Before You Panic, Read This
Last night someone close to me was scrolling Facebook and stopped on a post from a pet parent. The post said the dog had finished all three melarsomine injections and then tested positive for heartworm. They read it twice, and then looked up, scared. Scared for that dog, for that family, and for every owner in the comments who was about to go through the same moment.
If that is you tonight, this post is for you. Take one slow breath first.
A positive result is real information, and it deserves a conversation with your veterinarian. But the timing of a test matters, and it is worth asking about. A test run too soon after treatment may still be picking up what the treatment left behind. Your veterinarian is the one who can tell you which situation your dog is in.
You don't have to figure this out tonight.
Educational information only, not veterinary advice. Your veterinarian sets the plan for your dog, with your questions and input as part of the conversation.
The Third Injection Is the Start of the Clearing, Not the End
Here is the part that rarely gets said out loud: when the last injection is done, the work inside your dog's body is just getting going.
The American Heartworm Society (AHS) treatment protocol runs in stages. Doxycycline for four weeks, a macrocyclic lactone preventive, and then melarsomine: one injection, followed at least a month later by two more injections 24 hours apart. After that last injection, the AHS calls for 6 to 8 weeks of strict exercise restriction, because the lung changes tied to dead and dying worms get worse with activity.
Notice the words dead and dying. The worms do not all die on the same day. The AHS clinical FAQ on post-treatment testing says adult heartworms die "over a period spanning more than one month." While that is happening, the dog's body is still carrying the evidence: the AHS notes that leftover antigen can stay detectable in circulation for a time and may affect a test result.
So when someone asks whether the worms need time to "leave," the careful answer is this: adult worms live in the heart and the blood vessels of the lungs, they die over weeks, and the body then needs more time to clear what they leave behind. A test run too early can read that leftover material as if the problem were still active.
What the AHS Protocol Says About When to Retest
The current AHS guidance is to run the follow-up antigen test 9 months after the last melarsomine injection, and to screen for microfilariae at the same visit. The AHS protocol table puts that appointment at about day 365 from the start of treatment.
If you have seen "6 months" in a Facebook comment, an older article, or even an older AHS summary, you were not imagining it. That used to be the standard. The AHS moved the interval from 6 months to 9 months in 2018 because antigen tests became more sensitive, and the society notes that testing at 6 months could lead to unnecessary retreatment. Waiting the extra 3 months gives the antigen time to clear. A 2025 review in Today's Veterinary Practice repeats the same timing: the AHS recommends antigen testing no sooner than 9 months after treatment.
The guidelines also say that dogs who are still antigen-positive before that 9-month mark should be given more time before retreatment is considered.
There is a second test on the schedule earlier. The AHS protocol lists a microfilariae test around day 120, roughly a month after the last injection. If it is positive, the protocol describes a microfilaricide and a recheck in four weeks, and your veterinarian decides what fits your dog. The protocol anticipates this possibility, so a positive result there is worth a conversation with your vet.
So What Does a Positive Result Actually Mean?
I can't tell you what any one dog's result means. I don't know which tests were run, how many months had passed, or what that dog's chart says. Only the dog's veterinarian does. What I can do is lay out what the AHS materials say, so you walk into that conversation steady instead of scared.
There are two different tests, and they ask two different questions. The antigen test looks for proteins from adult female worms. The microfilaria test looks for the baby worms circulating in the blood. When people say a dog tested positive for "both," it is worth asking which two results were meant, and when each was drawn.
A positive antigen test before the 9-month mark may reflect residual antigen, and the AHS guidance is to allow more time before retreatment is considered, a decision that belongs to your veterinarian. A positive at 9 months or later is taken more seriously. The AHS says it confirms adult worms are present, which may mean some survived treatment, or may mean a new infection if preventive was missed along the way. Even then, the guidelines describe a next step, and your veterinarian will walk you through it. A positive microfilaria test soon after the injections is something the protocol anticipates; it describes a microfilaricide and a recheck, and your veterinarian decides what fits.
The point is the same each time: the date on the test matters as much as the result.
Questions worth bringing to your vet:
Which test was this (antigen, microfilaria, or both), and exactly how many days or months after the last injection was it drawn?
Does the AHS 9-month retest timing apply to my dog's timeline, and can we put that date on the calendar?
Should we do anything differently before that retest, such as keeping exercise restriction in place?
Has my dog's preventive been on schedule since treatment, and how do we keep it that way?
What result, at what date, would change the plan?
If You Want Another Set of Eyes
Options matter, and some owners want a second opinion. Some people choose to also see an integrative or holistic veterinarian for supportive care alongside their regular vet. These are licensed veterinarians who have added training in complementary approaches. The American Holistic Veterinary Medical Association (AHVMA) keeps a Find a Holistic Veterinarian directory if you would like to look. On the conventional side, the American Veterinary Medical Association (AVMA) offers resources for pet owners, and your regular veterinarian is always a good first call.
One thing to know before you go: melarsomine is the only FDA-approved treatment for adult heartworms, and it is the standard the AHS recommends. Integrative care is something people explore alongside the treatment plan, and nothing here suggests it replaces it. Bring your dog's full treatment and test dates to every appointment, and ask your veterinarians to share notes so everyone is working from the same timeline.
I am not recommending any particular provider, and listing a directory is not an endorsement.
What Matters Now. What Can Wait. What Comes Next.
What matters now: one slow breath, and one call to your veterinarian's office. Ask what the result was, which test it was, and how long after the last injection it was drawn.
What can wait: the late-night spiral and the comment threads. Decisions about retreatment belong in a conversation with your veterinarian, who has your dog's dates and chart.
What comes next: keep the preventive on schedule, keep the exercise restriction your vet gave you, and put the 9-month retest on the calendar together.
Heartworms develop over months, not days, and the clearing takes months too. Nobody in that comment section is failing their dog. The calendar is simply a part of testing that is easy to miss.
If you are in the middle of your own dog's treatment and want something steady to hold between appointments, the Heartworm Healing Guide is built from my own lived experience as a rescue dog owner, input from other dog owners, and the research I gathered along the way. It is not a clinical or scientific work. It supplements your veterinarian's care and never replaces it. You can find it at howzuribeatheartworms.com.
I'm not a veterinarian, and this post is for education only. This site is not affiliated with or endorsed by the American Heartworm Society, the American Holistic Veterinary Medical Association, or the American Veterinary Medical Association. Your veterinarian sets the protocol for your dog, with your questions and input as part of the conversation.
Sam Carter
Sources
AHS Clinical FAQ: Antigen Test Timing Following Melarsomine Treatment (American Heartworm Society)
AHS Current Canine Guidelines (2018 edition): confirmation of adulticide efficacy, elimination of microfilariae, Table 2 protocol
AHS Canine Guidelines (November 2024 edition): melarsomine three-dose protocol and exercise restriction
FAQs and Answers About Canine Heartworm Treatment, Today's Veterinary Practice, June 2025
AHS Canine Guidelines Summary (2014): the older 6-month interval
Find a Holistic Veterinarian, American Holistic Veterinary Medical Association
Resources for pet owners, American Veterinary Medical Association
Related Reading
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“This site is named in honor of Zuri’s story and is intended for orientation and support only. It does not suggest outcomes, methods, or treatment paths for heartworm disease.”
Sam Carter is a pen name used for privacy. This site offers decision‑support and lived experience, not medical advice. Read more from Sam Carter on Amazon.
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