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Ask the Vet: Vigilance needed to guard against heartworm disease

Dr. Amanda Bruce

Dog owners try to protect their animals from many kinds of parasites, but the recommendations for prevention, testing and treatment are often evolving. It all can become quite confusing.

One of our jobs as veterinarians is to be aware of the best thinking in our business and then relay that information to pet owners clearly.

This year, the American Heartworm Society updated its prevention recommendations again. So I want to focus on heartworm disease, telling you what this harmful and often fatal disease is, and sharing recommendations for how to test for it and prevent it.

Mature heartworms live in the lungs and sometimes in the hearts of infected dogs. Heartworm disease starts when an adult female heartworm produces offspring in a dog’s blood. When a mosquito feeds on an infected dog, the heartworm young also infect the mosquito. The larvae mature, and when the mosquito takes a blood meal from another dog, it leaves behind larvae that travel through the bite wound to mature in the new dog, starting the process all over again.

There is a silver lining to our long, cold winters in the Twin Ports. Heartworm larvae require temperatures above 57 degrees Fahrenheit to mature in mosquitoes. Because of this, veterinarians practicing in our area see very few heartworm infections in dogs.

But the newest heartworm recommendations discuss a phenomenon called “heat islands.” These microenvironments are found around buildings and parking lots where temperatures may be high enough for heartworm larvae to mature in mosquitoes, even when the rest of the world around it would be too cold. That’s why we need to continue to be aware of the heartworm risk, even in our cooler climate.

It takes about six months following a mosquito bite for heartworms to mature and start producing young. This is why we don’t conduct heartworm testing on puppies until they are at least 7 months old. A heartworm test involves taking a blood sample from a dog and looking under the microscope for evidence of heartworm infection.

Preventing heartworm disease is safe and easy. We can give our dogs a variety of medications to prevent infection, usually pills or tablets administered on a monthly basis.

I often get questions from pet owners looking for herbal therapies to treat disease. The American Heartworm Society does not report any safe or effective herbal methods to prevent or treat heartworm disease.

Historically, we recommended heartworm preventatives for a six-month period starting in May. But the new recommendations call for giving our dogs medication year-round. These medications not only prevent heartworm disease but also control other intestinal parasites, which can be a risk at all times of the year.

In recent years, veterinarians across the country have found subpopulations of heartworms that are resistant to some of the prevention medications we have been using. Also, more owners are traveling with their dogs, and shelters are transferring dogs in and out of different parts of the country. All of these factors have combined to keep heartworm infection a risk, even in areas like ours that typically have a low incidence of the disease.

Many owners are tempted to stop heartworm testing on a yearly basis when their dogs are on year-round preventative. But the American Heartworm Society also strongly urges against this.

The unfortunate truth is that heartworm infection is a problem we continue to see worldwide even though we have effective preventatives. The key for dog owners is to continue to be vigilant and not let the guard down for our pets with something as dangerous as heartworm disease.

Dr. Amanda Bruce is owner of PetCare of Duluth, 2701 W. Superior St., Suite 102, Duluth. You can reach her or ask questions for future columns at or 218-461-4400. For more information about this subject or pet care in general, go to