Importance of Year-Round Flea & Tick Prevention

Why Does My Dog Need a Heartworm Test?

It may be time to purchase some new heartworm preventive for your puppy, especially as they’ve either reached or are approaching their adult size. But this time, your veterinarian will require a heartworm test before prescribing the preventive.

The life cycle of heartworms

You may recall that as a puppy, your dog didn’t have to get a heartworm test to get preventive doses. The reason your puppy didn’t have to get a heartworm test when they were little, but does now, has to do with the life cycle of heartworms.

Recall that heartworm disease is caused by the parasite Dirofilaria immitis, which are long, hair-like worms that are passed to dogs through mosquito bites. The mosquitoes pass on microscopic infective larvae, which mature into worms that infest a dog’s heart and pulmonary artery, the large vessel that carries blood from the heart to the lungs.

The heartworm test

The heartworm test only detects adult female heartworms. This means that while a heartworm is maturing, the test will be negative. It takes heartworms around 5 to 6 months to complete their life cycle. So, even if your puppy was infected on day 1 of their life, they wouldn’t test positive until after they were 6 months old. Because of this, young puppies can receive heartworm preventives without a test.

After 7 months of age, a heartworm test is required before you can purchase more preventive. This test serves multiple functions, including:

Making sure the heartworm preventive program is working. Though heartworm preventive is highly effective, there isn’t a 100% guarantee that your puppy won’t get heartworms. For instance, some heartworms are developing resistance to heartworm preventives, which could affect a dog who regularly receives their preventives.
Catching heartworm disease if you’ve missed a month, if your pet secretly spit out or vomited up their heartworm medication, or if they rubbed off their topical.
Allowing for earlier intervention if your pet does test positive.
Preparing you and your pet’s veterinary team for potential reactions to the preventive if your pet does have heartworms. Though rare, some dogs with heartworm larvae in their bloodstream have reactions to the preventive.
Ensuring legal compliance. Use of preventives in a heartworm-positive dog is technically an extra-label use of the drug, so it’s only okay to give heartworm prevention to a dog with veterinary approval and supervision.

So, say “yes” to the heartworm test and keep your pet protected year-round from heartworms! If you have any questions about the test, contact your VCA care team.

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