Hello again everyone! Today I wanted to touch base about heartworm disease in dogs. Cats can also theoretically get heartworms, but it is pretty rare and generally their bodies only support 1-3 adult worms as they are not the ideal host, so this post is really going to address the issue in dogs. There is a link at the end of this post that specifically addresses heartworm in cats.

“But Dr. Muir, we live in Connecticut! Isn’t this more of a problem in the southern states?”

Well, it used to be. But thousands of dogs come up from the south to the north every year for adoption (for complex reasons that I won’t get into here) and with them sometimes come heartworms. One of the tricky things about heartworm is that with the antigen tests that we have available, it takes 6 months after exposure for a dog to come up positive. During that time there are larvae in the blood but it is the adult antigen that we routinely test for. That means that even if a dog tests negative before its trip up north it may still have some heartworm brewing. It is for this reason that when a newly adopted dog comes to our clinic, if it came from the south we recommend testing when it comes into the shelter or our clinic and then again 6 months later instead of waiting the whole year.

“Okay, that makes sense, but my dog is not from the south. Connecticut bred all the way! So we’re safe, right?”

Nope! We have heartworm up here now. It’s not as prevalent as in the south but we certainly have enough mosquitoes to keep it moving around from the few positive dogs and the likely many positive coyotes (who are thought to serve as a local reservoir now) to the local dogs. I live near the Timberlands here in Guilford and I personally swatted a few mosquitoes off of me during a warm spell during this past February (FEBRUARY!) and just killed one indoors in my bathroom (the farthest room from any door in the house) so we do have the opportunity for infection year round and to the dogs who “never leave the house.” This is one reason why year-round prevention is so important.

Another reason is the lifecycle of the larvae:

Larvae have several developmental months within the blood of a dog during which the prevention is effective. We give prevention once per month (unless you are using ProHeart injections). As long as we continue to give it, it’s pretty effective. But if we accidentally miss a month here or there, or we stop for the winter, or we don’t realize that our dog has outgrown the dose range for the pill he or she is taking and they are being underdosed there is room for a larvae or two to slip through into maturity. And that is all it takes.

“Okay, fine, but I give the preventative every month like clockwork – it’s even on my Google calendar! Why do I need to test for it every year?”

Yeah – so the heartworm preventatives that we have now are very effective. Like, 99% effective. About as effective as birth control when it is also used properly. But – just like every so often you hear about someone accidentally getting pregnant while on birth control – no drug is perfect. We test to make sure. Because we want to make sure that the medications we are giving aren’t going to cause a problem. And because heartworm disease is a big deal – there are worms growing in the heart. This weakens and damages the heart and can cause a cough and exercise intolerance. And the longer the damage goes on, the less reversible it is with treatment. Treatment itself is no picnic – toxic drugs, careful treatment to prevent reaction and try to prevent the dead worms from becoming emboli, months of strict exercise restriction, chest xrays, blood work – it is hard on the dog and expensive for the owner. As pricey as the preventatives can seem, they are a bargain compared to the cost of treating heartworm disease – financially, physically, and emotionally.

“Ewwwww! I don’t want my dog’s heart to look like that! But I hate giving him chemicals. It’s poison!”

Yes. Poison for the bugs. Worms and insects have bodies – particularly nervous systems – that are very different from mammals. The drugs that we have now are targeted to the larvae, worms, and other insects (like fleas and ticks). Any animal (or person) can have a reaction to any drug, but overall these preventatives are very safe. Heartgard has been around for years. I use that and Simparica Trio interchangeably in my own dogs currently. I have colleagues that prefer Proheart so they don’t have to remember to give a pill every month. Revolution is generally really well tolerated in cats and protects against fleas, ear mites, and other bugs as well. Even dogs with an MDR1 mutation can use most products safely.* There are options for everyone. And natural isn’t always better. You know what’s natural? Heartworm. Lyme disease. Anaplasma. In this particular case, I want to prevent natural. Because the treatment for a natural infection is so much worse than the prevention – and even with treatment, there can be lasting damage to the heart.

If you have specific questions about your dog’s risk for heartworm disease, heartworm testing, or heartworm preventatives, please call your vet!
–Dr. Dana Muir

For references/more reading about heartworm disease:
https://www.heartwormsociety.org/pet-owner-resources/heartworm-basics https://www.heartwormsociety.org/pet-owner-resources/infographics
https://d3ft8sckhnqim2.cloudfront.net/images/incidence-map-reverse/2022_AHS_Incidence_Ma p.pdf?1680711852
https://www.dupontvet.com/blog/10-things-you-didnt-know-about-heartworm-disease/ https://www.wormsandgermsblog.com/2010/03/articles/animals/dogs/heartworm-in-people/ https://www.heartwormsociety.org/pet-owner-resources/heartworm-in-cats https://www.heartwormsociety.org/pet-owner-resources/heartworm-prevention-for-dogs https://www.heartwormsociety.org/pet-owner-resources/heartworm-medicine-for-dogs https://www.heartwormsociety.org/pet-owner-resources/video-gallery
*prime.vetmed.wsu.edu/2021/10/19/are-heartworm-prevention-products-safe-if-my-dog-has-the mdr1-mutation/#:~:text=All%20heartworm%20products%20labeled%20for,and%20prevent%20p arasites%20like%20heartworms