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Q&A: Can Dogs Get Heartworms While on Heartgard?

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Dr. Mark is a veterinarian. He has been working with dogs for more than 40 years.

Several areas in the Mississippi valley are home to Heartgard-resistant heartworms.

Several areas in the Mississippi valley are home to Heartgard-resistant heartworms.

Why Did My Dog Develop Heartworm While on Preventative Medication?

"I have a Catahoula Leopard dog born in Mississippi. We found her at a shelter in Indiana and adopted her at a year old. She was on Heartgard when we got her, and we continued it. But in 2020, at 2 years old, she tested positive for heartworms. The company paid for her treatment and we switched to a different medicine.

I had my doubts that she had heartworm since she showed no symptoms whatsoever; I told them to have the test done again, and they told me it came back positive. Apparently there was no explanation as to how that could happen, and they had never had that happen at their office.

Now she is on Simparica, which isn’t cheap, and they strong arm you at the vet to get her tested frequently or you can't get the medicine. I’m not cool with that kind of tactic. Is there anything else I can give her that would prevent heartworms?" —Colleen

Heartgard-Resistant Heartworms

Are you currently living in Indiana? Your dog most likely became infected with a Heartgard-resistant strain of heartworms in Mississippi. (1) It is also been found in Louisiana, Arkansas, and Tennessee. There may be other areas with that infection, but all have been found in the Mississippi valley.

The resistant heartworm has not been found in Indiana (or anywhere in the Ohio valley). Since she has been treated and is now free of infection, you can prevent it from happening again by purchasing ivermectin (Heartgard), not just Simparica.

Heartgard still works in almost all areas of the United States. (2)

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Are Heartworm Tests Required for Medication?

Your dog does not legally need a heartworm test in order to be on preventative. There does need to be a doctor/client/patient relationship for the prescription so they can still charge you for an office call and physical examination.

There are non-prescription herbal heartworm preventatives, but none that have been tested or shown to work. Even most holistic veterinarians rely on conventional heartworm preventative.

Where to Buy Heartworm Preventative

You can sometimes buy ivermectin through Amazon, but it is not always available. For more information, check out my article about how to buy cheap heartworm preventatives without a prescription.

If you want to buy medication for just one dog, it may be simpler to buy Heartgard without a prescription from Australia or Canada.

Continue Giving Heartworm Preventatives

It sounds like they caught the infection early and your dog still has no clinical symptoms. Keep up the heartworm preventative!


  1. Bourguinat C, Keller K, Xia J, Lepage P, McTier TL, Woods DJ, Prichard RK. Genetic profiles of ten Dirofilaria immitis isolates susceptible or resistant to macrocyclic lactone heartworm preventives. Parasit Vectors. 2017 Nov 9;10(Suppl 2):504.
  2. Drake J, Wiseman S. Increasing incidence of Dirofilaria immitis in dogs in USA with focus on the southeast region 2013-2016. Parasit Vectors. 2018 Jan 17;11(1):39.

This article is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from your veterinarian. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.

© 2022 Dr Mark

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