When do Dogs Need Heartworm Prevention?

Dear Dr. B. What time of the year am I supposed to administer heartworm medication to my dogs? I thought it was sometime in the...


Dear Dr. B.

What time of the year am I supposed to administer heartworm medication to my dogs? I thought it was sometime in the Fall.

Tank you for your time,

Ottawa, ON

Heartworm is a blood parasite of dogs and cats. It is spread by mosquitoes. Heartworms live in the heart and the arteries leading out of the heart. It can cause heart failure and other health problems.

Heartworm is difficult to treat. The most effective medication used to treat heartworm contains arsenic. Dying worms can lead to serious breathing problems, but live worms are even more dangerous.

Although treating heartworm is hard, preventing it is easy. Heartworm preventatives are medications that generally are given once monthly. The most commonly used oral heartworm preventatives are Interceptor and Heartgard Plus. Advantage Multi is a topical heartworm preventative. Revolution, another topical product, appears to be an effective heartworm preventative in cats, but many experts doubt its efficacy in dogs.

Most heartworm preventatives have a fringe benefit: they eliminate intestinal worms, too. Some intestinal worms can spread to humans and cause serious disease, so heartworm preventatives don’t just help dogs and cats–they also help to protect human health. Note that Heartgard does not protect against intestinal parasites, but Heartgard Plus does.

Most heartworm preventatives have excellent safety profiles. The monthly pills are palatable–Buster thinks he’s getting a treat when I give him his Heartgard Plus.

And now, Karen, let’s answer your question. The American Heartworm Society and the Companion Animal Parasite Council recommend year-round administration of heartworm preventatives. That is my recommendation as well.

Since heartworm is spread by mosquitoes and mosquitoes don’t do well in Canadian winters, you may be tempted to stop the medication during the winter. I recommend against that. At least one species of mosquito that spreads heartworm lives primarily indoors and has the potential to spread the disease year-round, world wide.

Your best bet is to administer heartworm prevention to your dog every month.

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