heartworm disease in dogs and cats: prevention, testing, and treatment

Heartworm is a serious and potentially fatal disease spread to pets through mosquito bites. The disease may cause painful symptoms and lasting damage even if dogs are treated, and there is no approved treatment for heartworm in cats. Year-round prevention is the most important step you can take to keep dogs and cats safe from heartworm.

Early heartworm infection often causes no obvious signs. Indoor pets are also at risk because a single infected mosquito can enter a home and transmit heartworm. Annual heartworm testing is standard for dogs, but preventing the disease is far simpler and less costly.

Talk to a veterinarian about finding year-round parasite control for your dog or cat that covers heartworm and is right for your pet’s breed, age, and lifestyle. Or if you suspect your pet may have been exposed, book a heartworm test.

How do pets get heartworm?

Heartworm is spread by mosquitoes. A mosquito picks up immature heartworms, called microfilariae, when it bites an infected animal. The parasite develops inside the mosquito and can then be passed to another pet when that mosquito bites. Heartworm is not contagious directly from one pet to another. A dog or cat cannot get heartworm simply by living with an infected pet.

After a mosquito transmits the parasite, the larvae gradually develop into adult worms. It takes roughly 6 to 7 months for heartworms to mature into adults, which is one reason testing cannot immediately detect a very recent infection.

Adult heartworms live primarily in the heart, lungs, and nearby blood vessels. Mature female heartworms can reach about a foot long, while males are roughly half that size.

Heartworm disease occurs throughout the United States. Local climate and geography can affect mosquito activity and disease risk, but they do not eliminate the need for prevention.

What are the signs of heartworm in dogs?

Early heartworm infection in dogs may cause no noticeable signs, even though worms are developing inside the body. A dog that looks healthy can still have heartworm disease.

As the infection progresses, common signs can include:

• soft, persistent cough
• tiring easily during normal walks or activity
• reduced appetite
• weight loss
• labored breathing
• fluid buildup associated with advanced heartworm disease

The severity of signs depends on factors such as the number of worms present, how long the infection has been developing, and the dog's activity level and overall health.

Because early disease can be silent, waiting for symptoms is not a reliable way to determine whether a dog has heartworm. Annual heartworm testing is recommended even when a dog receives year-round prevention.

Can cats get heartworm?

Yes. Cats can get heartworm disease, even if they live indoors. Heartworm can be particularly difficult to recognize in cats because its signs can resemble other respiratory problems, including asthma or what owners may mistake for a hairball problem.

Even one or two heartworms can cause serious disease in a cat because the cat's heart and lungs are smaller than a dog's. Many infected cats also never develop fully mature adult heartworms, which makes diagnosis more complicated.

Signs of heartworm disease in cats can include:

• coughing
• wheezing
• rapid or difficult breathing
• intermittent vomiting
• weight loss

Some cats show no obvious warning signs before developing serious complications.

Heartworm diagnosis in cats is more complicated than in dogs and may require more than one type of test. Your veterinary team can determine which combination of testing is appropriate based on your cat's history and signs.

How does heartworm prevention work?

Heartworm prevention is a veterinarian-prescribed medication given year-round. Depending on the product and the pet, prevention may be available as a monthly chewable, a monthly topical medication, or a longer-acting injection for dogs.

Heartworm preventives work by eliminating immature heartworm larvae acquired relatively recently from mosquito bites. They do not kill established adult heartworms, which is why giving prevention consistently is important. It’s important to have dogs tested before putting them on heartworm prevention, since it can cause a severe reaction if heartworm microfilariae are circulating in their bloodstream.

Many heartworm preventives also protect against common intestinal worms. Learn more about worms in dogs and cats

If your pet's preventive dose is late or you miss a dose, let your veterinary team know and ask for guidance. Depending on how late the dose is and your pet's history, the veterinary team may recommend a specific schedule or additional testing.

Get affordable year-round parasite prevention with Optimum Wellness Plans®. By adding this customization to your plan, you get it at a discounted price. Learn more about OWP customizations

Dogs older than about 7 months should be tested before starting or restarting heartworm prevention if they have not been appropriately screened. A dog that tests positive for heartworm can still be placed on prevention, but this should happen under direct veterinary supervision. Dogs with circulating heartworm microfilariae can have a reaction to a preventive dose, so your veterinary team should determine the appropriate next steps.

Puppies younger than 7 months can generally start heartworm prevention without first testing because a recent infection may not yet be detectable. Testing too early could produce a falsely reassuring negative result. After starting prevention, puppies are tested at about 6 months, again 6 months later, and annually after that.

How is heartworm treated in dogs?

Heartworm disease can be treated in dogs, but the treatment is lengthy, costly, and demanding, often taking several months. The treatment plan depends on how advanced the disease is and whether the dog needs to be stabilized before the adult worms are treated.

Exercise restriction is an important part of treatment. Dogs generally need immediate and continued restriction of activity from diagnosis through the end of treatment because dead and dying worms can cause inflammation in the lungs and can lodge in the pulmonary arteries. Physical activity increases the risk of serious complications.

Treatment generally involves:

1. Assessing how advanced the heartworm disease is and stabilizing the dog if needed.
2. Starting medication that includes a heartworm preventive and doxycycline before adult worms are treated.
3. Giving a series of injections to kill adult heartworms, usually in stages rather than all at once.
4. Performing follow-up testing for microfilariae and then antigen to confirm that the infection has cleared.

The cost of heartworm treatment varies based on factors such as the dog's size, the stage of disease, and the number of follow-up visits and treatments required. Your local Banfield team can provide an estimate.

How is heartworm managed in cats?

There is no approved treatment that kills adult heartworms in cats. The medication used to kill adult heartworms in dogs is not safe for cats.

Because adult heartworms cannot be eliminated with an approved medication, treatment for cats focuses on managing the disease and its effects on the lungs. The veterinary team may monitor the cat over time and use medication to control lung inflammation and other signs.

Corticosteroids or other supportive medications may be used when respiratory signs flare. A cat experiencing respiratory distress may need emergency stabilization and oxygen.

In rare, carefully selected severe cases, surgical removal of heartworms may be possible.

Adult heartworms can survive roughly 2 to 3 years in cats. This makes heartworm prevention especially important for cats because preventing infection is much safer than trying to manage established heartworm disease.

Heartworm FAQs

When to contact your veterinary team

Book an appointment if your dog develops a persistent cough, tires easily during normal activity, or has labored breathing. For cats, contact your veterinary team about coughing, wheezing, repeated vomiting, weight loss, or unexplained breathing changes.

Routine reasons to see your vet:

• annual heartworm testing for dogs
• testing before starting or restarting prevention when appropriate
• a missed or late preventive dose
• starting heartworm prevention for a new puppy or kitten
• discussing the appropriate prevention option for an indoor or outdoor cat

Sudden collapse, severe difficulty breathing, pale or blue gums, or dark coffee-colored urine requires emergency veterinary care. Go to the nearest emergency animal hospital right away.

Important Notice: The information provided on this page is for informational purposes only and is not a substitute for professional veterinary advice. Always consult a veterinarian with any questions regarding your pet's health or medical condition. Never disregard or delay seeking professional veterinary advice based on information from this page.