Heartworm Disease in Dogs: Symptoms, Treatment & Prevention

Quick Answer
  • Heartworm disease is caused by Dirofilaria immitis, a mosquito-borne parasite that lives mainly in the pulmonary arteries and can also affect the heart.
  • Many dogs have no early signs, so yearly testing matters even when your dog takes prevention consistently.
  • Standard treatment usually uses doxycycline, a macrocyclic lactone preventive, and a 3-dose melarsomine protocol with strict exercise restriction.
  • Prevention is usually far less costly than treatment. Year-round prevention often runs about $75-$300 per year, while treatment commonly ranges from about $1,000 to $5,500+ depending on severity and location.
Estimated cost: $75–$5,500

What Is Heartworm Disease?

Heartworm disease is a serious parasitic infection caused by Dirofilaria immitis. Dogs get infected through mosquito bites, not from direct contact with another dog. After infection, immature worms migrate through the body and mature over about 6 months. Adult worms then live mainly in the pulmonary arteries, and in heavier infections they can extend into the right side of the heart.

The damage is not only from the worms taking up space. Heartworms trigger inflammation in the blood vessels and lungs, strain the heart, and can reduce oxygen delivery throughout the body. Over time, this can lead to coughing, exercise intolerance, weight loss, lung disease, heart failure, and injury to organs such as the liver and kidneys.

Vets often describe heartworm disease by severity class. Class I dogs may have no visible signs. Class II dogs often develop cough and reduced stamina. Class III dogs can have marked breathing problems, weakness, weight loss, and radiographic evidence of heart and lung changes. Class IV, called caval syndrome, is an emergency where worms obstruct blood flow through the heart and can cause collapse, pale gums, and dark urine.

Heartworm disease occurs across the United States, and risk is not limited to warm southern states. Mosquito exposure, travel, missed prevention doses, and local wildlife reservoirs all matter. That is why year-round prevention and routine screening remain important for dogs in every region.

Signs of Heartworm Disease

  • No visible signs early on, especially with lower worm burdens
  • Mild, persistent cough
  • Tiring more quickly on walks or during play
  • Reluctance to exercise or reduced stamina
  • Decreased appetite
  • Weight loss or muscle loss
  • Rapid breathing or increased effort to breathe
  • Lethargy or weakness
  • Swollen belly from fluid buildup in advanced disease
  • Collapse, fainting, pale gums, or dark red-brown urine in caval syndrome

Early heartworm disease can be easy to miss, and some dogs look normal until the infection is more advanced. Mild cough and lower stamina are common early clues, while breathing trouble, belly swelling, or collapse suggest more severe disease. See your vet immediately if your dog has labored breathing, fainting, weakness that comes on suddenly, pale gums, or dark urine, because those signs can fit advanced heartworm disease or caval syndrome.

How Do Dogs Get Heartworm?

Dogs get heartworm disease from the bite of an infected mosquito. When a mosquito feeds on an infected canid, it can pick up microscopic larvae called microfilariae. Those larvae develop inside the mosquito, then enter the next dog during a later bite. From there, they migrate through tissues and eventually mature into adult worms in the pulmonary arteries.

This means heartworm is not spread by sharing bowls, grooming, coughing, or normal contact between dogs. A mosquito is required for transmission. Wildlife such as coyotes can help maintain heartworm in an area, which is one reason infection risk can persist even when a neighborhood has relatively few pet dogs.

Risk goes up when dogs are not on consistent prevention, spend time outdoors during mosquito season, or live in areas with long mosquito activity. Still, indoor dogs are not risk-free. Mosquitoes get inside homes, and weather patterns have made mosquito exposure less predictable in many parts of the country.

A missed preventive dose can create a window for infection. Because the parasite takes months to mature, pet parents may not realize there was a problem until a routine screening test turns positive long after the mosquito bite happened.

How Is Heartworm Disease Diagnosed?

Most dogs are diagnosed with a blood antigen test during routine screening. This test detects proteins from adult female heartworms, so it usually does not turn positive until about 6 months after infection. That delay is one reason yearly testing matters, even for dogs on prevention.

If a screening test is positive, your vet will usually confirm the diagnosis and look for circulating microfilariae. Additional staging often includes chest X-rays and sometimes echocardiography, especially in dogs with more advanced signs. Bloodwork and urinalysis may also be recommended to check organ function before treatment starts.

Testing helps answer practical questions that shape the care plan: how severe the disease is, whether there is lung or heart damage already present, whether the dog is stable enough for adulticide treatment, and whether referral is needed. Staging also helps your vet talk through prognosis and realistic monitoring needs.

False negatives can happen. Examples include testing too early, infections with only male worms, or very low worm burdens. That is why prevention, annual screening, and follow-up testing all work together rather than relying on one test alone.

Treatment Options for Heartworm Disease

Spectrum of Care means you have options. Here are treatment tiers at different price points.

Medical management when adulticide is not feasible

$600–$1,800
Best for: Dogs who are not good candidates for melarsomine right away, dogs with major financial constraints, or situations where adulticide treatment must be delayed
  • Confirmatory testing and basic staging
  • Monthly prescription heartworm preventive to stop new infections and target susceptible larval stages
  • Doxycycline course, commonly about 4 weeks, to reduce Wolbachia-associated inflammation
  • Strict exercise restriction tailored by your vet
  • Prednisone or other supportive medications when indicated
  • Repeat antigen and microfilaria testing, plus recheck exams
Expected outcome: Variable. Some dogs remain stable for a time, but adult worms can continue damaging the lungs and blood vessels while they die off slowly over many months to years.
Consider: This approach is not the preferred first-line option for most dogs because it is slower, may allow ongoing damage, and requires prolonged monitoring and exercise restriction.
Most Common

AHS-style 3-dose melarsomine protocol

$1,000–$3,500
Best for: Most stable Class I, II, and many Class III dogs whose health status allows adulticide treatment
  • Heartworm confirmation, bloodwork, and staging chest X-rays
  • Monthly macrocyclic lactone prevention started under veterinary guidance
  • Doxycycline, typically for 28 days
  • Prednisone taper when indicated by your vet
  • Three melarsomine injections given as 1 injection, then 2 injections 24 hours apart about 1 month later
  • Pain control, hospitalization or day-stay monitoring around injections, and follow-up testing
  • Strict exercise restriction during treatment and for 6-8 weeks after the final injections
Expected outcome: Good in many dogs when treatment is completed and activity restriction is followed closely. The 3-dose protocol is the standard recommendation because it improves adult worm kill and may reduce complications compared with less complete protocols.
Consider: Requires multiple visits, careful timing, and very strict rest. The biggest risk period is when worms die and fragments can lodge in lung vessels, so active dogs can be hard to manage safely at home.

Specialty care for severe disease or caval syndrome

$3,000–$5,500
Best for: Dogs with Class IV caval syndrome, severe Class III disease, marked cardiopulmonary compromise, or complications that need referral-level support
  • Internal medicine or cardiology consultation
  • Advanced imaging such as echocardiography
  • ICU-level stabilization for dogs in respiratory distress or heart failure
  • Jugular extraction of worms for caval syndrome when indicated
  • Hospitalization, oxygen support, transfusion or intensive monitoring if needed
  • Post-stabilization transition to a melarsomine-based plan when appropriate
Expected outcome: Guarded to fair depending on how quickly the dog is stabilized and how much heart, lung, or organ damage is already present. Prompt intervention can be lifesaving.
Consider: Higher cost range, referral logistics, and greater procedural risk because these dogs are often critically ill before treatment begins.

Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.

Questions to Ask Your Vet About Heartworm Disease

Bring these questions to your vet appointment to get the most out of your visit.

  1. What severity class do you think my dog is in, and what findings support that?
  2. What tests do you recommend before treatment, and which ones are most important for my dog's situation?
  3. Is my dog a candidate for the 3-dose melarsomine protocol right now, or do we need to stabilize anything first?
  4. How strict should exercise restriction be at home, and what activities are off-limits?
  5. Which medications will my dog need before, during, and after treatment, and what side effects should I watch for?
  6. What warning signs mean I should call the clinic the same day or seek emergency care?
  7. When should we repeat antigen and microfilaria testing to confirm treatment success?
  8. What prevention plan do you recommend long term, and how can we make missed doses less likely?

How to Prevent Heartworm Disease

Heartworm prevention is one of the clearest examples of how preventive care can protect both health and budget. Most dogs do best with year-round prescription prevention, because mosquitoes can appear outside the expected season and indoor exposure still happens.

Preventive options include monthly oral products, monthly topical products, and longer-acting injections given by your vet. Common active ingredients used for prevention include ivermectin, milbemycin oxime, selamectin, and moxidectin. Some products also cover intestinal parasites, fleas, or mites, so the best choice depends on your dog's lifestyle, age, and other parasite risks.

A realistic US cost range for prevention is often about $75-$300 per year, depending on product type, dog size, and whether the medication also covers other parasites. By comparison, heartworm treatment commonly costs far more and asks much more of the dog and household.

Dogs should be tested according to your vet's recommendations before starting or restarting prevention, and they should continue routine annual screening even when doses are given on schedule. If you miss a dose, contact your vet promptly. The safest next step depends on timing, product type, and your dog's testing history.

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