Heartworm Disease in Dogs: How Testing and Prevention Work

What is heartworm disease in dogs?

Heartworm disease is a parasitic infection that dogs get from mosquito bites. When an infected mosquito bites a dog, it passes immature heartworm larvae into the bloodstream, where they mature into adult worms that settle in the heart, lungs, and surrounding blood vessels. Left untreated, the infection can cause lasting damage to those organs.

Dr. Long describes the transmission simply: "Mosquitoes bite the dog, and they transmit the parasite through their bite into the bloodstream of the dog." From there, the larvae spend roughly six months maturing before they become adult worms. Adult heartworms can live for several years inside a dog, and a single dog can carry dozens of them.

That is what sets heartworm apart from parasites like fleas or intestinal worms. Those are unpleasant and worth preventing, but they are usually straightforward to treat. Heartworm is different. "Heartworm disease is one of the worst parasite diseases there are because it can cause irreversible problems in the heart and or lungs," Dr. Long said.

Why the damage can be permanent. Adult heartworms physically occupy the pulmonary arteries and, in heavy infections, the right side of the heart. Their presence inflames and scars the blood vessels and forces the heart to work harder to push blood through the lungs. Even when the worms are eventually killed with treatment, the scarring and strain they caused can remain. Dogs that go undiagnosed for a long time can end up with lasting cardiovascular and respiratory problems that affect their quality of life well after the infection itself is gone.

Why prevention is the better path. Treating an established heartworm infection is possible, but it is hard on the dog. It involves a series of injections to kill the adult worms, weeks of strict rest so that dying worms do not cause dangerous blockages in the lungs, and repeat testing to confirm the infection is cleared. It is also expensive. Compared to that, prevention is easy. As Dr. Long put it, "The great news is, however, it's really easy to prevent, and it starts with a routine test to make sure that your dog does not have heartworm disease."

Where heartworm is a risk. Heartworm disease has been diagnosed in all 50 states. It is most common in warm, humid regions with long mosquito seasons, including much of Texas, but dogs in California, Colorado, and Arizona are not immune. Mosquitoes get indoors, dogs travel, and climate patterns shift. Our veterinarians recommend year-round prevention regardless of where you live.

What are the signs of heartworm disease in dogs?

Early heartworm disease usually causes no symptoms at all. Most dogs look and act completely normal for months after infection, which is exactly why routine testing matters: by the time signs appear, the worms are already adults and the damage is underway.

When symptoms do show up, they tend to be subtle at first and easy to attribute to something else. The most common early signs are:

• A mild but persistent cough
• Getting tired more quickly on walks or during play
• Reluctance to exercise
• Reduced appetite and gradual weight loss

As the disease progresses, dogs may develop a swollen abdomen from fluid buildup, labored breathing, a pale or bluish tint to the gums, or fainting episodes after exertion. In the most advanced cases, a large mass of worms can block blood flow through the heart, which is a life-threatening emergency.

What to do if you notice these signs. Because these symptoms overlap with many other conditions, from kennel cough to heart disease unrelated to parasites, they are worth a visit rather than a wait-and-see approach. If your dog is coughing, slowing down, or losing weight without an obvious reason, book an appointment so our team can examine them and run the appropriate tests. If you are unsure whether something warrants a visit, members can reach a Modern Animal veterinarian anytime through 24/7 Virtual Care.

How is heartworm testing done?

A heartworm test is a blood test. Your vet draws a small blood sample, runs it on an in-clinic or laboratory test, and typically has an answer the same day. There is no fasting, sedation, or special preparation required, and most dogs tolerate the draw without any fuss.

"The test for heartworm disease is simple," Dr. Long said. "It's a blood test, and we're going to collect blood from maybe a leg, a front leg or a back leg, sometimes even your dog's jugular vein, depending on the size of your dog." The jugular is often used for smaller dogs because it is easier to get a clean sample from a larger vessel.

What the test actually looks for. The standard screening test is an antigen test. It detects a protein produced by adult female heartworms, which means it identifies established infections rather than brand-new ones. Because the larvae take about six months to mature into adults, a dog bitten by an infected mosquito last month will still test negative today. This lag is why timing matters so much for puppies and for dogs that have had a gap in prevention, which we cover in the next section.

Your vet may also check the blood sample for microfilariae, the immature offspring that adult worms release into the bloodstream. Finding them confirms an active infection and tells your vet more about how to approach treatment safely.

What happens if the test is positive. A positive result is not a diagnosis on its own. Your vet will usually confirm it with a second test, then recommend chest X-rays and additional lab work to stage the disease and check how much strain the heart and lungs are under. Treatment is tailored to that staging. The most important thing is not to start or continue standard monthly prevention on your own before talking to your vet, since some preventives can cause a serious reaction in a dog with a heavy microfilariae load.

What happens if the test is negative. This is the outcome most dogs get, and it is the green light to start or continue prevention. Your vet will talk through the options below and help you pick the one that fits your dog and your routine.

How often should a dog be tested for heartworm?

Dogs should be tested for heartworm once a year, even if they are on prevention year-round. Testing is also required before starting prevention for the first time, and again after any significant lapse in doses. Your vet will confirm what schedule applies to your dog.

The yearly test is easy to fold into your dog's annual wellness visit, where it is typically run alongside other routine screening lab work. You can read more about what that panel includes in our post on routine screening lab work.

Why test a dog that is already on prevention? Prevention is highly effective, but not perfect. A dose can be given late, spit out unnoticed, or vomited up after a meal. Some dogs are harder to dose consistently than others. The annual test is a safety net that catches any infection early, while it is still far more treatable. It also protects your dog's eligibility for the manufacturer guarantees that most prevention products carry, which generally require documented annual testing.

Why test before starting prevention? Heartworm preventives work by killing the larval stages of the parasite that a dog picked up over the previous month. They do not kill adult worms. Putting an infected dog on prevention without knowing it means the adults continue to live and cause damage while everyone assumes the dog is protected. In some cases, it can also trigger a dangerous reaction. A quick test before the first dose avoids both problems.

What about puppies? Puppies can start heartworm prevention as early as eight weeks of age, and they do not need a test first because they are too young to have a detectable adult infection. Our vets will typically recommend a first heartworm test around six months and then annually from there. If your puppy started prevention late, your vet may adjust that timing. For more on what to expect in the first year, our resource hub has guides on puppy vaccines, parasite screening, and more.

What if my dog missed a few doses? If the lapse was short, your vet may simply have you resume and retest in about six months, since a new infection would not show up on a test any sooner than that. If the gap was longer, they will usually want to test before restarting. Either way, it is worth a quick call or a message through the app rather than guessing.

What are my options for heartworm prevention?

At Modern Animal, dogs have two main prevention options: a flavored monthly chewable called Simparica Trio, or a ProHeart injection that lasts a full 12 months. Both prevent heartworm disease effectively. The difference is in what else they cover and how often you have to remember them.

Monthly chewable: Simparica Trio. This is the option our clinics use most often. Dr. Long describes it as "a flavored chewable tablet that you just give your dog every month as a treat." Most dogs take it readily. Beyond heartworm, it also protects against fleas, ticks, and several common intestinal parasites. "So it's a one stop shop for all of the parasite prevention your dog needs," she said. For most owners, that means one product, one dose, once a month, with no need to layer on separate flea, tick, or deworming products.

12-month injection: ProHeart. ProHeart is an injection given at the clinic that provides heartworm protection for a full year. "There's also an injection that we can give called ProHeart that lasts a full 12 months," Dr. Long explains. The appeal is obvious for anyone who struggles to remember a monthly dose, or whose dog refuses chewables. The trade-off is scope. "It only prevents heartworm disease, and if you choose to go that route with your dog, you will need to do additional prevention for other parasites," she said. That usually means a separate flea and tick product, and your vet may recommend periodic deworming as well.

How to choose. There is no single right answer. A monthly chewable suits owners who like the all-in-one coverage and can keep a schedule. The injection suits owners who want to set it and forget it for heartworm specifically and are fine managing flea and tick prevention separately. Your dog's age, weight, health history, and any past reactions to medications also factor in. Your vet will help you decide which option makes the most sense for your dog's lifestyle and your own preferences.

A note on cats. Cats can get heartworm disease too, though it is less common and works differently. This post focuses on dogs, but if you have a cat, it is worth asking your vet about feline prevention at your next visit.

Why does heartworm prevention need to be continuous?

Heartworm prevention only works if it is given on time, every time. Each dose clears the larvae your dog picked up during the previous month. Skip or delay a dose, and larvae from that window can survive long enough to mature into adults that the preventive can no longer kill.

Dr. Long's advice on this is simple: make sure "your dog stays on the continuous monthly prevention that he or she needs."

Why year-round matters. Some owners stop prevention in colder months on the assumption that mosquitoes are gone. In practice, mosquito seasons are unpredictable, warm spells happen in winter, mosquitoes survive indoors, and dogs travel. Restarting after a break also means retesting before the first new dose. Year-round prevention removes the guesswork and is what our veterinarians recommend across all of our regions.

How Autoship helps. The most common reason dogs end up with a gap in protection is not a decision to stop. It is simply running out. Modern Animal members can set up Autoship in the Modern Animal app so that Simparica Trio arrives each month before the next dose is due. As Dr. Long put it, "Don't forget that you can use our Autoship feature in the Modern Animal app to make sure that your dog stays on the continuous monthly prevention that he or she needs."

A few habits that help. Pick a consistent day each month and set a phone reminder. Give the chewable with a meal, and watch your dog for a moment to make sure it went down. If your dog vomits within an hour or two of a dose, call your vet about whether to redose. And keep your dog's vaccine and prevention records in the app so you can check the date of the last dose at a glance.

What does heartworm testing and prevention cost?

The cost of a heartworm test varies by location. It is typically run as part of a broader wellness visit or screening lab panel, and the total for that appointment depends on factors like your dog's age, health history, whether it is a first visit or a routine recheck, and whether your vet recommends additional diagnostics based on what they find.

Prevention costs depend on which product you choose and your dog's weight, since both Simparica Trio and ProHeart are dosed by size. Current pricing for both can be found in the pharmacy hub in the Modern Animal app.

For a full breakdown of service and test pricing at Modern Animal, visit modernanimal.com/pricing. Everything is listed transparently so you know what to expect before you come in.

The bottom line. Heartworm disease in dogs is serious and the damage it causes can be permanent, but it is also one of the most preventable conditions we see. A yearly blood test confirms your dog is clear, and a consistent prevention plan, whether a monthly chewable or a 12-month injection, keeps them that way. If your dog is due for a heartworm test or has never started prevention, book an appointment through the Modern Animal app or online. If you have questions about missed doses, which product fits your dog, or a symptom you are not sure about, members can reach our team anytime through 24/7 Virtual Care.