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. Our dog received her HW injections 7 weeks ago. We are taking her on vacation with us to a cabin in MN, but continuing do activity restriction. I’m concerned about ticks. Should she be on heartworm prevention?

    • The American Heartworm Society prefers dogs with heartworms be on a milbemycin-based heartworm preventive. Her microfilaria status and your attending veterinarian’s recommendations need to be taken into account! We usually give ProHeart6 at the time of diagnosis or after pre-treatment laboratory testing and chest radiography. Our first choice for flea and tick control is Simparica. Ask your veterinarian for her advice. Thanks for reading, Dr. Randolph.

  2. I just adopted a 100lb spayed Anatolian Shepherd dog about 2mo ago. She was in a shelter for several months, but tested HW negative in Aug 2022.
    We went for her first vet visit last week and she tested HW positive. Retest also positive.
    Started her on the Doxycycline and HeartGuard+ 3 days ago. She vomited up her morning meal about 2hrs after eating. Only change in her diet is the Doxy and HG and the pill pockets. Wondering if it’s the antibiotic.
    Since she did test negative not so long ago would I be safe in “assuming” that her HW burden is fairly low? As far as I can tell, she is asymptomatic. (Good energy level, no cough, good appetite.) We did the antigen tests but no radiographs, no ultrasound, etc.
    Thanks for this website. Great info.

    • Good that you got a confirmation heartworm test. Ask your pet’s doctor whether he has access to a milbemycin-based heartworm preventive; the effect on adult heartworms is greater than the ingredient in Heartgard and that’s why it’s recommended by the American Heartworm Society. ProHeart injection is fine as are the many oral formulations. Some patients have GI upset with doxycycline. Ask your veterinarian for a probiotic of his choice. And, no, you can’t assume the heartworm burden is low. I take pre-treatment chest radiographs of our heartworm treatment patients. Not everyone does. If you want to read more, look at the right side of each page; we have a Heartworm Category, in which I’ve created a collection of all of the heartworm-related articles for you. Read one, click on the BACK button on your browser and continue up or down the list. Thanks for reading, Dr. Randolph.

  3. Hi – My dog (Allie) just had her first round of heartworm treatments at the beginning of May. I asked my vet for something to keep Allie (basset hound/lab mix) calm. I just adopted her and she deals with separation anxiety and leash aggression. The vet prescribed Trazodone and it’s working pretty well. If you have a Costco membership it was only about $8 for a 30 day supply with the membership discount. Prior to the meds, she would howl every time I left the apt. Now she whines but quickly settles. Previously, I was giving her calming chews (which kind of worked) and the vet said I could give the calming chews and the Trazodone. It might be worthwhile to get a script. After about a week, Allie was no longer in pain from the shot and the Trazodone has been a lifesaver to chill her out. Also, the snuffle mat, kong wobbler, and puzzles have been helpful. I bought my puzzles on Amazon but a friend gifted me some she found at TJ Maxx for a fraction of the price of Amazon. Allie’s still bored because she loves being outside but it gives her a bit of enjoyment while she rests.

  4. My 3 yr old German Shepherd is a rescue and was diagnosed with heartworm. He has another 1.5 weeks of Doxycycline. It has caused him to have a loss of appetite, hence, I have resorted to boiled chicken/rice mixed with his grain. He beings the injections in June. The clinic that is treating him advised no exercise pre injections. He is so bored and wants to play. He began running around the house due to excess energy. With that said, once he begins the injections, I realize the importance of keeping him still. Some friends recommended CBD oil to calm him. I am very skeptical of this and perhaps a prescribed medication to calm him would be better. What are your thoughts…
    I have read your reply and words of wisdom to various questions, thank you for taking the time to answer our questions!

    • There is NO standardization on the ingredients of CBD oil, therefore, I can’t possibly recommend it. Ask your veterinarian if Benadryl might help. Sedation is about all it’s good for in dogs. Restriction of activity is SO important! Best wishes with that heartworm treatment. Thanks for reading, Dr. Randolph.

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