Heartworm 101: What Every Dog Parent Should Know This Summer

Summer means longer walks, more time outside, and unfortunately, more mosquitoes. And where there are mosquitoes, there's heartworm risk. As JoyFull's veterinary advisor, I want to make sure every dog parent has the information they need to keep their pup safe this season.

So, what exactly is heartworm?

Heartworm is a parasitic infection caused by worms called Dirofilaria immitis. These worms travel through an infected dog's bloodstream and take up residence in the heart and lungs, causing permanent organ damage over time. In severe cases, adult worms can block arteries in the lungs entirely — and without treatment, it can be fatal.

The good news: heartworm has been diagnosed in all 50 states, but it's entirely preventable. The less good news: it's been diagnosed in all 50 states, so no dog is truly off the hook regardless of where you live. Transmission does decrease in colder months, but in urban areas the risk never fully reaches zero.

How does a dog get heartworm?

Only through mosquito bites — your dog cannot catch it directly from another dog. A mosquito bites an infected animal, picks up microscopic heartworm larvae, and then transmits them to your dog on its next bite. From there, the larvae mature into adult worms over the course of about six months.

Cats can also be infected, but they tend to be more resistant and rarely show clinical signs. And while it's technically possible for humans to be infected, it doesn't lead to clinical disease the way it does in dogs.

What are the signs to watch for?

Early infection often looks completely normal — which is part of what makes heartworm so insidious. As the worm burden increases, you might notice:

• A persistent cough
• Your dog tiring more easily during exercise
• Lethargy or weight loss

In more advanced cases: difficulty breathing, a swollen belly from fluid buildup, collapse, or in the most severe cases, sudden death. If your dog is showing any of these signs, please see your vet right away.

When and how should you test?

The American Heartworm Society recommends annual heartworm testing for all dogs over 7 months of age — and this applies even if your dog is on a preventative. No preventative is 100% effective, and annual testing is the only way to catch an infection before it progresses.

Spring is generally considered the best time to test, since adult worms take at least six months to develop after transmission — meaning spring catches any exposure from the previous fall. That said, if you haven't tested yet this year, now is still a good time.

Your vet will use a blood test to check for both heartworm antigens and microfilaria (microscopic larvae). One test alone isn't enough for a complete picture, which is why the Society recommends both.

What if my dog tests positive?

First: don't panic, but do act quickly. Treatment is available, but it's significant — the full course takes a minimum of 91 days, involves multiple injections, antibiotics, and complete exercise restriction throughout. It's also considerably more expensive than prevention. This is why I always say: prevention isn't just easier, it's genuinely kinder to your dog.

One critical note: never start a heartworm preventative without testing first. Starting prevention on a dog with an active infection can be dangerous. Always test before starting or switching preventatives.

Prevention: the best medicine

Year-round FDA-approved heartworm preventatives (dispensed by your vet) are the gold standard. These medications, called macrocyclic lactones, are highly effective and safe for long-term use. You can also reduce risk by limiting outdoor time during peak mosquito feeding hours and using EPA-registered mosquito repellents.

If your dog isn't currently on a preventative, the first step is a heartworm test — then talk to your vet about the right prevention protocol for your pup.

Heartworm is serious, but it's also one of the most preventable conditions out there. A quick annual test and a consistent preventative is all it takes. Your dog is worth it.

Dr. Natascha Hedegaard is a practicing veterinarian and JoyFull Pet's veterinary advisor. She formulates and reviews every JoyFull product for safety and efficacy, guided by peer-reviewed clinical research.