Question

My 3-month old puppy was given ‘Heartguard’ at her first checkup after she was adopted from a shelter. Is that enough to treat her potential exposure as we don’t know her previous medical history.

Answer

It is recommended to give the first dose of heartworm prevenative by 8 weeks of age.  This is due to the life cycle of the larva and the time it takes for it to reach start developing into an adult which is ~50 days.

So, in this case, it should be treated the same as if any other pet has missed a dose of heartworm preventative.  In these cases it is recommended to have a heartworm test performed after 7 months to ensure your puppy does not have heartworm. This will let you know if your puppy is not infected with heartworm and in the event that your puppy is infected with heartworm, allow for treatment to begin while minimizing the risk of complications related to treating the adult heartworm.

Below is a diagram of the heartworm lifecycle:

As shown in the diagram, the mosquito picks up the larva and then infects the host.  Once infected, the heartworm larva remains in the tissue between 45-65 days.  This is the window of opportunity for heartworm preventatives to work.

After this period of time, and the larva enters the bloodstream where it migrates to the heart and matures into an adult.  This generally takes between 4-5 month period.  It is also not detected by heartworm tests.

It is only once the larva matures into an adult that heartworm tests will pick up the heartworm.  The longer it stays an adult, the larger the heartworm becomes and the more likely the chance of complications when receiving treatments.

For more information about Heartowrm in dogs, we recommend our VetBabble article on Heartworms or the American Heartworm Society website.