Heartworm is a parasitic disease caused by foot-long worms that live inside a dog’s heart, lungs, and surrounding blood vessels. Left untreated, it causes lasting damage to these organs and can be fatal. Dogs get heartworm exclusively through mosquito bites, not from contact with other dogs, and the infection takes months to become detectable, which is why routine annual testing matters even for dogs that seem perfectly healthy.
How Dogs Get Heartworm
The parasite responsible is called Dirofilaria immitis. Its life cycle depends entirely on mosquitoes as a go-between. When a mosquito feeds on an infected animal, it picks up microscopic baby worms (called microfilariae) circulating in the blood. Inside the mosquito, these larvae develop over roughly two weeks into an infective stage. The next time that mosquito bites a dog, the larvae exit the mosquito’s mouthparts and crawl across the dog’s skin into the bite wound.
Once inside the dog, the larvae migrate through tissue and eventually reach the heart and pulmonary arteries, where they mature into adults. This journey from initial bite to full-grown worm takes about six months. Adult heartworms can live five to seven years in a dog, grow up to 12 inches long, and reproduce, releasing new microfilariae into the bloodstream to continue the cycle.
Several factors influence how common heartworm is in a given area: mosquito species present, temperature, humidity, and how many infected animals are nearby to serve as reservoirs. Warmer temperatures speed up larval development inside mosquitoes, which is one reason the disease is most prevalent in the Gulf Coast and southeastern United States. The American Heartworm Society’s most recent incidence survey, reflecting 2025 data, found that Texas leads the nation in heartworm cases. But the same survey flagged troubling increases in areas far from these traditional hotspots, meaning heartworm is a realistic concern across the entire country.
Symptoms at Each Stage
Heartworm disease is classified into stages based on how many worms are present and how much damage they’ve caused. In the early stage, a dog with a low worm burden may show no symptoms at all. This is one of the disease’s biggest dangers: by the time you notice something is wrong, significant internal damage may already exist.
As the disease progresses to a mild or moderate stage, the most common signs are a persistent cough and reluctance to exercise. Your dog may tire more quickly on walks or seem winded after normal activity. These symptoms develop because worms physically obstruct blood flow through the lungs and trigger inflammation in the surrounding vessels.
In severe disease (stage 3), the heart begins to fail. Dogs at this point often have labored breathing, a swollen belly from fluid accumulation, and dark or reddish urine caused by the destruction of red blood cells. Large numbers of worms pack the right side of the heart and the major blood vessels leading to the lungs, creating a condition called caval syndrome. Dogs with caval syndrome show sudden loss of appetite, extreme weakness, pale gums, and visibly distended neck veins. Even with emergency intervention to physically remove the worms, caval syndrome carries a mortality rate of 30 to 40 percent.
How Heartworm Is Diagnosed
Heartworm is detected through a simple blood test at your vet’s office. The primary test looks for a protein released by adult female heartworms. It’s the most sensitive method available, but it has an important limitation: the protein doesn’t show up until at least five months after infection, because the worms need time to mature. This is why a dog can test negative and still be harboring an early infection.
A second type of blood test checks for microfilariae circulating in the bloodstream. This can be detected starting around six months after infection. The American Heartworm Society recommends running both tests together, since some dogs have adult worms but no circulating microfilariae (a situation called an occult infection), while the microfilaria test helps confirm active reproduction and rule out other harmless parasites that can look similar under a microscope.
Puppies under seven months old don’t need testing because any infection they picked up wouldn’t be detectable yet. Puppies started on preventive medication after eight weeks of age should be tested six months after the first dose, then annually. For dogs seven months or older that haven’t been on prevention, the recommended schedule is to test immediately, again at six months, again at 12 months, and annually after that.
What Treatment Looks Like
Treating heartworm is considerably harder, riskier, and more expensive than preventing it. The process typically spans several months and involves multiple vet visits. Treatment kills the adult worms, but as they die and break apart, fragments can block blood vessels in the lungs. This is why strict exercise restriction is the single most important part of the treatment process. Even mild exertion increases blood flow through the lungs and raises the risk of a potentially fatal blockage.
For most dogs, treatment includes an antibiotic course to weaken a bacterium that lives inside the heartworms. Killing this bacterium first makes the worms more vulnerable and reduces the inflammatory reaction when they die. The adult worms are then killed with a series of injections given deep into the muscles of the lower back. These injections are painful, and your dog will likely be sore for a few days after each one.
The full treatment timeline, including the antibiotic phase, injections, and recovery period, often stretches to four or five months. During this entire time, your dog’s activity needs to be severely limited: leash walks only, no running, no roughhousing, no off-leash time. For active dogs, this prolonged confinement is one of the most challenging parts for owners to manage. After treatment, your vet will retest to confirm the infection has cleared.
Dogs with advanced disease or caval syndrome face a much harder road. Some require emergency surgical removal of worms before standard treatment can even begin, and the prognosis at that stage is guarded at best.
Prevention Options
All heartworm preventives belong to a single drug class that works by killing the larval stages of the parasite before they can mature into adults. These medications are available in oral, topical, and injectable forms. Oral preventives are given monthly and come in chewable tablets. Topical versions are applied to the skin monthly. An injectable option administered by your vet provides protection for six or 12 months depending on the product.
The active ingredients across these products fall into two related groups. One group includes ivermectin and selamectin. The other includes milbemycin oxime and moxidectin. All are highly effective when given consistently and on schedule. Many combination products also protect against intestinal parasites like roundworms and hookworms, or external parasites like fleas.
Year-round prevention is recommended regardless of where you live. Even in northern climates with cold winters, unseasonably warm spells can allow mosquitoes to emerge earlier than expected. The geographic spread of heartworm cases documented in recent surveys reinforces that skipping prevention during “off-season” months carries real risk. A single missed dose creates a window where larvae deposited by a mosquito bite can survive and develop beyond the point where preventive medication can kill them.
Annual testing remains important even for dogs on year-round prevention. No medication is 100 percent effective, and a late or missed dose, a tablet that was spit out unnoticed, or a topical that was washed off can leave gaps in protection. Catching a breakthrough infection early, before it causes organ damage, makes treatment far safer and more straightforward.