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Heartworm Testing for Dogs: Why It’s Important

Dog lies in grass with mosquitoes in the air
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There is nothing more sad than seeing a dog with heartworm disease because it is completely preventable. By the time a dog is suffering from symptoms, he is really sick and he may not survive the treatment. 

Annual heartworm testing for dogs can help catch infections early and ensure safe and effective treatment. It’s also a crucial step required before starting your dog on a monthly heartworm preventative, like Heartgard Plus, or broad-spectrum parasite prevention, like NexGard Plus. But what’s involved in a dog heartworm test and why is testing so important? Let’s dive in.   

What is a Dog Heartworm Test?

A dog heartworm test is an antigen test that requires collecting a small blood sample. Heartworm tests in dogs detect proteins that are unique to adult female heartworms. 

Heartworm tests for dogs are performed on small blood samples. If your dog is also having other blood tests performed, then your veterinarian will only collect one blood sample so your dog doesn’t have to have more than one needle stick. 

Does My Dog Need a Heartworm Test?

Closeup of mosquito

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Heartworm disease is transmitted by mosquitoes and is becoming very common in the United States. Adult heartworms live in a dog’s lungs where they cause damage to the lungs and the heart. Heartworm disease can be fatal, but typically dogs don’t show any symptoms in the early stages of the disease. 

Every dog in the United States should be tested for heartworm disease once a year and should be placed on a monthly heartworm preventative, like Heartgard Plus, to ensure year-round protection. Heartworm prevention should not be started (or re-started) without first confirming a dog’s heartworm status by testing for these deadly worms. The only exception is puppies less than 7 months of age—they can be started on heartworm prevention without a test. 

This precaution also applies to broad-spectrum flea and tick control medications, like NexGard Plus, that also help prevent heartworm disease and fight intestinal worms. A dog would need a negative heartworm test prior to starting a combination parasite protection product like this.

Dogs who have not been on heartworm prevention or who’s prevention has lapsed should be tested six months after re-starting heartworm prevention and again at one year. This includes dogs with unknown histories, such as those adopted through shelters and rescues.

Not that long ago, dogs in some parts of the United States were considered very low risk for contracting heartworms, and so veterinarians only recommended testing every two years. However, heartworm disease has been spreading rapidly across the United States, and so the recommendations have changed to annual testing and year-round prevention. 

A heartworm infection caught early, before there are any symptoms, is much easier to treat and your dog won’t be left with long-term consequences, such as damage to the heart and lungs. 

Types of Dog Heartworm Tests

Heartworm test using microscope

There are several types of tests for heartworms in dogs that your veterinarian can perform. The most common test performed in a veterinary office is called a SNAP test. These are similar to an at-home COVID test. A SNAP heartworm test catches about 85 percent of mild cases of heartworm disease and is more accurate in dogs with more severe disease. [1

Testing for heartworms in dogs can also be performed at off-site laboratories. These tests are considered even more accurate than SNAP tests. Sometimes if your dog shows no symptoms of heartworm disease but tests positive on a SNAP test, the test will be confirmed at one of these types of laboratories. This is because the treatment for heartworms is expensive and has potential side effects, and therefore should not be administered unless necessary. 

Other types of dog heartworm tests include the filter method and a Modified Knott’s test. These tests detect microfilaria, or larval heartworms, that circulate in the blood. These are only present if adult heartworms in your dog have been there long enough to mate and produce offspring. These larval heartworms can be seen under a microscope as tiny wriggling worms just a few red blood cells in size. However, many dogs can be positive for heartworms without having microfilaria, so this is not considered a sensitive test and is not relied upon to determine whether a dog has heartworms. 

If a dog is found to be heartworm positive on a SNAP test, then your veterinarian may check for microfilaria to determine the stage of disease and select the best, most effective treatment plan. 

There are no accurate at-home heartworm tests for dogs. One of the main reasons is that accurate testing requires a blood sample, and that is not something most pet parents can or should collect at home. 

Dog Heartworm Testing Benefits

Heartworm tests for dogs are both inexpensive and highly accurate. These tests can either detect infection early before there is any disease, or show a negative result so that it’s safe to get your dog on proper heartworm or broad-spectrum parasite preventative medication, like Heartgard Plus or NexGard Plus.

Early heartworm detection means a better prognosis for your dog. This means your dog won’t suffer from shortness of breath, lethargy, weight loss, or any of the other signs of severe heartworm disease, such as cough and sudden collapse. And, if your dog is heartworm positive, then early treatment is safer, more effective, and can prevent the damage adult heartworms create in the lungs and heart.

Dog Heartworm Test Cost

Heartworm tests for the presence of adult heartworms cost between $10 and $100, depending on the type of test and whether it is added on to other tests already being performed or is being run as a stand-alone test. 

Heartworm tests added onto blood work that you are already purchasing is likely at the low end of this range. Testing done as an emergency due to symptoms consistent with heartworm disease are more expensive, especially if performed at an emergency clinic. 

Testing for microfilaria is in the range of $10-$50, because there are no special chemicals or special equipment required. You are paying for the knowledge of an experienced veterinary technician and their time at the microscope. 

The cost of annual testing, done every year for a dog’s 10-15+ year lifespan, is significantly less than the cost of heartworm treatment.

What if My Dog Tests Positive for Heartworm?

Vet looks at X-ray of dog

If your dog tests positive for heartworms, your veterinarian will discuss a plan. The first step is confirmation by a follow-up test. This might include a microfilaria test or a confirmatory lab test.  

Once your dog’s heartworm infection is confirmed, then your veterinarian will recommend a work-up to make sure they are healthy enough for treatment. This typically consists of blood work, including testing for anemia as well as assessing kidney and liver values. X-rays (radiographs) of your dog’s heart and lungs (thorax) are next. If your dog already has evidence of severe changes due to heartworm disease, then your veterinarian may recommend an alternative treatment or management plan.

Heartworm Treatment for Dogs

Dog rests in crate

If you are lucky enough to have caught your dog’s heartworm infection early, then your veterinarian will follow the treatment plan recommended by the American Heartworm Society

This involves starting a medication called doxycycline that weakens the heartworms to make them easier to kill. Your vet may also prescribe oral steroids to minimize the reaction of your dog’s immune system to the dying and dead worms.

Heartworm treatment itself involves the injection of a very strong medication into muscle. This medication contains arsenic and must be handled carefully and injected only into the big muscles that run along your dog’s spine. 

You must limit your dog’s activity during treatment and for at least one month after. 

During treatment, your dog will be started (or re-started) on heartworm prevention to reduce the risk of new infections. Most types of heartworm prevention are safe to give to heartworm-positive dogs. However, you shouldn’t start or re-start heartworm prevention unless your veterinarian instructs you to do so. 

Nine months after the heartworm treatment, your vet will likely test for heartworms again to confirm that the treatment was successful.


  1. Atkins CE. Comparison of results of three commercial heartworm antigen test kits in dogs with low heartworm burdens. J Am Vet Med Assoc. 2003 May 1;222(9):1221-3. doi: 10.2460/javma.2003.222.1221. PMID: 12725308.