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My Dog Is Heartworm Positive: Signs, Diagnosis, and Treatments

If heartworm happened to my dog, it can happen to yours.
By Julia Lane, September 2021, Updated December 2021
Dalmatian dog with heartworms

My vet can't remember the last time she had a heartworm positive case. Until now. My 8-year-old Dalmatian, Jolie, tested positive for heartworms at her annual check up last week. We retested the blood in hopes that it was a false positive. But there was no need to send the sample back to the lab. Through a microscope, my vet could see microfilaria swimming in her blood sample.

I’m shocked and upset. My husband and I take excellent care of our dogs. How could this have happened? 

What is Heartworm in Dogs?

Heartworm in dogs (dirofilariasis) is a serious and potentially fatal disease caused by a parasitic Dirofilaria worm that grows up to 14 inches long. These worms grow to live in a poor infected dog’s heart, lungs, and pulmonary artery. Mosquitos infect dogs with heartworms through their bites, so warmer areas are more prevalent with this pest.

Heartworms can live up to five years and can produce millions of offspring. This disease causes long-term damage to a dog’s internal organs, so it’s crucial to use heartworm prevention medications. So, you are probably wondering, how can you tell if a dog’s got heartworms?


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  • Mild persistent cough
  • Shortness of breath
  • Fatigue
  • Resistant to exercise
  • Decreased appetite
  • Swollen belly, due to fluid accumulating in the abdomen
  • Nose bleeds
  • Sudden death

How is Heartworm Disease diagnosed?

So, where did I go wrong? Apparently, despite living in the Chicago area, we needed to give our dogs heartworm preventative through the winter, not just the warmer months. When we lived in New Orleans’ subtropical climate, it was a given that the dogs received heartworm preventative year-round.

In our case, we didn’t notice any significant changes in behavior. But, before administering preventative medication, vets will perform a simple blood test. If your dog tests positive, as our dog did, additional testing will be required to determine the severity of the infection. In some cases, your vet may need to perform an ultrasound to determine your dog’s ability to tolerate treatment. 

If your dog is showing any unusual behavior or listlessness, head to your vet to follow up. The American Heartworm Society recommends a dog be tested annually. Doing so will ensure the infection can be detected quickly, minimizing damage caused by the disease.

How Risky is the Treatment?

By the time most dogs show signs of heartworm, they are typically already in advanced stages. Due to the life cycles of the worms, it takes anywhere from five to seven months for young heartworm to become detectable by a blood test.

Dr. Ashley Saunders, a veterinary cardiologist and professor at the Texas A&M College of Veterinary Medicine & Biomedical Sciences explains, “The recovery rate with treatment depends on how bad the disease is at the time it is treated and which treatment protocol is used,” She continues, “Dogs can recover following treatment, but those with severe damage may have long-term clinical signs or develop them at a later date.” 

There are three stages of heartworms—adults, young adults, and microfilariae larvae—each requiring a unique treatment. Your vet will prescribe the best course of action. In addition, antibiotics and steroids will likely be necessary. Depending on the severity of the disease, the outlook may be grim. Some pets may even require a life-long regimen due to these horrible worms, and, in other cases, dogs may only have mere months to live.

What seems particularly unfair is that Jolie has already been through a lot. We adopted her through a Dalmatian rescue when she was ten months old. She had been abandoned by her family, left in a backyard without food, water or shelter. She was emaciated, infested with fleas, and hung her head, too sad to lift her eyes to meet ours. She didn’t know how to play. Our older Dalmatian, Darby, helped her come out of her shell. She needed surgery, rehab, chiropractic, and supplements. We helped her get well.

“Heartworm preventatives come in all forms that can be given orally as a tablet or beef chew, topically on the skin, or as an injection. There are also options for pets with food allergies or when it is difficult to give oral medications,” Saunders said. “Most preventatives also include medications against various other parasites, like hookworms, roundworms, fleas, ticks, and others. Consult your veterinarian to identify the best preventative for your pet.” 

To think that for less than $50, we could’ve given her a few more doses of Heartgard (low-dose ivermectin) and kept her free of heartworms and the risky, expensive treatment required to kill them. It all makes me sick to my stomach.  My poor girl has been through enough, and now this. 

Despite the growing trend to keep toxins to a minimum in our dogs (and for a good reason), please give your dog monthly heartworm preventative year-round.  The risk is not worth it.

Photo: Bethany Ferr from Pexels

Julia Lane owns Spot On K9 Sports, a training facility in the Chicago area, and offers online dog-sport coaching. She is the author of several travel books, and her byline has appeared in The New York Times Magazine, Poets & Writers and elsewhere.