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. About 2 months ago, we adopted a senior dog (about 8 1/2 y.o.) heartworm and lymes +. He was immediately started on monthly heartworm medication and has completed 1 month doxy. He’s scheduled to start immiticide next week. He has no obvious limiting issues (did well with his Vet exam). Given his age and visibly good health, I’m having a hard time justifying immiticide treatment over slow-kill. It’s not about the expense but rather the stress the treatments could cause in an elderly dog requiring clinic observation stay after each injection. Will we be doing him any favors by choosing the aggressive treatment versus staying the course with monthly heartworm medication? Thanks for reading and would greatly appreciate your perspective!

    • READ THIS ARTICLE. Dr. Ray Dillon, one of the most eminent heartworm experts in the world says, “No Slow Kill.” The importance of the ongoing damage to the cardiopulmonary system and immune system cannot be overstated. Get the adulticide therapy and follow your pet’s doctor’s instructions, please. Thanks for reading, Dr. Randolph.

  2. I have a dog who is about to turn 1 and we found out in May 2022 that he is HWP. We’re going to do the fast kill: we’ve done the 28 days of doxycycline and then day 30 had him at the vet all day for them to administer heartgard plus and do x rays – they said he was fine and no significant findings!

    This past month was just resting before the melarsomine treatment. They vet said to activity restrict him such as no running, jumping, using the stairs, or rough playing before injections. Because it can increase his heart rate which the heartworms can damage his heart and lungs. So they only allowed for like 10 min walks. He hasn’t showed any signs of distress during this month but we have gone on longer walks to get his energy out. Have I made a mistake by doing that even though he hasn’t gotten his injections yet?

    He’s definitely gotten excited when other dogs pass by or played with a few. However when I read your article it made me a feel a bit better since he doesn’t start his injections until this Sunday. When he starts, I know he will either need to be crated or limit any type of activity that may get him excited.

    What do you recommend for a 1 y/o active puppy as he will need to be indoors and calm for the next 2.5 months essentially? He’s pretty active.

    • And I know my vet’s mentioned activity restricted and crate rested. What does activity restricted help with? It seems like activity restriction is before and after the melarsomine injections. Now I understand they need to be crate rested aka calm at all times, low heart rate, no excitement while during 1st-3rd melarsomine injections so that there’s not washing cycle and increasing the possibility of a dead adult worm getting lodged into one of his vessels and causing an embolism. Just so I’m understanding clearly before and after his injection treatments!

      Looking forward to hearing back!! Thanks for all ur help!

      • “What does activity restricted help with?” Embolism is explained in the article. You want to keep his heart rate as low as possible. You may also benefit from reading the replies I’ve posted to other readers’ questions. Thanks for reading, Dr. Randolph.

    • It’s unlikely you’ve done any damage. The BIG thing is to keep his heart rate as low as possible during the treatment and into the ultimate post-treatment phase for the time your veterinarian recommends. Some dogs need to be crated, but some dogs get so frustrated in the crate that the fretting raises the heart rate. Ultimately, it comes down to common sense. Thanks for reading, Dr. Randolph.

  3. Hello. We just adopted a rescue puppy (about 1 year old) a month ago. She was finishing up the medicine part of heartworm treatment and we just completed her second shot treatment yesterday. She was very lethargic, skittish and growly/snappy after both days of the treatment. This morning she is back to her normal puppy self (She is a terrier mix). We were told when she was rescued she was positive for the microfilariae, so they were doing this treatment as a precaution to make sure no issues arise. The vet told us we were told to keep her calm for 30 days and she is able to have 2 sedatives a day if needed. We hate to do that if we don’t have to due to her reaction. She also will not go to the bathroom if leashed. We have a fenced in yard that she typically goes in while we walk with her (off leash). How critical is it to stay totally calm for 30 days if she did not test positive for worms but a trace of the microfilariae? I very much enjoyed your article-very informative. Thank you!

    • I’m not completely clear on your dog’s status. The antigen test was negative for adult heartworms? Was a well test (Dirochek) also performed? Did she have a test on a heat-treated sample (usually done in a laboratory)? Did she have adulticide therapy? It’s possible that she had adult heartworms that produced those microfilaria, then those adults died, either from age or having had adulticide therapy through an (unknown) doctor, but the microfilaria were never addressed. It would not be safe to assume that there are no adult heartworms, and failing to restrict activity could result in a fatal embolism. The main factor in restriction of activity is keeping the heart rate low. If she can walk around the back yard just long enough to use the bathroom then go right back inside, and you can keep her from running, jumping and vigorous activity, she can probably do fine without the sedative. The bottom line is to be sure her heart rate stays low. Let us know if there are any parts of the story we don’t have right. Thanks for reading, Dr. Randolph.

  4. We just adopted a 2yo heart worm positive dog. We didn’t really understand what we were getting ourselves into until we read through the folder we were given after we brought her home. In any case we love her and are preparing for her first injection.

    We will have to move our vacation (cabin in the woods) until after her treatment. Is it safe for us to go on this trip with her 8 weeks after her first injection? There will be another dog present. Is there a risk of infection to the other dog at this time?

    Thank you very much!

    • ASSUMING there are no complications 8 weeks after treatment, there is no reason she couldn’t go on vacation with you. No risk to your other dog IF he takes his heartworm preventive on schedule. One dog cannot infect another with heartworms directly, the parasite requires life cycle stages in the mosquito. Thanks for reading, Dr. Randolph.

  5. Hi, Dr. Randolph, we adopted a rescue the end of Oct of last year. In March she turned 1 and was diagnosed with heartworm. Long story short, she is gaining alot of weight due to restrictions and is a bit stir crazy. My question is how important is the Trazadone and Gabapentin for her staying calm? Her 2 & 3rd shots we’re last Wed and Thurs. She did quite well. My husband said we don’t need to give her meds she’s fien. I hate giving them to her but, I feel that it keeps her from getting so worked up from boredom, keeping her Blood pressure low and the chances of her bolting out the door. we did close off 95% of thr backyard so she can’t run far but I prefer she not even attempt that. Should we medicate her or is it not going to help with that or her weight. not trying to get her to the point where she goes through withdrawls later. Clearly, I’m getting overwhelmed. Thank you for your time.

    • As I am not the attending veterinarian for your pet it would be inappropriate for me to make suggestions about her medication. However, if weight gain is accompanying decreased activity, you can simply cut back on the “groceries” proportionately to get her weight back in line. Share your concerns about the medication with your veterinarian and get that advice directly from him. Thanks for reading, Dr. Randolph.

  6. 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.

  7. 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.

      • The confirmatory test is free? Is this the 4Dx plus SNAP test? My vet charged me $135 for that. Had to also pay for the antigen test! On another note, how long after the 3rd shot will my dog have to be crate rested for? I’m aware that after the 1st injection until 3rd injection my dog will have to be crate restricted then my vet said after 3rd shot they can be activity restricted (meaning not in the crate all day). My dog is almost 1 y/o and we just found out in May he’s heartworm positive. His first injection will be next Sunday. Fingers crossed!! Just stumbled across this article – super helpful I appreciate all your time and content!

        • Additionally, do you recommend administering CBD to help keep a young pup calm? We have another dog at home so when he doesn’t get enough exercise he will end up trying to bother the older dog to play. I’ll mostly keep him in my room during that time but I’m sure he will go stir crazy from lack of walks and outdoor exposure 😩. Would love your thoughts on that!

          • I don’t recommend CBD for ANYTHING. Look at this product! There is no testing, there is no standardization, there is no approval from FDA or USDA and little has been done in the way of scientific trials. I’ve treated a kazillion dogs for heartworms and I’ve never seen one go stir crazy yet.

        • The only confirmation test that’s free is provided by the company we purchase our heartworm tests from, and it’s a Dirochek well test, performed only in laboratories. It’s generally recommended to restrict activity 4-6 weeks after the third injection, BUT, you should be guided by YOUR veterinarian’s recommendations, as he knows your pet’s complete medical history. Thanks for reading, Dr. Randolph.

  8. 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.

  9. 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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