Last updated: June 1, 2026

Heartworm Disease in Dogs: A Complete Guide to Prevention and Treatment

Heartworm disease is one of the most serious parasitic conditions affecting dogs in North America and many parts of the world. Caused by the parasitic worm Dirofilaria immitis and transmitted through the bite of an infected mosquito, heartworm disease can lead to severe lung disease, heart failure, organ damage, and death if left untreated. The adult worms — which can grow up to 12 inches long — live in the heart, lungs, and associated blood vessels, causing progressive damage over months to years.

The most frustrating aspect of heartworm disease is that it is almost entirely preventable. Monthly heartworm preventatives are safe, effective, and affordable, yet heartworm disease continues to affect hundreds of thousands of dogs each year due to missed doses, lapsed prevention, and lack of awareness. Treatment of an established heartworm infection is expensive, prolonged, carries risks, and requires significant activity restriction. Prevention is, without question, the better path.

The Heartworm Life Cycle

Understanding the heartworm life cycle explains how dogs become infected, why prevention works, and why treatment is so complex.

The cycle begins when a mosquito bites a dog (or other canid) that is already infected with heartworms. The mosquito ingests microscopic baby heartworms called microfilariae that are circulating in the infected dog’s blood. Inside the mosquito, the microfilariae develop over 10 to 14 days into infective larvae (L3 stage). This development within the mosquito is temperature-dependent — it requires sustained temperatures above approximately 57 degrees Fahrenheit (14 degrees Celsius) — which is why heartworm transmission is seasonal in temperate climates.

When the mosquito carrying infective larvae bites another dog, the larvae are deposited on the skin surface and enter the body through the bite wound. Once inside the dog, the larvae undergo a remarkable migration. Over the next two to three months, they develop through additional larval stages while migrating through subcutaneous tissue and muscle. Approximately 70 days after infection, the now-juvenile worms enter the bloodstream and travel to the pulmonary arteries (the blood vessels leading from the heart to the lungs). Over the next several months, the worms mature into adults, reaching their full size. The entire process from mosquito bite to mature adult worms in the heart and lungs takes approximately six to seven months.

Adult female heartworms begin producing microfilariae approximately six to nine months after the initial infection. These microfilariae circulate in the dog’s blood, ready to be picked up by the next mosquito that bites the dog, completing the cycle. Adult heartworms can live five to seven years in dogs, and a single dog can harbor dozens to hundreds of worms in severe infections. The worms cause physical obstruction of blood flow, inflammation of the blood vessel walls, and progressive lung and heart damage.

Symptoms of Heartworm Disease by Stage

Heartworm disease is classified into four stages based on the severity of symptoms and the degree of organ damage. In the early stages, dogs may show no signs at all — this is why regular testing is so important even in dogs that appear healthy.

Class 1: Asymptomatic or mild disease. In the early stages of infection, most dogs show no symptoms at all. The worm burden is low, and the body has not yet developed significant inflammatory or obstructive changes. Some dogs may have an occasional mild cough. This stage is often detected only through routine screening tests. Dogs diagnosed at this stage have the best prognosis with treatment.

Class 2: Moderate disease. As the infection progresses and the worm burden increases, dogs begin showing more noticeable signs. A persistent cough — especially after exercise or exertion — is common. Exercise intolerance develops: the dog tires more easily, is reluctant to play or walk as far, and may breathe more heavily than normal after moderate activity. Mild weight loss may occur. Chest radiographs at this stage may show enlarged pulmonary arteries and early lung changes.

Class 3: Severe disease. Dogs with severe heartworm disease have a significant worm burden and substantial lung and heart damage. Symptoms include a persistent, sometimes harsh cough, marked exercise intolerance (some dogs become reluctant to move at all), difficulty breathing even at rest, weight loss and muscle wasting, a pot-bellied appearance from fluid accumulation in the abdomen (ascites), fainting or collapse during or after exercise, and signs of right-sided heart failure. Chest radiographs show significant enlargement of the pulmonary arteries, lung inflammation and damage, and enlargement of the right side of the heart. Blood work may reveal elevated liver enzymes and kidney values.

Class 4: Caval syndrome. Caval syndrome is the most severe and life-threatening form of heartworm disease. It occurs when a massive number of worms physically obstruct blood flow through the heart and major vessels. Signs include sudden onset of labored breathing, pale gums, dark brown or red-tinged urine (from red blood cell destruction), and cardiovascular collapse. Caval syndrome is a medical emergency requiring immediate surgical removal of worms from the heart and veins. Without emergency surgery, caval syndrome is almost always fatal.

Testing and Diagnosis

Regular heartworm testing is essential — even for dogs on year-round prevention — because no preventative is 100 percent effective, doses may be given late or missed, and dogs may spit out or vomit oral preventatives without the owner’s knowledge.

Antigen test. The primary screening test for heartworm disease is the antigen test, which detects proteins produced by adult female heartworms. This is the test used in routine annual screening and is available as a point-of-care test (SNAP test) that provides results in minutes. The antigen test becomes positive approximately five to seven months after infection (when female worms mature and begin producing antigen). It is highly specific — a positive test reliably indicates heartworm infection. However, false negatives can occur with very low worm burdens, all-male infections (which produce less antigen), or very early infections. Annual testing is recommended, ideally in the spring.

Microfilaria test. Testing for circulating microfilariae (baby heartworms) in the blood is often performed alongside the antigen test. A positive microfilaria test confirms the presence of reproducing adult worms. Some heartworm-positive dogs may have no detectable microfilariae despite having adult worms — this can occur with single-sex infections, immune-mediated clearance of microfilariae, or the effects of certain heartworm preventatives.

Chest radiographs. X-rays of the chest are an important part of evaluating a heartworm-positive dog. Radiographs reveal the severity of lung and heart damage, including enlarged pulmonary arteries, lung inflammation, and heart enlargement. The radiographic findings help stage the disease and guide treatment decisions.

Echocardiography. An echocardiogram (ultrasound of the heart) may be performed in moderate to severe cases. Echocardiography can visualize worms within the heart and pulmonary arteries, assess heart function, and identify complications such as pulmonary hypertension (high blood pressure in the lungs) and right-sided heart failure.

Blood work. A complete blood count and chemistry panel assess overall health and organ function. Common findings in heartworm-positive dogs include eosinophilia (elevated eosinophils, a type of white blood cell associated with parasitic infections), anemia in severe cases, elevated liver enzymes from congestion, and protein loss. These results help assess the dog’s readiness for treatment and identify concurrent problems.

Treatment of Heartworm Disease

Treatment of heartworm disease is effective but complex, expensive, and not without risk. The goal is to kill the adult worms while minimizing the complications that can occur as the worms die and are broken down by the body. Dead worm fragments can cause blood vessel blockages and severe inflammatory reactions in the lungs, which is why strict activity restriction during treatment is absolutely critical.

Pre-treatment stabilization. Dogs with moderate to severe heartworm disease may require stabilization before treatment begins. This can include anti-inflammatory medications (such as prednisone), antibiotics targeting Wolbachia (a symbiotic bacterium that lives inside heartworms — killing Wolbachia weakens the worms and reduces the inflammatory reaction when they die), and treatment of secondary conditions such as heart failure or fluid accumulation.

Melarsomine (Immiticide) injections. The drug used to kill adult heartworms is melarsomine dihydrochloride, administered as deep intramuscular injections into the lumbar (lower back) muscles. The current recommended protocol involves three injections: an initial single injection, followed by a 30-day rest period, then two additional injections 24 hours apart. This three-injection protocol kills the worms more gradually than a two-injection protocol, reducing the risk of dangerous pulmonary embolism (blockage of lung blood vessels by dead worm fragments). The injections can be painful at the injection site, and the veterinarian may prescribe pain management.

Strict exercise restriction. This cannot be emphasized enough — exercise restriction is the most critical component of heartworm treatment success and safety. Physical activity increases heart rate and blood flow through the lungs, which can dislodge dead or dying worm fragments and cause pulmonary embolism, a potentially fatal complication. Dogs must be strictly confined — leash walks only for bathroom needs, no running, jumping, playing, or excitement — for the entire treatment period and for six to eight weeks after the final melarsomine injection. This period of restriction is challenging for both dogs and owners but is genuinely life-saving. Providing mental stimulation through calm training exercises can help manage restlessness during confinement.

Microfilaria treatment. After the adult worms are killed, a separate treatment targets the circulating microfilariae. This typically involves administration of a heartworm preventative (which kills microfilariae) under veterinary observation, as the die-off of large numbers of microfilariae can occasionally cause reactions.

Follow-up testing. A heartworm antigen test is performed approximately nine months after treatment completion to confirm that all adult worms have been eliminated. If the test is still positive, additional treatment may be necessary. Microfilaria tests are performed at the same time.

Prevention: Year-Round Protection

Heartworm prevention is one of the most important aspects of responsible dog ownership. Preventing heartworm disease is dramatically simpler, safer, and less expensive than treating it.

Monthly oral preventatives. Products containing ivermectin (Heartgard), milbemycin (Interceptor, Sentinel), or moxidectin (ProHeart) are given monthly and work by killing the larval heartworms that were transmitted by mosquitoes during the preceding 30 days. They do not prevent infection — they retroactively eliminate larvae before they can mature into adult worms. This is why consistent monthly dosing is essential; a gap of even a few weeks can allow larvae to develop past the stage that preventatives can kill. Many monthly preventatives also protect against intestinal parasites.

Injectable preventative. ProHeart 12 (moxidectin) is an injectable heartworm preventative administered by a veterinarian that provides 12 months of continuous protection. This eliminates the risk of missed or late monthly doses and is an excellent option for owners who struggle with monthly compliance. ProHeart 6, which provides six months of protection, is also available.

Topical preventatives. Topical heartworm preventatives are available and are applied to the skin monthly. These may be combined with flea and tick prevention in some products.

Year-round prevention. The American Heartworm Society recommends year-round, lifelong heartworm prevention for all dogs, even in regions with seasonal mosquito activity. The rationale includes unpredictable weather patterns (warm spells in winter can activate mosquitoes), the risk of missed doses creating gaps in protection, the presence of mosquitoes inside heated homes during winter, and the fact that many preventatives also protect against intestinal parasites year-round. Year-round prevention also protects against other parasitic diseases depending on the product used.

Annual testing. Annual heartworm testing is recommended even for dogs on consistent prevention. Testing confirms that prevention is working, detects any breakthrough infections early (when they are easiest to treat), and is required by many veterinarians before prescribing or refilling heartworm prevention. Testing is particularly important if any doses were given late, if the dog may have spit out or vomited a dose, or if the dog was off prevention for any period.

Frequently Asked Questions

Can heartworm disease be cured completely?

Yes, heartworm disease can be cured in most dogs with appropriate treatment. The standard three-injection melarsomine protocol, combined with doxycycline (to kill Wolbachia bacteria), heartworm preventative (to prevent new infections and kill microfilariae), and strict exercise restriction, successfully eliminates adult heartworms in the vast majority of cases. However, any damage that the worms have already caused to the heart, lungs, and blood vessels may not be fully reversible. Dogs treated in the early stages of disease typically recover completely with no lasting effects. Dogs with severe or longstanding infections may have permanent lung or heart damage that requires ongoing management even after the worms are eliminated. This is why early detection and prevention are so important.

How much does heartworm treatment cost?

The cost of heartworm treatment varies by geographic region, the dog’s size, and the severity of disease, but typically ranges from $1,000 to $3,500 or more. This includes diagnostic testing, pre-treatment medications (doxycycline, prednisone), the melarsomine injections, hospitalization for injection days, pain management, follow-up testing, and heartworm preventative. Severe cases requiring stabilization, hospitalization, or treatment of complications cost more. By comparison, year-round heartworm prevention costs approximately $50 to $200 per year depending on the dog’s size and the product used — a fraction of treatment cost. This dramatic cost difference is one of the most compelling arguments for consistent prevention.

Can cats get heartworm disease?

Yes, cats can be infected with heartworms, though feline heartworm disease differs significantly from canine heartworm disease. Cats are less susceptible hosts — infections typically involve only one to three worms, and many larvae are killed by the cat’s immune system before reaching maturity. However, even one or two worms can cause significant disease in a cat because of the cat’s smaller cardiovascular system. Feline heartworm disease primarily causes respiratory signs (coughing, difficulty breathing) and is often misdiagnosed as feline asthma. There is no approved treatment to kill adult heartworms in cats (melarsomine is not safe for use in cats), so management is supportive. Prevention with feline heartworm preventatives is the best approach for cats at risk, especially those in regions where respiratory disease may be misattributed.

Why is exercise restriction so important during treatment?

Exercise restriction during heartworm treatment is literally a life-or-death matter. When adult heartworms die — whether from melarsomine treatment or naturally — their bodies break apart into fragments that are carried by blood flow into the smaller blood vessels of the lungs, where they become lodged. This creates blockages (pulmonary thromboembolism) and triggers intense local inflammation. During rest, blood flow through the lungs is relatively slow and the fragments are absorbed gradually over weeks. During exercise, blood flow increases dramatically, which can dislodge larger fragments, overwhelm the lungs’ ability to manage the debris, and cause acute, potentially fatal respiratory distress. Even moderate excitement or activity can be dangerous. This is why dogs must be strictly confined — crate rest with only brief leash walks for bathroom needs — for the duration of treatment and for six to eight weeks after the final injection.

What if my dog misses a dose of heartworm prevention?

If you realize your dog has missed a dose of monthly heartworm preventative, give the dose immediately and resume the regular monthly schedule. If the gap was longer than six weeks, contact your veterinarian — they may recommend giving the missed dose immediately, continuing monthly prevention, and testing for heartworm infection six months later (it takes approximately six months after infection for the antigen test to become positive). If prevention has been lapsed for several months, a heartworm test should be performed before restarting prevention, as giving certain preventatives to a dog with circulating microfilariae can potentially cause a reaction. The best strategy is to set a monthly reminder, use a calendar or app, or consider switching to the 12-month injectable preventative (ProHeart 12) to eliminate the risk of missed doses entirely.