Starting prevention
A puppy can generally begin prevention without a pre-start heartworm test because a new infection is not yet detectable. Test about 6 months after the first visit, again 6 months later, then annually.
DOG HEALTH · CLIENT GUIDE · REVIEWED AUGUST 15, 2026
Heartworm disease
in dogs.
How infection happens, why year-round prevention and testing matter, what the blood tests find, and what to do when a dose is missed.
IN THIS GUIDETesting scheduleThe two blood testsMissed or late doses“Grace period”Understanding resultsCommon questions
WHY TEST A DOG WHO LOOKS HEALTHY?
Infection can be silent for months.
Heartworms are transmitted by mosquitoes. Early infections usually cause no visible signs, and preventives kill susceptible immature stages—not adult worms. Testing is how we confirm that prevention is working and find infection before avoidable heart and lung damage progresses.
5–7 months before infection may become detectable12 months between routine screens50 states report heartworm disease
01 · TESTING SCHEDULE
Who should be tested—and when?
The 2024 American Heartworm Society canine guidelines recommend annual screening with both an antigen test and a microfilaria test. Timing also depends on age and any gap in prevention.
Never on prevention
Test before starting. Begin prevention under veterinary direction, then test again approximately 6 months and 12 months later before moving to annual screening.
Routine screening
Test every 12 months—even with perfect year-round prevention. A dose can be missed, vomited, spit out, applied incorrectly, or rarely fail.
Follow-up matters
Contact the clinic and restart as directed. A test performed immediately cannot rule out a very recent infection, so a repeat test about 6 months after the lapse is essential.
Changing preventives?
AHS recommends antigen and microfilaria testing when a change in heartworm preventive is planned. Do not create a gap while switching.
02 · THE TWO BLOOD TESTS
Antigen and microfilaria tests answer different questions.
Heartworm antigen test
Looks for proteins produced primarily by adult female heartworms. It is the familiar rapid blood test used for screening.
- Best single screening test for adult female infection
- Can miss very early infections, male-only infections, or low female worm burdens
- A positive screening result is confirmed before treatment planning
Microfilaria test
Looks directly for circulating baby heartworms in the bloodstream, usually with a concentration method or microscopic examination.
- Finds dogs capable of infecting mosquitoes
- Helps identify infection that an antigen test may miss
- Guides safe case management and documents removal of microfilariae
Why both? Some infected dogs have adult-worm antigen without circulating microfilariae; others may have microfilariae with a negative antigen screen. Using both improves detection and helps the veterinary team interpret unexpected results.
03 · MISSED OR LATE PREVENTION
What to do when something goes wrong.
Call the clinic
Tell us the product, the date the last dose was definitely given, and whether any dose may have been spit out, vomited, or applied incorrectly.
Resume promptly
For monthly products, AHS client guidance says to consult your veterinarian and immediately restart prevention. Follow the exact product label and clinic instructions; do not double-dose unless specifically told to.
Plan the right tests
Your veterinarian may test now to establish current status, but a second test about 6 months after the missed-dose window is needed to detect an infection acquired during the gap.
Stay monthly afterward
Use reminders and verify that oral doses were swallowed and topical doses stayed on the skin. Keep the annual screen even after the follow-up test.
Vomited or spit out?
Do not automatically give another full dose. How soon vomiting occurred, whether the chew is visible, and the specific label all matter. Call for product-specific instructions.
Topical rubbed or washed off?
Contact us before reapplying. Bathing windows and absorption differ by product, and accidental double-dosing should be avoided.
Late injection?
ProHeart appointments have product-specific intervals. Call as soon as the due date is missed so the veterinary team can determine interim prevention and testing.
THE “GRACE PERIOD” QUESTION
There is no universal
guaranteed grace period.
Monthly heartworm preventives work by eliminating susceptible larvae acquired previously. That “reach-back” is why a dose is given monthly—but it should not be interpreted as extra time that is always safe.
AHS explains that larvae can develop into a stage poorly eliminated by preventives in as little as 51 days. Product ingredients, label directions, the true date of the last swallowed or absorbed dose, and the timing of mosquito exposure all matter.
If a dose is late:Do not wait for an arbitrary grace deadline. Contact your veterinarian, resume as directed, and document the date.
Being a few days late is not the same as proof of infection, but no website can promise that a specific number of late days is risk-free. The safest habit is to dose on the labeled schedule.
04 · UNDERSTANDING RESULTS
A result is the start of interpretation—not the end.
No infection detected today
Continue year-round prevention. If there was a recent gap, repeat testing at the veterinarian-recommended interval because early infection may not yet be detectable.
Possible occult infection
Adult female worms may be present without circulating microfilariae. The team confirms the antigen result and evaluates the dog before treatment.
Needs confirmation
The sample is rechecked and microfilariae identified because other filarial species or an antigen result affected by test limitations may be possible.
Heartworm infection supported
Further evaluation determines disease severity and prepares the dog for AHS-recommended treatment. Exercise restriction begins promptly.
A positive test is not a reason to panic—or to “wait and see.”
Your veterinary team will confirm the diagnosis, assess health and disease severity, and explain a treatment plan. Do not increase exercise or discontinue medications without veterinary guidance.
05 · COMMON QUESTIONS
Heartworm testing FAQ
Why test yearly if prevention is highly effective?
No preventive is 100% effective in real-world use. Annual testing catches failures caused by missed, late, vomited, spit-out, or improperly applied doses and allows earlier treatment.
Can an indoor dog skip prevention or testing?
No. Mosquitoes enter homes, and heartworm disease occurs in all 50 states. AHS recommends year-round prevention and annual testing regardless of an indoor lifestyle.
Can a test today clear a missed dose from last month?
No. A new infection usually is not detectable immediately. That is why follow-up testing about 6 months after a lapse is so important.
Does a negative antigen test mean there are definitely no heartworms?
It substantially lowers concern but does not answer every scenario. Early infection, male-only infections, low female burdens, and other test limitations are reasons AHS now recommends annual microfilaria testing alongside antigen testing.
Should I stop prevention while waiting for a test?
Usually no—creating a longer gap increases risk. Contact the clinic for instructions based on your dog’s age, product, and history.
Can preventives treat adult heartworms?
No. Preventives target susceptible larval stages and are not a substitute for the recommended adulticide treatment of an infected dog.
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