Heartworm in Dogs: Signs, Testing, Treatment and Year-Round Prevention

A positive heartworm test is hard news, and it's normal to want to know how bad it is and how fast you need to act. I'd call the vet first, today if you can, and ask three things: should my dog's activity be restricted starting now, has the positive result been confirmed, and what are the next steps for treatment.

Activity restriction starts early for a reason. AHS guidance, summarized in its Canine Guidelines Summary, calls for restricting exercise immediately after diagnosis. Worms that die, on their own or from treatment, can break apart and block blood vessels in the lungs, and activity makes that more dangerous. So, until your vet tells you otherwise, think short leash walks for bathroom breaks and no running, fetch or rough play.

Confirmation matters too. AHS advises confirming a positive antigen test with an additional, different test, because treatment is complex and costly. Your vet may also want to check how severe the disease is and whether your dog has other health problems before choosing a plan. A dog who is already sick may need to be stabilized first.

Heartworm disease is caused by a parasite called *Dirofilaria immitis*. According to the Merck Veterinary Manual, adult worms live mainly in the pulmonary arteries, the vessels that carry blood from the heart to the lungs, and in advanced disease they can reach the heart itself. Over time they damage the heart and lungs.

A mosquito is the only way it spreads. The life cycle, as Merck describes it, goes like this: a mosquito bites an infected animal and picks up tiny baby worms called microfilariae. Inside the mosquito, those develop into infective larvae. When that mosquito bites another dog, the larvae enter through the bite wound, mature over the following months, travel to the pulmonary arteries and grow into adult worms.

That has two practical meanings. First, your other dogs can't catch heartworm directly from the positive dog; AHS is clear that a mosquito has to carry it. They do share the same mosquitoes, though, so every dog in the house belongs on prevention. Second, indoor dogs are at risk. Merck notes that mosquitoes get into homes and bite indoor dogs.

Many dogs with heartworm look perfectly fine at first. Merck explains that early on, the worms and vessel damage haven't yet caused obvious problems, and a quiet, low-activity dog may hide the fact that it tires easily. How sick a dog gets depends on how many worms it has, how long it has been infected, its immune response and its activity level. A dog can carry a heavy infection and show few outward signs.

Vets often group heartworm disease into four classes, per Merck and the AVMA's 2025 heartworm brochure:

• Class 1: no signs, or mild ones like an occasional cough. Often found on routine screening.
• Class 2: mild to moderate signs, such as a cough that won't go away, tiring quickly on walks, and weight loss.
• Class 3: more severe disease, with marked exercise intolerance, labored breathing, weakness, fainting, or fluid buildup such as a swollen belly.
• Class 4: caval syndrome, where a large number of worms blocks blood flow. This is life-threatening.

Go to an emergency vet now if your dog collapses, struggles to breathe, has pale or blue gums, coughs up blood, or suddenly becomes very weak, especially with a swollen belly. Those can be signs of caval syndrome or a blockage in the lungs. It's worth knowing where your nearest emergency vet is before you need one, and our Dog CPR article covers general emergency preparedness.

A cough or tiredness alone can't tell you whether a dog has heartworm, since plenty of other problems cause them. Only testing can. Our Signs of a Healthy Dog article is a good baseline for what normal looks like.

AHS updated its canine guidelines on April 9, 2024, and its testing guide now recommends two tests every year for every dog: an antigen test and a microfilaria test.

The antigen test detects proteins mainly from adult female heartworms. It's the preferred routine test, but it can't reliably catch a very new infection. The microfilaria test looks for the baby worms that adults produce. A dog can be antigen-positive and microfilaria-negative, so one negative test doesn't rule out heartworm. X-rays and heart ultrasound can help your vet judge how severe the disease is, but they don't replace the blood tests.

Timing also matters, because an infection takes months to show up. Merck reports that antigen can appear about five months after infection and microfilariae about 6.5 months after. A negative test right after a risky stretch may simply be too early. The schedule AHS recommends:

AHS recommends yearly testing even for dogs on steady prevention, because prevention is highly effective but not perfect, and a dose can be missed, vomited, spit out or rubbed off without anyone noticing.

AHS treatment follows a set sequence, and your vet will decide the timing for your dog. Based on the AHS Canine Guidelines Summary, the plan includes:

1. Confirming the diagnosis and checking how severe the disease is.
2. Restricting exercise right away.
3. Stabilizing a dog who is already sick before the adult worms are killed.
4. Giving a macrocyclic lactone preventive and doxycycline.
5. Killing adult worms with a series of three melarsomine injections.
6. Keeping activity strictly limited through treatment and the recovery period after the last injection.
7. Retesting afterward.

Melarsomine is the only FDA-approved drug that kills adult heartworms in dogs, per Merck. Doxycycline targets *Wolbachia*, bacteria that live with heartworms, and can reduce the inflammation that follows. Your vet chooses and gives these drugs; the doses are theirs to set.

If you've seen natural remedies suggested online, here is what actually treats heartworm. Prevention products stop developing larvae; they don't kill established adult worms, and melarsomine is what does that. AHS's medicine page lists prescription preventives from your vet, not natural products. Livestock dewormers aren't a safe home treatment either. Anything used for heartworm should come through your veterinarian.

The rest part is the hardest for most families. AHS says plainly that treatment can be difficult, takes multiple vet visits and means months of exercise restriction. The AVMA brochure explains why: as the worms die, pieces can lodge in the lung vessels, and activity raises the risk of serious breathing complications. Short leashed potty breaks and a quiet space make the rest period easier to keep.

Watch closely after each injection. According to the AHS Canine Guidelines Summary, possible problems include pain or swelling at the injection site, coughing, coughing blood, labored breathing, fever, low energy, poor appetite and collapse. Call your vet promptly for any of these, and treat collapse or severe trouble breathing as an emergency.

You'll probably hear about "slow kill," especially from rescue groups or online. AHS describes it as giving a preventive-type drug for a long period, sometimes with doxycycline, while the adult worms die off gradually without melarsomine.

AHS does not recommend slow kill as the routine approach. Its reasons: adult worms may survive for years this way, lung disease can keep getting worse in the meantime, nobody can predict when the worms will die, and resistance is a concern. If cost or your dog's health makes the standard plan hard, say so to your vet and ask what options fit your dog.

Cost depends on your dog's size, where you live, how severe the disease is, and whether your dog needs imaging, a hospital stay, extra medication or follow-up visits. PetMD, updated April 29, 2025, reports roughly $500 to $1,500 for melarsomine injections and related care in some cases. A peer-reviewed 2021 article in Frontiers in Veterinary Science cites an AHS estimate of about $1,200 to $1,800.

Treat those as rough, dated ranges, not quotes. Severe cases can cost more, and your own vet's estimate is the number to plan around. If money is tight, ask for an itemized estimate, and if you adopted the dog, ask the shelter or rescue whether it helps with treatment.

Prevention is far easier than treatment, and AHS recommends it year-round for every dog. Its reasons: heartworm has been diagnosed in all 50 states, mosquitoes can show up in any state, and preventives work against infections picked up over the previous month or longer.

AHS lists four preventive formats: monthly oral, monthly topical, a six-month injection and a 12-month injection. All are prescription products in the U.S. Your vet picks one based on your dog's age, weight, health and exposure, and on what your household can give reliably. Some also cover other parasites, but coverage differs by product. Our articles on ticks and Lyme disease and on getting rid of fleas cover those parasites.

If a dose was missed or late, call the vet who prescribed it rather than guessing. AHS recommends testing right away, again six months later, and yearly after that. The same goes if your dog spit out or vomited a pill or rubbed off a topical. I'd write each dose date on a calendar, so that call is easy.

After treatment, your dog still needs prevention. A treated dog can be infected again.

Historical AHS incidence maps show higher average case rates across broad parts of the Southeast and the lower Mississippi Valley, with big differences between states and localities. AHS now posts a 2025 map. Texas has documented heartworm risk and isn't a low-risk exception, so year-round prevention there follows the same AHS advice as everywhere else. Moving to a northern state doesn't remove the risk either, since dogs have been diagnosed in all 50.

Shelter and rescue dogs can test positive because many had spotty prevention, unknown histories, or lived in higher-risk areas. A positive result doesn't make a dog untreatable. The AVMA and AHS describe treatment as generally successful for many dogs. Before you adopt, ask about disease severity, whether stabilization is needed, who pays for treatment, and whether your home can handle months of strict rest.

These come up constantly, and the answers go a step past the sections above.

Often, none at all. When early signs appear, Merck lists a mild, occasional cough, and many infections turn up only on a routine blood test. As disease progresses, the cough can become persistent and the dog may tire quickly.

Most infected dogs can be treated successfully, according to AHS, but treatment is complex and must be run by a vet. Success is checked with an antigen test about nine months after the last melarsomine injection. Your dog stays on prevention afterward, since a treated dog can be infected again.

The AVMA describes treatment as generally successful for many dogs. How any one dog does depends on its disease class, whether it needed stabilization, and how strictly rest is kept after injections. Class 4 caval syndrome is life-threatening and needs emergency care.

Without adulticide treatment, AHS notes that adult worms may survive for years while lung disease can progress. That long survival is a main reason AHS doesn't recommend slow kill as the routine approach.

If your dog tested positive, call your vet today, start restricting activity, and ask about confirming the result and planning AHS-based treatment. If your dog is fine, the next step is simpler: get the yearly antigen and microfilaria test on the calendar and keep prevention going every month of the year. Our directory lists vets, emergency vets and shelters near you when you need one.

• American Veterinary Medical Association: AVMA's 2025 heartworm brochure
• heartwormsociety.org: AHS incidence maps
• heartwormsociety.org: AHS's medicine page
• heartwormsociety.org: AHS
• heartwormsociety.org: American Heartworm Society
• heartwormsociety.org: Canine Guidelines Summary
• heartwormsociety.org: testing guide
• Merck Veterinary Manual: Merck Veterinary Manual
• Merck Veterinary Manual: Merck
• PetMD: PetMD
• public-pages-files-2025.frontiersin.org: Frontiers in Veterinary Science