awareness Month.

Lee Pickett, V.M.D. practices companion animal medicine in North Carolina. Contact her at

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