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. HI Dr. Randolph,

    My 7 year old adopted dog’s preliminary antigen test has come back positive and he is going for confirmatory bloodwork after the weekend. I am reading about exercise restrictions. He is a very calm dog, but won’t do his business in the yard. We typically walk about 1 hour each day. Is keeping him on a leash and walking him for about 10 minutes around the block slowly ok so that he does his business? Is it most likely his confirmatory bloodwork will be positive too? Thank you.

  2. Hi Dr. My 2 y/o dog has tested + for heartworms with the antigen test, however no microfilariae were detected. She has been on preventive for a year and the veterinarian says she must have gotten it prior to the year and microfilaria not showing up probably because of prevention. They would like to do treatment and said to do research first and it seems the next step is confirming. Should I have another veterinarian test again? She is very, very active, a border collie mix. So keeping her calm is a major issue if we begin treatment. Aren’t there any mild sedatives to help? Can you address the use of calming meds during treatment? Ty

    • The company that makes the test we use will perform a confirmation test at no charge. Your pet’s doctor may have access to the same service. If not, ask him to submit a “well test” for confirmation. No “research” is necessary. The science is settled. Read this article. I don’t like giving long-term sedatives to patients. I’ve yet to see even the most excitable dog fail to be adequately controlled during heartworm treatment by leash-walking alone. Thanks for reading, Dr. Randolph.

  3. I adopted a 4 y/o male pound pup 11-12-21. He was treated for heart worms beginning 10-29-21. No one told me to keep him calm. The pup slipped out of his collar the first day and took off in the woods for 3 days. I took the pup to my vet 11-19-21 and advised me to limit his activity. The vet said he appears to be doing well. I just came upon this site and now I’m more worried about the 3 day hike in the woods. How long before I can stop worrying about long term damage from running so soon after treatment? Thank you!!

    • If he’s doing well now, you don’t have anything to worry about from what has ALREADY happened, but, do continue to restrict activity going forward. Thanks for reading, Dr. Randolph.

  4. My pup is going through heartworm treatment right now. She has 1 more month left to go. 3 days after her first injection she started to get a cough & also had nausea. She is petite & they gave her an injection for her nausea as well as am antibiotic injection to help with her cough. Now, 3 weeks later, her cough is gone for the most part (she did have a small cough before treatment, I do give her benadryl for her allergies). Now she doesn’t really cough anymore, but should I be concerned about that cough?

    • If she’s not coughing, she’s eating fine and having good bowel movements, you’re probably OK. However, to be sure, I’d ask the veterinarian who is treating her. Thanks for reading, Dr. Randolph.

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