Why Dogs Are Tested for Heartworm
Heartworm disease is caused by parasitic worms that live in a dog’s heart and lungs after being transmitted through a mosquito bite, and left untreated, it can cause severe, potentially fatal damage to the heart and lungs.
Because infected mosquitoes can transmit heartworm larvae even in dogs on preventive medication if a dose is missed or given late, annual testing is recommended even for dogs consistently on prevention.
Testing before starting or renewing a heartworm prevention prescription is also a safety requirement, since giving certain preventives to an already-infected dog can trigger a dangerous reaction.
Early detection through routine testing gives dogs the best chance at successful treatment, since heartworm disease is far easier and safer to treat in its early stages than after significant damage has occurred.
Types of Heartworm Tests
The antigen test is the standard screening test used at most vet clinics, detecting proteins released by adult female heartworms, and it can typically be run in-house with results ready in a matter of minutes.
A microfilaria test examines a blood smear under a microscope for the presence of baby heartworms (microfilariae) circulating in the bloodstream, which helps confirm an active infection and guide treatment planning.
Some clinics use a combination test that checks for heartworm along with tick-borne diseases like Lyme disease and ehrlichiosis in a single blood draw, which is efficient for dogs due for a general wellness screen.
Antigen tests are most reliable when heartworms have matured enough to be detected, generally around five to seven months after an infective mosquito bite, which is why very recent infections can sometimes be missed on a single test.
What Happens During Testing
The test requires only a small blood sample, typically drawn from a vein in the leg, and most dogs tolerate it well with minimal discomfort, similar to any routine blood draw.
In-house antigen tests usually produce results within 10 to 15 minutes, allowing your vet to discuss the outcome and next steps during the same appointment.
If the initial screening test comes back positive, most vets will run a confirmatory second test, sometimes a different type, since false positives, while uncommon, can occur.
No special preparation like fasting is typically needed before a heartworm test, making it easy to combine with other routine wellness bloodwork during an annual exam.
Understanding Your Dog’s Results
A negative result means no heartworm antigens were detected in the sample, and combined with consistent use of a monthly preventive, this is the expected and reassuring outcome for most tested dogs.
A positive result means adult heartworms are likely present, and your vet will typically recommend additional tests such as chest X-rays or an echocardiogram to assess the severity of infection before starting treatment.
Heartworm treatment for dogs is a multi-step process involving strict exercise restriction and a series of injections to kill adult worms, and it typically spans several months from diagnosis to being heartworm-free.
Because treatment is intensive, costly, and carries its own risks, an early positive diagnosis caught through routine testing is significantly easier to manage than one caught after symptoms like coughing or exercise intolerance have already developed.
Cost & Testing Frequency
A standalone heartworm antigen test typically costs between 35 and 75 dollars at most veterinary clinics, though combination tests that also screen for tick-borne diseases usually run somewhat higher.
Annual testing is recommended by the American Heartworm Society for all dogs, even those consistently taking monthly preventive medication, since no prevention method is 100 percent guaranteed against a missed or late dose.
Dogs starting heartworm prevention for the first time, or those who’ve had a gap in preventive coverage of more than a month or two, generally need a negative test result before starting or resuming medication.
Puppies under seven months old can typically start preventive medication without an initial test, since heartworms take several months to mature to a detectable stage, but should be tested at their next annual visit.
Frequently Asked Questions
How often should my dog be tested for heartworm?
The American Heartworm Society recommends annual testing for all dogs, regardless of whether they’re consistently on monthly preventive medication.
Can a dog test negative for heartworm and still be infected?
Yes, very recent infections may not yet be detectable, since antigen tests generally can’t pick up heartworms until they’ve matured for about five to seven months after the infective mosquito bite.
Is the heartworm test painful for dogs?
No, it only requires a small blood draw similar to routine bloodwork, and most dogs show no more than brief discomfort from the needle stick.
Do indoor dogs need heartworm testing?
Yes, mosquitoes can get indoors, and heartworm has been diagnosed in dogs with little to no outdoor exposure, so testing is recommended for all dogs regardless of lifestyle.
What happens if I skip a month of heartworm prevention?
A missed or late dose creates a window where an infective mosquito bite could establish an infection, which is part of why annual testing is recommended even for dogs generally consistent with their prevention.