Treatment Of Heartworm Microfilaria In Dogs

Microfilaria are the offspring of adult heartworms. They are produced in massive numbers in canines infected with Dirofilaria immitis (the taxonomic name for the heartworm parasite).
In some ways microfilaria can be worse than the adult canine heartworm. When large numbers of them die suddenly they can cause a shock reaction and death. That is why you should never give heartworm preventive to a pet who has missed a dose or who might have heartworms.
Microfilaria can cause reactions that affect veins and arteries in the body, make deposits in kidneys and even cause eye and brain damage.

Elvis' heartworm treatment should be uncomplicated.
Elvis' heartworm treatment should be uncomplicated.

There are almost as many protocols for treating heartworms as there are doctors of veterinary medicine. The system we follow calls for microfilaria to die at the beginning of the heartworm treatment, which is why Elvis is in the hospital with us today.
We have administered an oral medication to kill the offspring, and will monitor Elvis every ten minutes throughout the day, so that if he has any reactions to the dying microfilaria we can quickly inject medication to reverse the reaction.

The observation for microfilaria reactions is very intense.
The observation for microfilaria reactions is very intense.

Elvis is one of those dogs who had a negative heartworm test six months ago, but, having been a stray, had not been on a heartworm preventive prior to his negative test. So, we were not unduly surprised when his six-months-later test came up positive.
Thanks to youth, good pre-treatment laboratory test results (CBC, Chemistry Profile and Urinalysis) and a chest radiograph with minimal heartworm-related changes, Elvis should come through the treatment with few or no difficulties.

Heart and lung damage from heartworms is minimal for Elvis.
Heart and lung damage from heartworms is minimal for Elvis.

See you tomorrow, Dr. Randolph.


  1. Our 2 year old boxer was negative on his he test. However further tests revealed the microfligella. Vet wants to treat with 300mg x2 daily of doxy. He is on heartguard as of now. He was picked as a stray in Honda texas. This dose seems abit high and they say they can here a murmur. Your option? Lost at this point

    • Please ask your pet’s doctor to consult with the American Heartworm Society for the best heartworm preventive to remove microfilaria. Otherwise, based on the information you’ve given, it sounds like he’s on a good path. Thanks for reading, Dr. Randolph.

  2. Hi there!! Appreciate your post and replying.

    My 14.5 month dog just passed his day 120 this past weekend. My vet suggested retesting for microfilaria, is that too soon to test? How long after his 3rd injection should be test negative or rid of adult worms + microfilaria?

    I just got a sneak peek of his test results and it says abnormal. I haven’t gotten a call from my vet yet but really worried about this. My vet and vet techs were sure it’d be negative since we caught it so early on (10 months). What would the next steps be? Im reading that we would need to administer a microfilaricide + retest 4 weeks after. I really thought we were close to being done as my pups been acting as normal as can be and haven’t been showing signs of distress or discomfort. Man I hope we get rid of these soon!!!

    • We are a little confused by some of what you wrote. However, we will address your questions in the order you wrote them. The American Heartworm Society (AHS) recommends microfilaria test around Day 120. Did he take 30 days of doxycycline in the very beginning? AHS recommends testing for adult heartworms at Day 270 and not to retreat a positive test in an asymptomatic patient in less than one year and then only if the positive test persists. “I just got a sneak peek of his test results and it says abnormal.” Was this a microfilaria test or an occult antigen test for adult heartworms. We cannot comment on this statement until we know the answer to that question. So glad he’s still asymptomatic! Thanks for reading and let us know the answer to the above question, Dr. Randolph.

      • Ah apologies there. Happy to clarify!!

        Yes he did 30 days of doxycycline then went in for x rays and our vet administered the heartgard+ and had him stay for the day to monitor. Waited another 30 days until injection #1 of the melarsamine (Diroban) + 30 days of prednisone but tapering doses. Injection #2 and #3 were done 24 hrs apart. Day 120 was this past weekend and they took his blood sample for a confirmatory + antigen test from IDEXX services: heartworm panel-canine. The vet said it tested for the microfilaria and antigen (only tests for adult female worms) he’s positive for both. On the results it says 1-5/LPF microfilaria seen.

        The vet said the finding is a bit unusual because the heartgard+ should be getting rid of the microfilaria. We’ve administered heartgard+ since June 24 every month (4 doses). We administered it this sat 9/24 and he drew blood for the test on 9/25. The vet said because we gave it to him the day before it should have came back negative but I asked if the heartgard+ works that instantly?

        A few options we have right now is:
        1. My vet is checking with Heartgard+ to see if that should work instantly or if there’s some resistance from the microfilaria. And since I’m giving him Heartgard+ and credelio, it’s missing the moxidectin component that may help.
        2. Ask the manufacturer if it’s ok to follow heartgard+ with simpartico trio instead of credelio (todays his is 30th day for flea/tick medication) so waiting to hear back by tonight.
        3. If we don’t hear back, my vet said to give my dog credelio then we can Advantage Multi as a source of moxidectin.

        Hope that clarifies my situation a bit more! Yes I’m glad too, however he’s getting a bit more energetic than before since he’s not on any other medication so trying to keep him calm has been a bit tough!

        Thanks so much for taking the time to understand my situation. You’re very much appreciated!!

  3. Our dog came to us as as rescue stray as well back in December 2020. The rescue’s antigen test was negative on 11/25/20 and he was given a dose of Heartgard. He received a dose of Proheart 6 on 12/18/20 when he came to us. We went for our next Proheart and checkup recently on 6/16/21 and he tested positive on HW antigen. They said it was an “occult” test, so I’m hoping I understand this correctly – that it means there is confirmed presence of adult female heartworm, but no microfilaria?

    I am devastated, having lost our 7-year-old lab to CHF due to heartworms in November 2020. He was on “slow kill/no kill” because he was too unstable for Immiticide treatment. He was on a ton of meds, including antibiotics, diuretics, blood pressure meds, steroids, etc. He lived for nearly 3 years after his initial diagnosis. I did my best to keep him comfortable, but ultimately watched him suffer through it.
    He was on Heartgard, but due to our lack of education on HWD, there were days skipped by mistake here and there. I feel horrible. Never again.

    Our rescue baby has just started doxy for 30 days, had his second Proheart 6, and will undergo his first Immiticide in August 2021, followed by 2 more Immiticide 24 hours apart a month later. I am utterly terrified. I’ve heard and read so many horror stories of dogs dying after this treatment. I would never be able to forgive myself, but I know I can’t go with “slow kill/no kill” because I’ve seen the damage the worms will do first hand. Are there any statistics on survival rates in dogs in a comparable situation?

    I am also worried about re-infection after “fast kill” treatment – I see the main protocol for killing microfilaria is with Heartgard/ivermectin, but my doggo is getting Proheart 6. Will that be just as effective at killing microfilaria/larva so that he isn’t reinfected by any worms left over after adulticide?
    Should I ask that he have a microfilaria test in addition to antigen test before he is started on the Immiticide so that there is no risk of him being reinfected? Is that a possibility?He is roughly 18 months old and in otherwise wonderful health. Vet says his heart and lungs still sound good and strong, there is no considerable damage thus far, and that he should have great results and minimal risk with Immiticide. He shows no symptoms whatsoever compared to our lab, who was unfortunately very advanced.

    He is also very playful and excitable, and I am very worried about keeping him calm during treatment and afterward. He is, thankfully, crate-trained and very comfortable with it. We have leash walked him in the yard until he gets comfortable here (he’s a Border Collie and we have cattle and horses). He does a great job on a leash considering he was a stray. He’s obviously incredibly intelligent and obedient. Being a BC, though, I am worried about adverse reactions to the Immiticide. He has not been tested for MDR1, but so far has had no reactions to Heartgard, Bravecto, Proheart 6, doxy, or the Clavamox and prednisone he was on when he came to us with bronchitis. I’m hoping that’s a good sign.

    How long does it take for the worms to start breaking down after Immiticide? The first shot I’m not too worried about because he will be coming home with us afterward. However, with the second set of shots, he will get the first, stay at the vet’s office overnight, then receive the second 24 hours later. He LOVES going to the vet’s office, crazy as it sounds! He loves people and other animals and he just gets so excited. I have to put a harness on him and really hang on whenever we go. I’m scared all of this excitement will cause a PE while he is at the vet’s office on his second set of shots, and if he does make it through that alone, I’m afraid that when we go pick him up, he will be so excited to see us that it will pose a risk also. Should I talk to my vet about possibly mildly sedating him to keep him calm? Is that safe and is that ever done at all with these shots?

    I am so thankful to have come across your blog! I have been a mess and what I have read here has really helped to calm my nerves. I know the more educated I am about this disease process and treatment, the better the outcome will be. I feel the pain of grief, regret, and guilt every time I look in the field and see the oak tree where my lab is buried. He suffered and died too young because of my ignorance and I wish more people were aware of how horrible HWD is. Thank you so much for taking the time to write this and to answer questions here!

    • Your pet’s doctor is NOT going to let him have too much activity on his overnight stay, so you don’t have to worry about that. Adult heartworms start dying about 5-7 days after the adulticide injection, so, none will be dying that night anyway. Limit his activity, DON’T let anyone talk you into slow kill, and say a prayer at least once a day. After all, we want God on our side in EVERY task we undertake. Nothing is too small or too big for Him. Sedation is rarely needed. ProHeart 6 has limited microfilaricidal activity. The MDR-1 gene has no interaction with any of the medications you listed except Heartgard, and the dose of Heartgard is below the threshold of reaction for the vast majority of MDR-1-positive dogs. Continue to preach the dangers of heartworm disease to everyone who will listen, especially the people who refuse to give their dogs heartworm preventive! Thanks so much for reading, Dr. Randolph.

      • Dr.Randolph – HELP!! What can you tell me about the microfilaria that are not produced by heartworms? They are not contagious and, if I understand correctly, have something to do with fleas. We have a dog at our shelter that was treated with the three injections of Immiticide in Oct/Nov., 2021, and is negative for adult heartworms, but we cannot get rid of the microfilaria. We have been giving Advantage Multi or Ivermectin (both microfilariacides) every two weeks since Jan., 22., but he still has a few. This has made me wonder if he does not have the other kind of microfilaria. I realize you cannot know what it is without seeing test results, but I’d like to hear what you know about this other microfilaria. Thank you.

        • I JUST got this from my favorite parasitologist: ” Acanthocheilonema, yes, we would want to definitively identify it before determining that these are not still residual Dirofilaria immitis microfilaria. The Advantage Multi should eliminate D. immitis mf but depending on the load, it may take a little longer. Also, though the dog is Ag negative, I would ask whether they also did heat treatment of the sample, in case the [adulticide] treatment did not eliminate all adult worms, and if there are only a few left they may be below detection limits. Regardless, if the microfilaria are identified are Acanthocheilonema, there is no approved treatment for Acanthocheilonema and since it is non-pathogenic, we would not pursue treatment.”

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