Your vet can test for heartworms with a simple antigen test, a quick blood test that detects proteins released by adult female heartworms. Results come back in about 10 minutes, and it's typically included in an annual wellness visit.
A few important things to know about testing:
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Annual testing is recommended even for dogs on prevention. No preventive is 100% effective, and catching an infection early makes an enormous difference in outcome.
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A negative test doesn't always mean the all-clear in a newly infected dog. Antigen tests don't turn positive until adult female worms are present, which takes about 6–7 months from initial infection. If your vet suspects recent exposure, they may recommend retesting.
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Your vet may also check for microfilariae (baby heartworms in the bloodstream) using a separate test, combining both tests gives the most complete picture.
If your dog tests positive, your vet will likely recommend chest X-rays, bloodwork, and sometimes a heart ultrasound before starting treatment, to understand how severe the infection is and how to treat it safely.