Heartworm in Dogs: Signs, Treatment and Prevention

What it is

Heartworm spreads through mosquito bites. A mosquito feeds on an infected dog, fox, coyote, or wolf and picks up baby worms called microfilariae. Over roughly 10 to 14 days these become infective larvae inside the mosquito, which then enter the next dog through the bite wound.

The larvae spend about six to seven months migrating and growing before they settle as adults in the heart and the blood vessels of the lungs. Adult heartworms can live five to seven years, and each mosquito season can add more worms to an unprotected dog, so damage builds over time.

Early on, many dogs show few or no signs. As the worm burden grows, you may see a mild, persistent cough, reluctance to exercise, tiring after moderate activity, reduced appetite, and weight loss. Advanced disease can lead to heart failure, with fluid collecting in the belly. Caval syndrome is a rare emergency in which worms block blood flow near the heart, causing sudden labored breathing, pale gums, and dark, coffee-colored urine. If your dog is coughing, see coughing in dogs for other possible causes too.

Causes and risk factors

What you can do at home

Heartworm cannot be treated at home, but what you do at home matters once your dog is diagnosed. The American Heartworm Society advises restricting exercise as soon as the diagnosis is confirmed, because physical exertion speeds up damage to the heart and lungs, and the more severe the signs, the less activity your dog should have. During treatment, CAPC advises limiting excitement and exercise to slow walking for at least a month after each set of injections. Your vet will tell you exactly how strict to be.

Keep watching your dog's breathing, cough, energy, and gum color, and report changes promptly. Sudden labored breathing, collapse, pale gums, or dark urine call for an emergency vet at once.

Do not try to clear an infection with a regular preventive alone; CAPC does not recommend this slow-kill approach. Do not start or restart a preventive on an untested dog on your own. The FDA warns that in a dog with baby worms in the blood, a preventive can kill them suddenly and trigger a shock-like reaction. To lower mosquito exposure, limit outdoor time when mosquitoes are most active, and use mosquito repellents only if they are approved for dogs.

What happens at the vet

Diagnosis starts with blood tests. An antigen test detects mature adult heartworms, and a second test looks for microfilariae in the blood. No single test finds every infection, and tests cannot reliably detect an infection until the worms are about seven months old, so your vet may repeat testing. Chest X-rays can show changes in the heart and lungs. A positive antigen test is usually confirmed with a different test before treatment starts.

Before treatment, your vet assesses the damage with a physical exam, chest X-rays, and lab work, and may stabilize a very sick dog first, which can take months in severe cases. Treatment follows a multi-step protocol set by your vet, built around an injectable medication that kills the adult worms. Some protocols begin with an antibiotic and a preventive beforehand, and microfilariae may need separate treatment. Exercise restriction continues throughout, because dying worms can inflame the lungs and arteries.

Dogs with few or mild signs generally do well, while severe disease carries a higher risk of complications. Your dog is retested about nine months after treatment to confirm the worms are gone. Dogs in caval syndrome usually need emergency surgery to remove the worms. Prevention then continues for life.

Prevention

Prevention is safer and more affordable than treating adult heartworm, and it is the best protection for your dog's heart. Heartworm preventives require a prescription. According to the FDA, they come as monthly oral tablets, monthly spot-on liquids applied to the skin, and longer-lasting injections given by your vet, and your vet can help you choose. Give the preventive year-round, exactly as prescribed, because mosquitoes can transmit heartworm any time they are active. CAPC advises prevention for all dogs regardless of lifestyle.

Test before you start. Dogs 7 months and older should have a negative heartworm test before beginning prevention, then be tested every year, even if no dose was ever missed. The AHS warns that a single missed or late dose, a pill that was spit out or vomited, or a spot-on that rubbed off can leave a dog unprotected, and CAPC notes some heartworm strains can mature even in dogs on routine prevention.

Keep your dog's weight within the range on the product package and tell your vet if it changes. Reducing mosquito exposure and using dog-approved repellents add a layer of protection. If your dog develops a cough or tires easily, bring it up with your vet instead of waiting for the next routine visit.