Heartworm in Dogs

Heartworm disease is a serious parasitic infection caused by Dirofilaria immitis. Mosquitoes spread immature larvae when they bite a dog. Over about six months, those larvae mature into adult worms that live mainly in the pulmonary arteries and can also affect the right side of the heart. Even dogs that look normal can have ongoing inflammation and injury inside the chest.

The disease is often quieter than pet parents expect at first. Some dogs only show a mild cough or lower stamina, while others develop weight loss, breathing trouble, fainting, or signs of heart failure. In severe cases, a life-threatening emergency called caval syndrome can occur when large numbers of worms interfere with blood flow. Because damage can continue before obvious symptoms appear, early testing and prevention matter.

Heartworm is found across the United States, and risk is not limited to one region. Indoor dogs, small dogs, and dogs in cooler climates can still be exposed because mosquitoes can get indoors and travel patterns have changed where infected dogs live. That is why many veterinary groups recommend year-round prevention and routine screening, even for dogs that seem low risk.

Dirofilaria immitis has a complex lifecycle that requires both a mosquito intermediate host and a mammalian definitive host. Understanding this lifecycle helps explain why prevention timing matters and why treatment is complicated.

The cycle begins when a mosquito feeds on an infected animal and ingests first-stage larvae called microfilariae (L1). Inside the mosquito, these larvae develop over roughly 10 to 14 days through two molts, progressing from L1 to L2 and then to the infective third-stage larvae (L3). This development within the mosquito is temperature-dependent and requires sustained warmth, which is why transmission is seasonal in some climates. The mosquito is the obligate intermediate host, meaning that Dirofilaria immitis cannot complete its lifecycle without passing through a mosquito.

When the mosquito carrying L3 larvae feeds on a dog, the infective larvae enter through the bite wound. Over the next one to two weeks, the larvae molt into fourth-stage larvae (L4) in the subcutaneous tissue near the bite site. The L4 larvae then migrate through body tissues over the following weeks, eventually reaching the bloodstream and traveling to the pulmonary arteries. By roughly 70 to 120 days after infection, the larvae arrive in the pulmonary vasculature and molt into immature fifth-stage adults (L5). These immature adults continue growing and typically reach full maturity at approximately six to seven months after the initial mosquito bite. It is only at this point that standard antigen testing can reliably detect the infection.

Adult heartworms can grow 10 to 30 centimeters long. Female worms are larger than males. In dogs, worm burdens can range from a single worm to over 250 in severe cases, though typical infections involve 15 to 30 worms. Adult worms can live five to seven years in dogs. Once mature, female worms begin producing microfilariae, which circulate in the bloodstream and can persist for two to three years if not cleared. These circulating microfilariae are what mosquitoes pick up to continue the transmission cycle.

Heartworm has been documented in all 50 US states. While historically concentrated in the Southeast and Mississippi River valley, the geographic range has expanded. Globally, Dirofilaria immitis is found in tropical and subtropical regions on every inhabited continent, as well as in temperate zones where mosquito seasons are long enough to support larval development. Factors such as transport of rescue dogs between regions, urban wildlife reservoirs like coyotes, and changing climate patterns have contributed to wider distribution.

Diagnosis usually begins with a heartworm antigen blood test, which looks for proteins from adult female heartworms. In many clinics, this test can be run in-house during a visit. If the result is positive, your vet will usually recommend confirmatory testing rather than moving straight to treatment, because treatment is lengthy and needs a clear diagnosis.

Confirmatory testing often includes a microfilaria test to look for immature heartworms in the bloodstream. Your vet may also recommend chest X-rays, bloodwork, and sometimes an echocardiogram to understand how much heart and lung damage is present. This staging step helps guide risk assessment, treatment planning, and monitoring.

Timing matters with heartworm testing. It takes about six months after infection for standard testing to reliably detect disease, so recently infected dogs may test negative early on. That is one reason dogs starting prevention later than puppy age, dogs with missed doses, and newly adopted dogs often need a testing plan that includes follow-up at specific intervals.

Heartworm disease is caused by Dirofilaria immitis, a parasite transmitted by mosquitoes. A dog does not catch heartworm directly from another dog through normal contact. Instead, mosquitoes pick up microscopic larvae from an infected animal and later pass infective larvae to another dog during a bite.

Any dog exposed to mosquitoes can be at risk. That includes indoor dogs, dogs in suburban neighborhoods, and dogs in areas with seasonal cold weather. Risk may be higher in places with warm, humid conditions and long mosquito seasons, but national transport of dogs, travel, and changing climate patterns have widened exposure beyond traditionally high-risk regions.

Other practical risk factors include missed preventive doses, delayed testing, unknown preventive history, and adoption from shelters or rescue groups where prior medical records may be incomplete. Puppies can start prevention early, but older dogs usually need testing before beginning a preventive plan. Your vet can help match the testing schedule and prevention approach to your dog's age, lifestyle, and local mosquito pressure.

The American Heartworm Society (AHS) recommends a multimodal treatment protocol designed to eliminate adult worms, circulating microfilariae, and the Wolbachia endosymbiont bacteria that live within heartworms. Understanding each phase helps pet parents follow the plan and know what to expect.

The protocol typically begins with a 28-day course of doxycycline (10 mg/kg twice daily). Doxycycline targets Wolbachia, an intracellular bacterium that heartworms depend on for reproduction and survival. Eliminating Wolbachia weakens adult worms, reduces the inflammatory response when worms die, and decreases pathology in the pulmonary vasculature. Concurrently, the dog is started on a macrocyclic lactone heartworm preventive (such as ivermectin or moxidectin) to eliminate any new L3 and L4 larvae and to begin reducing microfilariae in the bloodstream.

After the doxycycline and preventive pretreatment phase, adulticide therapy with melarsomine dihydrochloride (brand names Immiticide or Diroban) is administered as a deep intramuscular injection into the epaxial lumbar muscles. The AHS recommends a three-injection protocol rather than the older two-injection approach. The first injection is given on day one. The dog then rests for 30 days before receiving two more injections 24 hours apart on days 60 and 61. This staggered schedule is designed to kill worms more gradually, reducing the risk of severe pulmonary thromboembolism that can occur when large numbers of worms die simultaneously.

Exercise restriction is one of the most critical parts of the entire treatment process. When adult worms die, their fragments are carried into the smaller pulmonary blood vessels, where the body breaks them down over weeks to months. Physical exertion increases blood flow and heart rate, which can dislodge worm fragments and trigger life-threatening pulmonary thromboembolism. The AHS recommends strict exercise restriction beginning at diagnosis and continuing for six to eight weeks after the final melarsomine injection. This means leash walks for elimination only, no running, no rough play, and crate or room confinement when unsupervised.

Prednisone is often prescribed in a tapering dose to manage the inflammatory response as worms die. The anti-inflammatory effect helps reduce the severity of pulmonary inflammation, coughing, and respiratory distress during the weeks following melarsomine injections. Dosing and duration vary based on the severity of disease and clinical signs.

Some practitioners and pet parents ask about the slow-kill method, which involves giving monthly heartworm preventive alone without melarsomine, relying on the preventive to gradually shorten adult worm lifespan. The AHS does not recommend this approach for several reasons. Slow-kill takes years rather than months, during which ongoing damage to the pulmonary arteries and heart continues. It also promotes the development of macrocyclic lactone resistance in Dirofilaria immitis populations, which is a growing concern. The fast-kill protocol with melarsomine eliminates adult worms within months and limits the duration of vascular injury.

Post-treatment monitoring follows a defined schedule. Approximately six to nine months after the final melarsomine injection, the dog should be retested with a heartworm antigen test and a microfilaria test to confirm clearance. If antigen or microfilariae are still detected, further evaluation and possible retreatment may be necessary. Dogs should remain on year-round heartworm prevention for life after treatment, as successful treatment does not confer immunity to reinfection.

Heartworm prevention is far safer, easier, and less costly than treating established disease. Preventive products are available by prescription in monthly oral, topical, and longer-acting injectable forms. Your vet can help choose an option based on your dog's age, weight, lifestyle, travel history, and how likely your household is to miss doses.

Major veterinary organizations recommend year-round prevention for most dogs. That advice helps cover unexpected mosquito exposure, reduces gaps caused by missed seasonal timing, and supports control of some other parasites depending on the product used. Puppies can usually start prevention as early as 6 to 8 weeks of age, while older dogs often need testing before starting.

Annual testing still matters, even when a dog is on prevention. No preventive plan is perfect in the real world because doses can be late, spit out, vomited, or skipped. Regular testing helps catch infection earlier and gives your vet a chance to review whether the current product and schedule still fit your dog's needs.

Although this article focuses on canine heartworm, it is important to understand how the disease differs in cats, because many households include both species and the implications for prevention are significant.

Cats are atypical or aberrant hosts for Dirofilaria immitis. The feline immune system mounts a strong response against developing larvae, which means most larvae do not survive to adulthood. When infection does become established, the worm burden is typically much lower than in dogs, usually one to three adult worms and sometimes only immature worms. However, even a small worm burden can cause severe disease in cats because their pulmonary vasculature is proportionally smaller.

A condition known as Heartworm Associated Respiratory Disease (HARD) can develop in cats even when immature worms die before reaching adulthood. The arrival and death of larvae in the pulmonary arteries triggers an intense inflammatory response that closely mimics feline asthma or allergic bronchitis. HARD can cause coughing, wheezing, difficulty breathing, and vomiting. Because the worms may never mature to a stage detectable by standard antigen testing, HARD can be difficult to diagnose and is likely underrecognized.

There is no approved adulticide treatment for heartworm disease in cats. Melarsomine, the drug used to kill adult heartworms in dogs, is not safe for use in cats due to the risk of fatal anaphylaxis and pulmonary thromboembolism even from a small number of dying worms. Management of heartworm-positive cats is limited to supportive care, monitoring, and treatment of clinical signs such as corticosteroids for inflammation and bronchodilators for respiratory distress. In some cases, surgical extraction of worms has been performed, but this requires specialized facilities and carries significant risk.

Because there is no safe treatment, prevention is the only reliable strategy for cats. Monthly preventives approved for cats are available in topical and oral formulations. The American Heartworm Society recommends year-round prevention for all cats, including indoor-only cats. Studies have shown that a meaningful percentage of cats diagnosed with heartworm infection were described by their owners as indoor cats, because mosquitoes readily enter homes through doors, windows, and gaps in screens.

Many dogs do well when heartworm disease is found before severe heart and lung damage develops and when treatment is completed exactly as directed. Prognosis depends on worm burden, how long the infection has been present, whether the lungs and heart are already affected, and how well exercise restriction can be maintained during recovery.

Recovery is not quick. Even after treatment starts, dead and dying worms can trigger inflammation and dangerous complications if a dog becomes too active. That is why your vet may recommend weeks to months of restricted activity, leash walks only, and careful monitoring for cough, breathing changes, weakness, or collapse.

Dogs with advanced disease, pulmonary hypertension, or caval syndrome have a more guarded outlook and may need specialty care. Still, many dogs improve with a thoughtful plan. Long-term success also depends on staying on year-round prevention after treatment and returning for the recommended follow-up tests to confirm the infection has cleared.