Why Test for Heartworm Before Starting Prevention?
Giving preventives to a dog that already carries adult heartworms can trigger reactions and mask the infection, so a blood test comes first.
It seems backward to test a dog before protecting it, but the order is deliberate. Preventives are designed for uninfected animals; in a dog already carrying microfilariae — baby worms circulating in the blood — a dose can set off a reaction, and continuing monthly doses can hide an adult infection that keeps damaging the heart.
When and how often the test is repeated
The standard screen is a small blood sample checking for adult worm antigen, sometimes paired with a microfilaria check. Because worms take about six months to mature after a mosquito bite, a very young puppy may start prevention first and be tested later — timing a veterinarian will map out at the consult.
Annual retesting is widely recommended even for dogs on year-round prevention, since missed doses, vomited pills, and individual differences in absorption leave small gaps. A quick yearly test is the cheap insurance that catches those gaps early.
MediIndex articles are for general information only and are not medical advice, diagnosis, or advertising. Outcomes vary by individual — consult a board-certified specialist for personal decisions.