Activity Restriction Needed During Heartworm Treatment

Activity restriction is important for dogs undergoing heartworm (Dirofilaria immitis) treatment. However, it is not usually needed for the entire process, just during the time after Immiticide (and, the newer medication, Diroban) injections are administered.


Willie gets his Revolution heartworm preventive/flea control EVERY month on time without fail.

Patients undergoing “slow kill” heartworm treatment need activity restriction only if there are complications or preexisting heart disease. Follow the advice of your veterinarian.

When Immiticide begins to cause adult heartworms to die, the healthy way for them to be disposed of is for white blood cells (WBCs) to munch away at them like little Pac Men. We want microscopic pieces of the heartworms to be disposed of.

During vigorous activity, blood flow through the heart is increased. That increase in “washing action” over the dying adult heartworms can cause large pieces of decomposing heartworm bodies to be freed into circulation. If an artery becomes occluded by this floating log, embolism occurs.

Embolism is defined as “the sudden blocking of an artery by a clot or foreign material which has been brought to the site of lodgement by the blood current.”  Embolism is a noun form, as is embolus, which refers to the actual object blocking the artery.   The plural form of embolus is emboli.

An embolus can clog an artery in the heart, brain, kidneys, lungs or other vital organ. If that happens, tissue death usually occurs because the obstructed artery can no longer supply oxygen and nutrients to the tissue. Adult Dirofilaria usually reside in the right ventricle of the heart and the pulmonary arteries that convey “used” blood from the right side of the heart to the lungs for releasing waste products and picking up new oxygen. Therefore, the lungs are the organ most likely to suffer embolism.

In the best-case scenario a pulmonary embolus is small and a minor area of inflammation occurs. The next-worse possibility is a larger embolus that results in a small area of tissue death and low-grade pneumonia. This picture is usually accompanied by a cough and requires attention by your veterinarian.

Even larger emboli can cause death of an entire lobe of the lung, requiring thoracic surgery to remove the damaged lobe. Sometimes the inflammatory reaction is so great and so sudden that fluid pours into the lungs and the patient dies within minutes.

None of this is meant to scare you, however we cannot overemphasize the importance of restricting activity during this phase of heartworm treatment.

How “restricted” is “restricted activity?” Running is out of the question. As are long walks. Venturing outside the house, including to fenced-in yard, must be on a leash. One quick burst of speed chasing a squirrel or stray cat could bring on an embolic complication.

For most heartworm-treatment patients the activity restriction period is only 2-3 months long. Considering that following the warning may avoid a fatal complication makes it a small price to pay.

See you next week, Dr. Randolph.



  1. Kathy says:

    Hi Dr. Randolph, I just adopted a rescue dog (34lbs) from Taiwan (last time he was tested was over a year ago (negative result)…he wasn’t on preventatives when he was there though)) few months ago and he was just tested again and is now (+) for heart worms. The vet we took him to here only prescribed him monthly heart worm preventatives and doxycycline, with mention of immiticide shots but they never confirmed with me whether they’re going to give it to him or not. I did have them give him a xray (vet said heart and lungs look fine/normal but he does have a heart murmur).
    Should I be concerned about only giving him the heart worm preventative and antibiotics as treatment or should I bring up that the shots are necessary for his full recovery and to prevent any damage to his organs in the future?
    I also, know it’s absolutely necessary to keep him quiet during the treatment process but he is a very active dog (won’t do his business in the yard so we usually take him for walks 2-3 times a day) and does not really like much treats/hates chew toys (he runs away from them when he sees them). Please advise on how to keep him quiet during these times.
    Thank you!

    • The very active dog is a challenge in heartworm treatment, however, few complications are unmanageable. Yes, pursue adulticide. Read this article on No Slow Kill. Please update us and let us know how he does. Thanks for reading, Dr. Randolph.

      • Kathy says:

        Thank you for your reply Dr. Randolph. I have another question regarding treatment as of right now as we wait for the Immiticide shots. We just received the the preventatives and antibiotics and was told to start him on that first. I am just wondering since he is (+), would taking the preventatives kill the microfilaria too quickly and cause him to go into anaphylactic shock, and if so, how would I watch for it/prevent it from happening (I was never notified of the status of his worm burden)? The vet only told me the reason why he would have a reaction to the preventatives is if he was exercising too vigorously. Also, if we were to get him another heartworm test to confirm, would using the preventative alter the results?
        Thanks a whole bunch in advance!

        • Has he already taken the first dose of heartworm preventive? Reactions usually occur within hours of the first dose. That you’ve used the plural implies he’s already had more than one dose, in which case the danger of anaphylaxis has already passed. A confirmation test isn’t a bad idea, but it’s unlikely to change the diagnosis. No, heartworm preventive does not affect the test result. To read about why he’s on an antibiotic for a worm infestation, click here. Thanks for reading, Dr. Randolph.

  2. Kaitlyn says:

    Our 10 month old lab just underwent hw treatment (2 shots may 8 and 9, 2020) today 5/15/20 he started having hives. Could this be a reaction to the injection or the heartworms dying and breaking down in his body? Or possibly the anti inflammatory he’s still on? No other changes recently so just curious if this could be related. Thanks!

    • I assume by now you’ve sought treatment for him. Sorry, this came in just before the weekend and we were so busy I couldn’t get to it. Hives can be a reaction to almost anything, from his injections to a sting (the MOST common offender) to something he ate. Thanks for reading, Dr. Randolph.

  3. Lisa says:

    Hi. We have a rescue red Siberian mix diagnosed hwd, put on doxycycline, then given 2 successive days’ injections (fast kill) on 3/2-3/2020. He has been doing well, except now losing his hair? There is a red rash in some areas (maybe fleas, which we did treat Frontline plus). Should we be concerned about hair loss? Continue doxycycline?(75lb dog, every 12 hours 100 mg)

    • Nothing you’ve described is a cause for hair loss. However, you will find that because Frontline is a very old molecule, the vast majority of fleas are resistant to it. Your veterinarian can dispense an effective, oral monthly flea control. If that doesn’t resolve his hair loss an appointment would be appropriate. Thank you for reading, Dr. Randolph.

  4. Julia says:

    Can you recommend any supportive sources/studies on the idea that “Patients undergoing “slow kill” heartworm treatment need activity restriction only if there are complications or preexisting heart disease”? I want to read up on this particular sub-topic of heartworm! Thank you!!

    • “Slow kill,” also called “No kill,” is not recommended by the American Heartworm Society. Click here to read an article about a lecture presented by Dr. Ray Dillon, a foremost expert on heartworm disease in dogs and cats and a board-certified veterinary internist. Thanks for reading, Dr. Randolph.

  5. Patricia says:

    I am having trouble finding out the degree of activity that is safe while my dog is on the initial phase of heartworm treatment with Doxycyline and Prednisilone… can we take a moderate walk of a mile as long as there is no running etc? and what about moderate play inside? My dog is keyed to activity before eating. If she does not eat all ‘breakfast” food she will generally eat after the noon walk or scarf it up after the dinner walk and sometimes eat ALL of her dinner after that.

    She does not currently exhibit symptoms… tho she may pant a little sooner than I would expect.

    I also understand that SEVERE restrictions are needed after the arsenic shots….


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