Heartworm Testing, Prevention, and Treatment

Heartworm Disease is a common and serious condition in dogs. Fittingly named, the disease is caused by worms which live in your dog’s heart and surrounding blood vessels. Heartworm disease can be severe, especially with higher worm burdens and worms that have lived in your dog for longer periods of time. Heartworm has been diagnosed in every U.S. state as well as many countries around the world. This article will discuss the nature of heartworm disease in dogs and how it is acquired, prevented, and treated.

Heartworm disease, or Dirofilariasis, is a parasitic infection in dogs and, less commonly, in cats. Mosquitoes spread heartworm disease, and dogs are the definitive host in that they support adult worms and help perpetuate the reproductive cycle.

How Does My Dog Get Infected with Heartworms?

First, a female mosquito bites a dog with untreated heartworm disease and thereby ingests stage I larvae called microfilaria (more on this later). There are then two stages of development inside the mosquito, which together take only about two weeks. That mosquito can now infect other dogs with its Stage III infectious heartworm larvae through bites. Over the next few months, immature heartworms inside the dog mature into stage IV and then young adult Stage V larvae while migrating through the tissues inside your dog. Over the next few months, the young adult heartworms take up residence inside the pulmonary arteries of your dog and begin producing stage I larvae (microfilaria) to continue the life cycle of the worm.

How Do They Affect My Dog?

The worms inside your dog damage the blood vessels surrounding your dog’s heart and causing high blood pressure. Additionally, damage to your dog’s heart may result in enlargement of the heart, which can lead to Congestive Heart Failure. In some cases, migrant worms may also damage a dog’s liver and kidneys. Additionally, heartworms have obligate bacteria inside of them, which can cause serious allergic reactions in dogs when the heartworms are damaged or destroyed.

How Is Heartworm Disease Diagnosed?

Heartworm disease can be detected with a tiny blood sample from your dog using a testing kit readily available at most veterinary hospitals. The test usually takes about 8 minutes to get a result. When a positive test result is identified, additional tests are required to confirm the diagnosis. It is important to note that heartworm testing can rarely have false-negative results and will only detect heartworm disease at least 5-7 months after the initial infecting mosquito bite. False negatives can occur in 3 conditions:

  1. Just a few worms are present inside your dog.
  2. Only juvenile-stage worms are present (the test detects the presence of adult worms only)
  3. All-male worm infection (the test detects female adult worms).

For this reason, dogs who miss at least 2 consecutive months of heartworm prevention, are at higher risk of acquiring the disease, or have unknown prevention status (rescues, recent shelter adoptions) should be re-tested 7 months after the last possible infectious bite from a mosquito. Yearly screening is also strongly recommended for all dogs, even for pets who are on heartworm prevention regularly.

How Do I Prevent My Dog from Getting This Horrible Disease?

Fortunately, heartworm prevention is readily available and is effective at preventing dogs and cats from acquiring heartworm disease. Heartworm prevention works by killing juvenile stages of heartworms before becoming adults inside your dog. Routine programs to control mosquito populations may also be helpful, especially as concern for resistance to heartworm preventative care grows.

Veterinarian for Treating Heartworm Disease

Heartworm is a treatable disease, but the damage caused by heartworms is permanent. Patients with heartworm disease are staged based on bloodwork changes and radiographs, which show a patient’s heart, vital organs, and blood vessels. Higher-class heartworm patients have a poorer prognosis, and treatment carries higher risks of side effects and complications. The actual treatment for heartworm disease centers around killing the bacteria inside the worms, reducing inflammation caused by the worms, and ultimately, killing the worms themselves. The American Heartworm Society has established a protocol for treating heartworm-positive dogs, and more information can be found on their Website. While treatment is extremely effective at eliminating the worms, it does carry the risk of serious side effects and risks. For this reason, prevention is vastly superior to treatment.

A Word about Cats

Cats can get heartworm disease, though indoor-only cats are at considerably less risk of exposure to mosquitoes. It’s much more common for cats with heartworms to exhibit symptoms than is the case with dogs. However, those that do have symptoms tend to be even sicker than their dog counterparts. Heartworm prevention for cats exists and should be strongly considered for outdoor cats or those who live in high mosquito or high heartworm risk areas.


Heartworm is a serious disease, and treatment for heartworm infections in dogs is both costly and seriously risky, sometimes resulting in dangerous complications and side effects. Of course, it’s better to treat heartworms when present, but prevention is essential to keeping your dog or at-risk cat safe. For more information on heartworm disease, or if you have lapsed in your dog’s prevention, I recommend scheduling an appointment with a member of our Veterinary Hospital who can discuss heartworm in more detail, answer any specific questions you have about heartworm and dogs, and of course, test your dog and begin monthly prevention. When it comes to keeping your pet safe, an ounce of prevention truly is worth a pound of cure. In the case of heartworm disease, this is especially good by which to live.


Jeffrey Stupine V.M.D.

Medical Director

World of Animals Veterinary Hospitals