Heartworm Test for Dogs

Heartworm testing serves as a cornerstone of proactive veterinary care, contributing significantly to the overall health and longevity of our furry friends. Heartworm disease, caused by the parasitic roundworm Dirofilaria immitis, poses a grave threat to dogs. By initiating a thorough understanding of heartworm disease and its testing protocols, pet owners can make informed decisions that lead to the best possible outcomes for their pets. In this article, we will delve into the intricacies of heartworm testing for dogs, from understanding the disease itself to the various testing methods, interpretation of results, and the significance of prevention.

Understanding Heartworm Disease

Heartworms are parasitic worms transmitted through the bites of infected mosquitoes. Once inside the host’s body, heartworms develop and migrate to the heart and lungs, potentially leading to severe health complications, including heart failure and organ damage.

Types of Heartworm Tests for Dogs

Heartworm Test for Dogs

There are several types of tests used to diagnose heartworm infection in dogs. These tests are important for detecting the presence of heartworms and determining the severity of the infection. The most common types of heartworm tests for dogs include:

Antigen Test (ELISA Test): This is the most widely used test for diagnosing heartworm infection. It detects the presence of heartworm antigens, which are proteins released by adult female heartworms into the bloodstream. A small blood sample is taken from the dog and tested for these antigens. If the test is positive, it indicates an active heartworm infection.

Antigen tests can yield inaccurate negative results in the following scenarios:

  • When the infection duration is less than 5 months, as it’s too early for adult worms to develop and release antigens into the bloodstream.
  • When the worms consist entirely of males or immature females, resulting in the absence of adult female worm antigens.
  • In cases of extremely low worm counts, where the level of detectable antigens is insufficiently low.
  • When technical challenges during the test procedure occur, necessitating a repeat of the test.

Microfilaria Test (Knott’s Test): This test is used to detect the presence of microfilariae, which are the juvenile forms of heartworms circulating in the dog’s bloodstream. A blood sample is taken and examined under a microscope to look for these microfilariae. If microfilariae are present, it indicates an active infection. While less commonly used due to their limitations in detecting infections in the early stages, they remain useful in certain situations.

False negative results in microfilaria tests can occur due to the following reasons:

  • None of the adult worms have reached maturity to engage in mating and produce microfilariae.
  • If all the adult worms are of the same sex, mating cannot take place.
  • In cases where the number of microfilariae in the bloodstream is too low to be detected—this could be due to the initial stages of mating or a small population of adult worms unable to generate significant microfilaria numbers.

Heartworm Antibody Test: This test detects the presence of antibodies produced by the dog’s immune system in response to heartworm infection. It can be used to identify if a dog has been exposed to heartworms in the past, even if the infection is no longer active.

Modified Knott’s Test: This is a variation of the microfilaria test that involves concentrating the microfilariae from a larger blood sample. This can improve the sensitivity of the test, making it more effective at detecting low levels of microfilariae.

Polymerase Chain Reaction (PCR) Test: PCR tests can detect the presence of heartworm DNA in a blood sample. This test is particularly useful in cases where the infection is in its early stages and the number of adult worms is too low to produce detectable antigen levels.

Other blood tests (CBC, blood chemistries, electrolytes): The presence of heartworm disease may be indicated by irregularities in the complete blood count (CBC) and blood tests assessing kidney and liver function. These examinations are commonly administered to dogs diagnosed with heartworm infection to evaluate organ function and health status prior to initiating treatment.

Radiographs (X-rays): Radiographs of dogs afflicted with heartworms typically display heart enlargement and swelling of the pulmonary artery, the major artery connecting the heart to the lungs. These indicators serve as preliminary evidence of heartworm disease. Radiographic images can also unveil the condition of the heart, lungs, and pulmonary vessels. This insight enables us to anticipate a higher likelihood of treatment-related complications.

Electrocardiogram (EKG or ECG): An electrocardiogram (EKG) traces the heart’s electrical currents and is particularly valuable for detecting abnormal heart rhythms. This test is also capable of identifying enlargements in heart chamber size, aiding in the assessment of a dog’s suitability for heartworm disease treatment.

Echocardiography (ultrasound of the heart): An ultrasonic assessment, provides visualization of the heart chambers and facilitates an overall evaluation of the heart’s condition. This technique is instrumental in detecting heartworms within the heart and pulmonary artery, contributing to the diagnosis of heartworm disease.

Frequency of Heartworm Testing

The frequency of heartworm testing is influenced by various factors, including geographic location, climate, and local prevalence of heartworm disease. Veterinary associations often provide guidelines to help pet owners determine the optimal testing schedule. In areas with a high mosquito population or a history of heartworm cases, annual testing is recommended.

When is it Appropriate to Test a Dog for Heartworms?

  • Puppies under the age of 7 months can initiate heartworm prevention without an immediate test (as it takes at least 6 months for a positive result to show after infection). However, testing should occur 6 months after the initial visit, followed by another test 6 months later, and subsequently once a year. This sequence ensures ongoing assurance of a heartworm-free status.
  • For adult dogs over 7 months who have not been on preventive treatment, testing is essential before commencing heartworm prevention. Similar to puppies, they should also be tested 6 months and 12 months later, followed by annual testing.
  • It’s imperative to seek guidance from your veterinarian and promptly resume monthly preventive treatment for your dog. Subsequently, retesting should occur 6 months later. This retesting timeline accounts for the fact that heartworms require around 7 months to mature sufficiently for accurate diagnosis.
Consistent annual testing remains crucial, even when dogs are on year-round heartworm prevention. This regular testing ensures the effectiveness of the prevention regimen. While heartworm medications are highly reliable, the possibility of infection still exists. A single missed or delayed dose of monthly medication can leave your dog vulnerable. Additionally, dogs might spit out or vomit a heartworm pill, or inadvertently rub off a topical medication. While heartworm preventives are potent, they are not infallible, and a 100 percent success rate cannot be guaranteed.

Preparing for a Heartworm Test for Dogs

Proper preparation for heartworm testing is crucial for accurate results. Timely testing, fasting requirements, and a thorough medication history are all key considerations. Fasting before the test can prevent food-related interference with test results.

Can I give my dog a heartworm test at home?

Yes, there are home heartworm test kits available for dogs that allow you to collect a blood sample at home and send it to a laboratory for analysis. These kits typically involve pricking your dog’s ear or using a small blood collection tube to obtain a blood sample. However, it’s important to note a few key considerations:

  • Accuracy: While home test kits can provide convenient testing, their accuracy might not be as reliable as tests conducted by a veterinarian in a controlled environment. False negatives or positives can occur due to factors like improper sample collection or handling.
  • Professional Interpretation: A veterinarian’s expertise in interpreting test results and recommending appropriate actions is valuable. If your dog tests positive, a veterinarian can guide you through the necessary steps and treatment options.
  • Follow-up Testing: A positive result from a home test should be confirmed with a veterinary visit and professional testing. Veterinarians have access to more advanced testing methods and can verify the accuracy of the result.
  • Consultation: Before using a home heartworm test kit, consult your veterinarian. They can provide guidance on whether it’s appropriate for your dog’s situation and help you make an informed decision.
  • Administering the Heartworm Test

Heartworm tests can be performed at veterinary clinics or, in some cases, using at-home testing kits. The most common method involves drawing a small blood sample from the dog, which is then analyzed for the presence of heartworm antigens or DNA fragments. Proper sample collection and handling are essential to ensure accurate results.

Interpreting Heartworm Test Results

Interpreting heartworm test results requires a comprehensive understanding of the testing method used. A positive result indicates the presence of heartworms, necessitating further confirmatory tests to establish the severity of the infection. A negative result, while reassuring, does not negate the need for ongoing preventive measures, as heartworm infections can be asymptomatic in the early stages.

Treatment Options for Heartworm-Positive Dogs

If a dog tests positive for heartworms, various treatment options are available, depending on the stage of the disease. Traditional treatment involves a series of injections to eliminate the worms, while the slow kill method aims to manage the disease over a more extended period. Both methods carry risks and require close veterinary supervision.

Additional medications like anti-inflammatory drugs, antihistamines, and antibiotics (such as doxycycline) can be employed to mitigate side effects and enhance treatment response. In cases of severe complications, hospitalization might be necessary for more intensive medical intervention.

Preventing Heartworm Infection

Prevention is undeniably the most effective approach to heartworm disease. Monthly heartworm preventatives are available in various forms, such as tablets, chewables, and topicals. Additionally, reducing a dog’s exposure to mosquitoes by keeping them indoors during peak mosquito activity hours can significantly reduce the risk of infection.


Heartworm testing is a crucial component of responsible pet ownership, allowing us to safeguard our canine companions from the potentially devastating consequences of heartworm disease. By staying informed about testing methods, interpreting results accurately, and prioritizing preventive measures, we can provide our dogs with the best chance of a healthy and heartworm-free life. Regular veterinary care, testing, and preventive strategies form the foundation of a happy and thriving partnership between dogs and their devoted owners.

Ryan Foley
Website | + posts

Dr. Ryan's journey in the field of veterinary medicine began with his graduation from the University of Adelaide in 2014. Armed with a solid educational foundation, he embarked on a career dedicated to the well-being and care of animals.