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Heartworm tests use a blood sample to detect either a specific antigen released by mature female heartworms or the offspring (microfilariae) of these worms.

It generally takes 6 months from the time the pet is infected with heartworm larvae by a mosquito bite until female worms can be detected. The administration of heartworm preventives to dogs with mature female worms can delay their detection for up to 9 months after infection.

This means that a dog may test “negative” for heartworms, even though it has adult worms in the heart and blood vessels of the lungs.

The tool uses the Window of Infection WOI time frame along with the pet’s heartworm test results and matches these with the purchase history of heartworm preventive.

The tool assumes that the first dose of heartworm preventive was given to the pet at the time of purchase, and that the remaining purchased preventive was administered at 30 day intervals.

The WOI tool is used to determine if there may have been gaps in administering heartworm preventive to a dog that has tested positive for the presence of mature female heartworms.