Heartworm in Dogs

Heartworm is caused by a parasitic roundworm, Dirofilaria immitis. It primarily infects dogs, cats, and ferrets, though it can affect a range of wildlife too. It's a potentially fatal parasite but importantly, it's also highly preventable. Humans can rarely become infected, though this is uncommon and typically doesn't cause illness.

Mosquitoes are the only way heartworms spread from animal to animal. When a mosquito bites an infected animal, it picks up young heartworms (microfilariae) along with the blood meal. Inside the mosquito, these develop into infective larvae over 2 to 6 weeks and migrate to the mosquito's salivary glands, ready to be passed on the next time that mosquito bites another animal.

Once a dog is bitten by an infected mosquito, it typically takes 6 to 7 months for those larvae to mature into adult worms living in the body; this window is called the prepatent period. The larvae spend their first 1 to 2 weeks developing under the skin near the bite site, then migrate into the chest and abdominal muscles. Around 45 to 60 days post-infection, they molt into their next larval stage, and by 10 to 15 weeks, they enter the bloodstream and travel through the heart to settle in the pulmonary artery of the lungs.

Over the following three months, the worms mature into adults. Adult female worms can grow up to 14 inches long, with males somewhat shorter. As they grow and feed, the worms damage blood vessels and reduce the heart's efficiency at moving blood through the body. In severe infections, adult worms can even migrate into the right side of the heart and large cardiac veins, leading to serious heart and lung disease. This is what's known as heartworm disease, and it can ultimately cause fatal congestive heart failure. Adult heartworms are notably long-lived, surviving 5 to 7 years in dogs.

Around 5 to 7 months post-infection, if both male and female worms are present, they mate and produce new microfilariae, continuing the cycle when another mosquito bites the infected dog.

In dogs, microfilariae are detectable in the bloodstream in roughly 80–90% of infections, meaning infected dogs are quite effective at passing the parasite on to biting mosquitoes, which is part of why widespread prevention matters so much for controlling heartworm at a population level.

Heartworm was once mostly confined to the southern United States, but it's now found in nearly every region where mosquitoes are present, as well as in many other countries. All dogs are susceptible, regardless of whether they spend most of their time indoors or out; any dog that has contact with mosquitoes, even occasionally, should be tested. If you're planning to travel with your dog, it's worth asking your vet about heartworm risk in your destination.

Dogs with recent or mild infections often show no signs of illness at all. Once adult worms have established themselves in the lungs, though, symptoms can include coughing, difficulty breathing, lethargy, reduced appetite, and tiring quickly after only light or moderate exercise.

Several diagnostic tools are available. An antigen test, which detects adult female heartworms, is highly accurate in dogs and is the primary screening tool. Antibody tests can detect prior exposure. To confirm a diagnosis and assess severity, your vet may also recommend chest X-rays, bloodwork, or an echocardiogram (heart ultrasound), all of which help shape the treatment plan.

In severe cases, surgery to physically remove heartworms may be necessary. This is a high-risk procedure and is generally reserved for the most serious infections, where it offers the best chance of survival.

Heartworm infection is almost completely preventable in dogs. A range of preventive medications is available in different formulations, and your veterinarian can help determine what fits your dog's risk factors and lifestyle best. Though of course, the medication only works if it's given consistently.

Preventives don't kill adult heartworms and won't clear an existing infection, which is why a blood test to check current heartworm status is recommended before starting a prevention program. After starting prevention, a follow-up test at 6 months confirms your dog wasn't already infected before starting (since tests only detect adult worms, not larvae). From there, annual testing is recommended, both to catch any new infections and to confirm the preventive is working as intended, since a small number of dogs can still become infected despite consistent year-round use.