My pet is on prevention but still tested positive for heartworms

Why did my pet test positive for heartworms despite prevention?

“When I adopted my pet, he was negative for heartworms and has been on prevention for over 6 months. However, my vet is telling me he is now positive for heartworms. How can this be?”

There is a therapeutic and diagnostic gap of three months when starting heartworm prevention.

Heartworm tests only detect adult infections and I believe your pet did not have an adult infection at the time you adopted him. Heartworm progression from early stage to adult lasts for a period of 6 months from the time of infection. Heartworm infections start with a mosquito bite which injects larval stage 3 into your pet. This stage lasts only a few days and molts into larval stage 4 and within two months to larval stage 5.

Heartworm prevention kills only early stage infections.

Heartworm prevention only kills larval stage 3 and early larval stage 4. It takes Larval Stage 5 three or four months to develop into adult heartworms. These heartworm larvae are not detected by heartworm tests and not killed by heartworm prevention. Therefore, there is a gap of 60 to 90 days when the pet could have been infected with heartworms. This infection might not have been killed by the prevention or detected by the heartworm test.

Early detection of heartworms is key

It is best to test new adopted pets three months after they are started on prevention. All pets in heavy heartworm areas such as the Mississippi river valley or costal areas should be tested every 6 months and pets not in the Mississippi river valley or costal areas can be tested yearly.