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 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.”

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.