Heartworm Testing, Prevention, and Treatment

Heartworm Disease is a common and serious condition in dogs. The name is an accurate description of what the disease is, worms that live in your dog’s heart and surrounding blood vessels. Heartworm disease can be very serious, 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. In the following article, I will discuss heartworm disease in dogs, how it is acquired, prevented, and treated.

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

How Does My Dog Get Infected With Heartworms? (The Science Part)

First, a female mosquito bites a dog with untreated heartworm disease and through that blood meal ingests stage I larvae called Microfilaria (more on this later). There is then 2 stages of development inside the mosquito which typically takes only about 2 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 Does This Affect My Dog?

The worms inside your dog damage the blood vessels surrounding your dog’s heart causing high blood pressure. Additionally, damage to your dog’s heart can result in enlargement of the heart and resulting 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 heartworms are damaged or destructed.

How Is Heartworm Diagnosed?

Heartworm disease can be detected with a very small 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 that initial infecting mosquito bite. False negatives can occur in 3 conditions:

  1. Just a few worms are present inside your do
  2. Only juvenile stages 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 they can become adults inside of your dog. Routine programs to control mosquito populations may also be helpful, especially as concern for resistance for heartworm grows.

Veterinarian For Treating Heartworm Disease

Heartworm is a treatable disease but the damage caused by the heartworms is permanent. Patients with heartworm disease are staged based on changes noted in bloodwork 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 of 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 greatly superior to treatment.

A Word About Cats

Cats can get heartworm disease although indoor-only cats, of course, are at less risk of exposure to mosquitoes. It should also be noted that more commonly cats can be positive for heartworms and have no symptoms than dogs, although those that do have symptoms tend to be even sicker than their dog counterparts. Heartworm prevention for cats does exist and should be strongly considered for outdoor cats or those who live in high mosquito or heartworm positive areas.


Heartworm is a serious disease and treatment for heartworm infections in dogs is costly and not without serious risk of complications and side effects. Of course, treatment is better than not treating, but prevention is by far the most recommended way to help keep 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. An ounce of prevention is worth a pound of cure, and in the case of heartworm disease, is good advice to live by.


Jeffrey Stupine VMD

Medical Director

World of Animals Veterinary Hospitals