What Is Heartworm Disease in Dogs? Causes & Treatment

If your vet recently brought up heartworm testing or prevention, you might be wondering what heartworm disease in dogs actually is and why it gets so much attention at every checkup. It’s a fair question, especially if you’re a new dog owner still learning which health risks are worth worrying about and which ones are more routine. The short version is that this is a serious condition, but it’s also one of the most preventable and well-understood parasitic diseases in veterinary medicine. Once you understand how dogs get infected, what treatment actually involves, and why prevention is so much easier than the alternative, the whole picture makes a lot more sense.

What Is Heartworm Disease in Dogs?

Heartworm disease is caused by a parasitic worm that lives inside a dog’s heart, lungs, and the blood vessels connecting them. These worms can grow to a foot or longer, and once they reach adulthood, they can live inside a dog for years.

This isn’t the same as the intestinal worms many pet owners are already familiar with, and that distinction matters. Intestinal parasites are uncomfortable and need treatment, but they don’t typically threaten the heart directly. Heartworms do. Because they settle in and around the heart itself, they interfere with how efficiently blood moves through the body. As the worm population grows, that strain compounds over time.

Left unmanaged, this ongoing strain can eventually lead to heart failure. The heart has to work harder to push blood past the worms, and that extra workload takes a real toll the longer it continues. This is part of why catching an infection early, before significant damage builds up, makes such a difference in how well a dog responds to treatment.

How Do Dogs Get Heartworms?

Dogs get heartworms from the bite of a single infected mosquito. There’s no other way for a dog to catch this parasite, and that single fact reshapes how most owners should think about their dog’s actual risk.

Here’s what causes heartworm disease in dogs to spread from one animal to another:

• A mosquito bites an already-infected dog or wild animal and picks up immature heartworm larvae in that blood meal
• The larvae continue developing inside the mosquito over the following one to two weeks
• The same mosquito bites another dog and passes the larvae into its bloodstream
• Over roughly seven months, those larvae travel through the body and mature into adult heartworms

A common misconception is that indoor dogs are somehow protected from this. They aren’t. Mosquitoes get inside homes constantly, whether through an open door, a torn screen, or simply riding in on a person’s clothing. It only takes one bite for infection to begin, regardless of how much time a dog spends outdoors.

Because a mosquito has to be involved at every step of this process, dogs also can’t catch heartworms directly from each other. Two dogs can share a yard, a water bowl, and constant close contact without any risk of transmission between them. The only path is through an infected mosquito.

Understanding how do dogs get heartworms also explains why the infection can go unnoticed for so long. That seven-month development window means a dog can be infected well before any signs appear, and well before a routine test would catch it if testing isn’t done on schedule.

Why Is Heartworm Risk Higher in Jacksonville?

If you live in the Jacksonville area, heartworm prevention gets emphasized more heavily here than it might in other parts of the country, and there’s a specific reason for that.

Mosquito activity depends heavily on temperature. In much of the country, a real winter freeze slows mosquitoes down for several months, which naturally narrows the window of risk. Northeast Florida’s humid, subtropical climate rarely gets cold enough for long enough to do the same. Mild winters mean mosquitoes stay active in far greater numbers, far more consistently, than they would somewhere with a harder seasonal shift.

That’s why local vets recommend year-round prevention instead of seasonal prevention. In a colder climate, skipping a dose in December might carry relatively low risk if mosquitoes are largely dormant anyway. In Jacksonville, that same skipped dose lands during a month when mosquitoes may still be actively biting. Your dog’s exposure window in this market is simply longer, and more consistent, than it would be across most of the country. That’s the whole reason “monthly, every month, all year” is the standard recommendation here rather than a seasonal one.

What Are the Signs and Symptoms of Heartworm Disease?

The signs and symptoms of heartworm disease to watch for include:

• Coughing, especially a soft, persistent cough that doesn’t have an obvious cause
• Lethargy or a noticeable drop in energy
• Trouble breathing or heavier breathing than usual, particularly after activity
• Decreased appetite

These signs develop gradually as the worms place ongoing strain on the heart and lungs. But here’s the part that catches a lot of owners off guard: a dog can carry an active infection for months, sometimes longer, without showing any of these signs at all. Some dogs, particularly those that are less active day to day, may show few or no outward signs even with a meaningful number of worms present.

That’s exactly why the absence of symptoms doesn’t mean the absence of risk. Annual testing exists precisely because you can’t rely on visible signs alone to know whether your dog is affected. A dog can look and act completely normal while an infection is actively developing.

If you do notice any of these signs, don’t wait to see if they pass on their own. Catching them early and getting your dog tested promptly leads to a much better outcome than waiting until the disease becomes advanced enough to be obvious without testing.

How Is Heartworm Disease Treated in Dogs?

Treatment isn’t a single vet visit. It’s a structured process that unfolds over weeks and months, with each phase building directly on the one before it. Knowing what to expect at each stage, rather than being surprised by it one step at a time, helps take a lot of the uncertainty out of a diagnosis.

Diagnosis and Staging

Treatment begins with a positive heartworm test, followed by chest X-rays and bloodwork. These steps confirm the infection and show its severity, including strain on the heart and lungs. Staging matters because it confirms treatment is safe for your dog’s specific condition, not one-size-fits-all.

Pre-Treatment Preparation

Before the worms themselves are ever directly targeted, dogs are typically started on:

• Doxycycline, an antibiotic
• Heartworm prevention medication
• Anti-inflammatories, as needed

Doxycycline targets bacteria that live inside the heartworms themselves. Removing that bacteria makes the worms easier to kill later and meaningfully reduces complications once treatment moves into the injection phase. This preparation step isn’t a delay before the real treatment starts. It’s what makes everything that follows safer and more effective.

The Injection Phase

Melarsomine injections are given deep in the back muscles. The first injection begins killing adult heartworms. Two more injections follow 24 hours apart to complete the process. Spacing the injections this way controls how quickly the worms die, which limits how much strain the body has to manage at any one time rather than all at once.

This is the phase where the infection is actually being eliminated, not just monitored or managed. It’s also the point where the strict rest period that follows becomes absolutely critical.

The Strict Rest Period

This is the single most important part of the entire treatment process. As the dead worms break apart, physical activity raises the risk of blood clots and serious lung complications. During this window:

• No running
• No playing
• No excitement
• Leash walks for potty breaks only

Here’s what matters most for owners to understand: your dog may start feeling and acting like themselves again during this period, sometimes within just a few days. Feeling better does not mean they’re healed. The worms are still breaking down internally, and resuming normal activity too soon is one of the most dangerous mistakes an owner can make during treatment. How closely you follow the rest restrictions during this window has a direct impact on whether your dog recovers safely, which is why this step gets emphasized more than almost any other part of the process.

Recovery and Retesting

Strict rest continues for six to eight weeks total, and your dog will be retested for heartworms about six months later to confirm the infection is fully cleared.

This stretch is calmer than the days immediately after the injections, but it isn’t a return to normal yet either. Your dog has moved past the highest-risk window, but the restrictions don’t lift all at once. The body needs that extended time to safely absorb the dead worms without added strain. Setting that expectation now, that recovery takes months, helps you plan ahead instead of surprises.

Is Heartworm Disease Curable?

Is heartworm disease curable? In most cases, yes, when it’s properly diagnosed and treated. But that answer comes with real nuance worth understanding.

Outcomes depend heavily on how advanced the infection was when it was caught. A dog diagnosed early, before significant heart and lung damage has occurred, generally has a much smoother path through treatment than one diagnosed after the infection has been present for a long time. The treatment process itself also carries genuine risk and requires total compliance with the rest period to go smoothly.

This isn’t a simple, fast, or guaranteed fix. It’s a multi-month commitment that asks a lot of both the dog and the owner. That reality is exactly why avoiding the need for treatment in the first place is worth taking seriously, rather than treating prevention as optional.

Is Prevention Really Easier Than Treatment?

Side by side, the difference is stark:

Prevention:

• Monthly protection
• Easy and stress-free
• Affordable
• Protects the heart, lungs, and overall health

Treatment:

• Multiple vet visits
• Injections and medications
• Strict crate rest for weeks
• Higher cost and higher risk of complications

Prevention works by stopping an infection before it ever takes hold in the first place. Treatment, by contrast, has to undo an infection that’s already established in the heart and lungs, which is inherently harder, slower, and riskier for your dog than preventing it would have been. There’s also an emotional cost to treatment that’s easy to overlook on paper. Weeks of confining an otherwise healthy-seeming dog, watching for complications, and waiting through a long recovery window is stressful for owners too.

When you weigh the two side by side, prevention isn’t just the more convenient option. It’s consistently the easier, safer, and less disruptive path for everyone involved.

Why Prevention Should Be a Habit, Not an Afterthought

Heartworm disease is serious, but it’s also highly preventable with monthly protection and annual testing. In Jacksonville’s climate, even one skipped month creates real exposure, since mosquitoes stay active nearly year-round.

The best way to protect your dog is to treat prevention as routine preventative care, not something to think about only when it comes up at an appointment. If it’s been a while since your dog’s last heartworm test, or you’re not sure they’re on a consistent prevention schedule, we’re happy to help you get back on track. Our team can walk you through testing and treatment options and answer any questions specific to your dog’s situation. Reach out to schedule an appointment and we’ll take it from there.