Valentine’s Day will soon be here and that gets us thinking about hearts! While the thought of heartworms isn’t very romantic, we feel it’s an important subject to discuss to help prevent this disease in the pets we love so much.

What is heartworm disease?

Heartworm disease is a serious illness caused by worms that live in the pulmonary vessels of infected pets. (Pulmonary vessels carry blood to the heart and lungs.) These foot-long worms—which look like strands of spaghetti—can cause severe lung disease, heart failure, and damage to other body organs. Heartworm disease is transmitted by mosquitoes. Dogs of any age, breed, or living condition, indoors or outdoors, are at risk in our area. It only takes one bite from an infected mosquito to infect your pet!

What animals are at risk for heartworm disease?

Heartworm disease affects dogs, cats, ferrets, and other non-companion mammals including wolves, coyotes, foxes, and sea lions. Foxes and coyotes are considered important carriers because of their potential to live in close proximity to humans and their pets.

Are dogs natural hosts for heartworms?

Dogs are a natural host, meaning they provide a welcoming environment for heartworms to mature into adults, mate, and produce offspring. If left untreated, one canine can potentially harbor hundreds of heartworms in her body.

What are the symptoms of heartworm disease?

Often dogs do not exhibit any signs of illness at all until the disease has progressed significantly. Some advanced symptoms include asthma-like persistent coughing or difficulty breathing, fatigue or lethargy, decreased appetite, and weight loss. Dogs harboring large numbers of heartworms can have sudden blockages of blood flow to the heart, causing cardiovascular failure.

By the time symptoms set in, prevention and treatment are usually too late. The medical work-up and treatment for heartworm disease in dogs is very expensive, painful, and requires strict rest for the pet for several months to limit the risks of complications (and complications can be very serious).

What can I do to prevent heartworm disease in my pets?

Because there are so few early signs of the disease, routine testing to detect heartworms is very important. For dogs, we recommend an annual blood test performed during your pet’s yearly exam. This simple test requires just a small blood draw and is processed in our in-house lab. (As a bonus, this blood test also screens for three tick borne diseases: Lyme, Ehrlichia, and Anaplasmosis.) We believe the best defense against heartworms is giving dogs a preventative every month of the year.

How do heartworm preventatives work?

Heartworm preventatives can be in the form of a chewable tablet or a spot-on topical medication. In both cases, they work by killing the immature (larval) stages of the heartworm parasite. This includes the infective heartworm larvae deposited by the mosquito, as well as the following larval stage that develops inside the animal. Once the immature larvae become adults, they cannot be effectively eliminated by preventives. Because of this, it is very important that heartworm preventives be administered strictly on schedule each month without interruption. Administering prevention late or skipping months altogether (like in the colder winter months) can allow immature larvae to molt into the adult stage.

I’m very good at administering my pet’s heartworm preventative each month and he had a heartworm test at last year’s exam. Does he really need another test so soon?

Yes, we do recommend a heartworm test every year for dogs. Even if you are a responsible pet owner keeping your pet on prevention year-round, an annual test ensures the prevention is working properly. Heartworm medications are highly effective, but like all medications, nothing works perfectly 100% of the time. And even if one monthly dose is missed, your pet will be unprotected. Some pets may also spit out their pills when you’re not looking, vomit, or rub off the topical medication. If your pet does contract heartworms, the earlier it is detected the better chances he has for a full recovery.

It is also important for your veterinarian to confirm your pet does not have heartworms before beginning or renewing a preventative prescription. Administering preventatives to a heartworm-positive pet can have severe or even fatal medical complications. It takes about six months for dogs with heartworms to test positive (this is the amount of time it takes the infective larvae to mature to adult heartworms), so your pet may have been infected with larvae at last year’s exam, but did not read positive on that test.

This is article contains contributions from Leesburg Veterinary Hospital’s Dr. Jennifer Boyle and Dr. Lauren Kloer. To get more advice on animal-related topics, visit LVH’s blog at