Heartworm Tests Explained

Heartworm Tests Explained

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Heartworm disease can be hard to detect in its early stages. That's why it's important that you test your dog regularly.

While dogs in warm climates with an active mosquito presence are particularly vulnerable to heartworm disease, the infection has been diagnosed in every state (even Alaska). In early stages of the disease, your dog may not show any symptoms at all. Or it’s possible that your pet will have relatively minor symptoms – a cough, or a reluctance to exercise – that could easily be mistaken for an inconsequential illness.

Are Heartworm Tests Necessary?

Given the risks of missing or misdiagnosing the early symptoms of the condition, veterinarians generally test dogs for heartworm disease annually. The test is a good backup option in case a preventative treatment dose was accidentally skipped. Regular testing helps ensure that the disease is caught early on, when treatment options are easier and more effective.

A more serious reason for testing for heartworms prior to preventative treatment is the dog’s health. In addition to killing larvae, preventatives will also destroy the microfilaria (baby heartworms, born to adults). A sudden, en masse destruction of all the microfilaria could have a negative, or even fatal, impact on a dog’s health. Even without a negative health impact, dosing preventatives to a dog with an adult heartworm infection could mask the seriousness of the disease’s presence and development, delaying treatment.

How Is Testing Done?

To test for the presence of heartworms, your vet will take a small blood sample from your dog and use this for one of these types of tests:

  • Antigen Test– Veterinarians will check for antigens left within the bloodstream by the female heartworm.
  • Microfilaria Test – In a Knott’s test, vets are looking for the presence of microfilaria, or the young heartworms, within the dog’s blood. Another microfilaria detection test is known as a Difil test.

Both of these blood tests have a small risk of a false negative; that is, there is a possibility that the tests will report that your dog does not have heartworm, when in fact the dog does. This is particularly true if your dog’s heartworm case hasn’t progressed to the point where adults are present, if there is a low number of worms, or if your dog is only infected with male heartworms.

Sometimes the two types of blood tests are done in conjunction, particularly if your vet suspects that the test results are incorrect. When dogs test positive for heartworms, x-rays of the lungs can be done to inspect for signs of the disease’s progress.

When to Test for Heartworm

Heartworm tests should be administered on a yearly basis because of the long inculcation period as heartworms mature. After mosquitoes transmit the heartworm larvae to dogs, it takes between six and seven months for the parasites to develop and infiltrate into the dog's bloodstream. While the larvae are developing, the disease is undetectable. When a dog is tested, generally in the early springtime, the test will determine if the dog picked up the disease during the preceding warm mosquito season.

Related Content

Heartworm Disease in Dogs: 5 Things You Should Know
Heartworm Symptoms
Heartworm Medicine
How to Prevent Heartworm in Dogs

This information is for informational purposes only and is not meant as a substitute for the professional advice of, or diagnosis or treatment by, your veterinarian. Always seek the advice of your veterinarian or other qualified professional with any questions you may have regarding a medical condition. Never disregard or delay seeking professional advice due to what you may have read on our website.

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