The short answer
You’ve created a window where heartworm larvae can develop past the stage that monthly preventives can kill. Give the next dose immediately and call your vet. They’ll likely recommend retesting in 7 months.
Why one month matters
Monthly heartworm preventives work retroactively. Each dose kills larvae that entered your dog’s body during the previous 30 days. If you miss a dose, any larvae that arrived during that gap get an extra month to grow.
Heartworm larvae need to reach a certain developmental stage before monthly preventives lose effectiveness against them. That threshold happens somewhere around 2 months after initial infection. A single missed month pushes some larvae into that danger zone.
Those surviving larvae continue developing. Within 6 to 7 months they’ll be adult worms living in your dog’s heart. At that point, you’re dealing with heartworm disease, not prevention.
What to do right now
Give the missed dose as soon as you remember. Don’t double up. Just resume the normal monthly schedule.
For a one-month lapse, the risk depends on when it happened. Missing a dose in January in Ontario carries almost zero risk because mosquitoes aren’t active. Missing a dose in July, at the peak of mosquito season, is a real concern.
Your vet will want to retest 7 months from the gap. That’s how long it takes for a new infection to become detectable on a blood test. So if you missed July, expect to be retested in February.
What if you’ve missed more than one month
A lapse of two months or more during heartworm season is more serious. Call your vet right away. They’ll likely restart prevention immediately and may add a course of doxycycline for 30 days as an extra precaution. Retesting at the 7-month mark is definite.
Do not assume that because your dog looks fine, everything is fine. Heartworm shows no symptoms for the first 6 to 7 months. A dog with an active developing infection looks completely normal.
How to avoid gaps
Set a monthly reminder on your phone. Some products like Simparica Trio and Heartgard Plus come with reminder stickers or apps. If you travel or have a busy schedule that makes monthly dosing unreliable, ask your vet about ProHeart, an injectable heartworm preventive that provides extended protection from a single shot. Availability varies in Canada, so your vet can tell you which formulation they carry.
Combo products that also cover ticks and fleas can make the routine feel more worthwhile, since you’re getting multi-parasite protection in one dose.
Key takeaways
- A missed month during mosquito season creates a real window for heartworm infection.
- Give the missed dose immediately and resume the normal schedule.
- Your vet will want to retest 7 months from the gap to check for infection.
- Missing a dose in winter in Ontario is low-risk. Missing one in summer is not.
- Set monthly reminders or ask about long-acting injectable options to avoid future gaps.
References
- dvm360. “Heartworm Prevention: Oops, I Missed a Dose!” dvm360.com
- American Heartworm Society. “Heartworm Basics.” heartwormsociety.org
- PetMeds. “What to Do If You Missed a Heartworm Dose.” 1800petmeds.com
- Dunedin Animal Medical Center. “Heartworm Prevention: Why Missing One Month Matters.” dunedinamc.com