4 Things to Know About Heartworm Tests for Dogs

Heartworm tests for dogs are routine but essential diagnostic tools that veterinarians use to screen for heartworms in dogs. Because heartworms live in a dog’s blood vessels, testing requires a small blood sample, usually taken during an office visit.

To understand how heartworm tests in dogs work, it’s helpful to first understand the heartworm’s life cycle.

Heartworm Life Cycle in Dogs

Dogs become infected with heartworms when they are bitten by mosquitoes carrying immature heartworm larvae. When the mosquito bites the dog, the larvae enter the skin and eventually make their way into the dog’s bloodstream and settle in the blood vessels of the lungs.

Over the course of about six months, these microscopic larvae mature into adult worms, which can grow up to 12 inches long. The adult worms mate and produce millions of microscopic offspring, called microfilariae, which also live in the dog’s bloodstream.

What Is a Dog Heartworm Test?

Testing for heartworms in dogs is a two-part process.

The first step is an antigen test that detects a specific protein released by adult female heartworms, indicating the presence of adult heartworms. This test is performed either at the veterinary clinic, in which case you may learn the results during your dog’s office visit, or the test is sent out to a diagnostic laboratory, in which case you’ll be notified after the appointment through a follow-up call. 

While antigen tests overwhelmingly accurate, they are not completely foolproof, as they can only detect mature heartworms, not the earlier life stages. If a dog has been infected for less than six months, the test will likely return a false-negative result.

If a dog tests positive on the antigen test, your veterinarian will likely want to confirm the result with a second such test. False-positive results on these tests are uncommon, but your veterinarian will want to be certain of the result before starting heartworm treatment.

The second step in heartworm testing in dogs is a separate test that screens for heartworm microfilariae (the “baby” heartworms) in the blood sample. This test helps determine whether the adult heartworms have mated. If the dog tests positive on this test, your veterinarian will recommend a treatment strategy with medications that eliminate both adult heartworms and microfilariae.

This is important, because if a mosquito bites a dog that is harboring microfilariae, the mosquito could then spread heartworm infection to other pets.

The American Heartworm Society® recommends annual screenings for both adult heartworms and microfilariae in dogs at least 7 months of age. And while most dogs test either positive or negative on both tests, it’s possible to get mixed results. Conducting both tests provides the veterinarian with the most accurate assessment of the dog’s heartworm status.

How Often Should My Dog Be Tested for Heartworms?

Heartworm testing for dogs is typically done once a year as part of a pet wellness checkup. By testing dogs every year, veterinarians can catch heartworm infections early, before the worms get the chance to cause long-term damage to a dog’s heart, lungs, and blood vessels.

This screening can also determine if an infected dog poses a threat to other pets, as the disease can be transmitted through mosquito bites to any other dog or cat in the vicinity.

Heartworm testing may also be recommended at other times and under specific circumstances, such as:

• When puppies are 7 months of age or older. While puppies can become infected early in life, they cannot test positive until the heartworms have matured into adults. It’s important to note that a negative heartworm test isn’t required to start a puppy on heartworm prevention. Puppies should be started on prevention as early as the product label allows. Your veterinarian can help you determine the right time to start your puppy on heartworm prevention.

• If a dog is switched from one heartworm preventive medication to a different one. Before making the change, your veterinarian will want to ensure your dog is free of adult heartworms.

• If you adopt a dog who wasn’t previously on heartworm prevention, or if the dog’s health history is unknown. If the dog tests negative at the time of adoption, a second test six months later is recommended to make sure an early infection wasn’t missed at the time of the first test.

• If one or more doses of heartworm preventive were skipped or missed. In this case, waiting six months after the missed doses(s) is necessary to determine if the lapse in prevention led to infection.

• When a dog shows possible signs of heartworm infection. Common symptoms of heartworm infection include coughing, vomiting, and fatigue.

Cost of Heartworm Test for Dogs

The cost of heartworm testing for dogs varies by location and between veterinary practices. Prices can vary, depending on whether the veterinarian is testing only for heartworms or screening for other parasites at the same time. Some veterinarians include annual heartworm screening as part of a pet wellness package. 

Annual testing, along with year-round heartworm prevention, should be considered an investment in a dog’s health. Given that heartworm treatment typically costs many times more than annual prevention and testing, it’s a cost-effective choice.

Why Annual Heartworm Testing for Dogs Is Important

Pet parents often ask why annual testing is recommended even when a dog has been on year-round heartworm prevention. There are several important reasons to make testing an annual event:

• The American Heartworm Society recommends year-round heartworm prevention for all dogs, but not all pet parents remember to implement this. Even the most diligent pet parent can sometimes forget to give a pup their dose of monthly heartworm medication or they may fail to give it on time, leaving the pet susceptible to infection.

• Dogs may sometimes spit out or vomit their heartworm medication, resulting in a lapse in their ongoing treatment.

• In the case of a growing dog whose weight is rapidly increasing, a pet parent may mistakenly give one or more insufficient preventive doses based on the weight change.

• In rare cases, dogs can become infected with heartworms even if they receive their heartworm medication on time each month.  

• Annual testing is the only way to know for certain if a dog is infected.

Heartworm testing in dogs is safe, highly accurate, and affordable. Routine screening assures veterinarians and pet parents that a dog’s heartworm prevention program is working.

By having your pup screened each year, you’ll be able to catch any consequences of missed doses or determine if the heartworm medication isn’t working, allowing you to explore next steps with your veterinarian.