Treating heartworms in dogs is a multi-step process that takes roughly four to six months from start to finish. The standard protocol involves an antibiotic phase, a series of three injections to kill the adult worms, and strict rest throughout. Total costs typically range from $600 to more than $3,000 depending on the severity of the disease and your dog’s size.
How Severe Is the Infection?
Before treatment begins, your vet will determine how far the disease has progressed. The American Heartworm Society classifies infections into four stages, and the stage shapes the treatment approach and urgency.
- Mild: No symptoms, or just an occasional cough. Many dogs are diagnosed at this stage through routine testing.
- Moderate: Persistent cough, reluctance to exercise, and abnormal lung sounds your vet can hear with a stethoscope.
- Severe: Coughing, difficulty breathing, fainting episodes, an enlarged liver, and fluid buildup in the abdomen. Some dogs at this stage are at risk of death.
- Caval syndrome: A sudden, life-threatening crisis where adult worms physically block blood flow through the heart. Without emergency surgical extraction of the worms, most dogs die within two days.
Dogs in the mild to moderate range are the best candidates for standard treatment. Severe cases need to be stabilized first, and caval syndrome requires immediate surgery, which can cost $3,000 to $6,000.
The Standard Treatment Timeline
The protocol recommended by the American Heartworm Society spans about 120 days and involves multiple phases. Here’s what it looks like from your perspective as an owner.
Days 0 Through 30: Antibiotic and Preventive Phase
Your dog starts a 28-day course of an antibiotic (doxycycline) that targets bacteria living inside the heartworms themselves. These bacteria, called Wolbachia, help the worms survive and reproduce. Eliminating them weakens the adult worms and reduces the inflammatory reaction when those worms eventually die. Your dog also begins monthly heartworm preventive medication at this point to kill any immature larvae circulating in the bloodstream. If your dog tests positive for microfilaria (baby heartworms), the vet will pre-treat with medications to prevent an allergic reaction.
Activity restriction starts now. The more symptoms your dog has, the stricter the rest needs to be.
Days 31 Through 59: Waiting Period
After the antibiotic course finishes, there’s a one-month pause before the injections begin. This gives the antibiotic time to fully take effect against the bacteria inside the worms. Your dog continues on monthly preventive during this window.
Day 60: First Injection
Your dog receives the first of three injections of the drug that kills adult heartworms (melarsomine). This is given as a deep muscle injection in the back. Your vet will also start your dog on a tapering course of a steroid (prednisone) over the following four weeks to control inflammation as worms begin to die. Activity restriction becomes rigid at this point: crate or pen rest at home, leash walks only to go to the bathroom, no running or playing.
Day 90: Second Injection
The second injection is administered, along with another round of tapering steroids.
Day 91: Third Injection
The third and final injection goes into the opposite side of the back from the day before. This back-to-back dosing on days 90 and 91 delivers the strongest punch against remaining adult worms. Strict rest continues for another six to eight weeks after this last injection.
Day 120: Follow-Up Testing
Your vet tests for microfilaria to see if immature worms are still present. A follow-up antigen test is typically done around six to twelve months after treatment to confirm the adult worms are gone.
Why Exercise Restriction Matters So Much
This is the single hardest part of treatment for most owners, and also the most important. When the injections kill adult heartworms, the dead worms break apart and get carried into the small blood vessels of the lungs, where the body gradually absorbs them. Physical activity increases blood flow and heart rate, which can push large worm fragments deeper into the lungs and cause a blockage called pulmonary thromboembolism.
Warning signs of this complication include sudden coughing, coughing up blood, rapid or labored breathing, extreme lethargy, and fainting. These can appear days to weeks after injections and require immediate veterinary attention. The risk is highest in the weeks following the day 90 and 91 injections, when the largest number of worms are dying.
Strict rest means crate confinement or confinement to a small room, short leash walks only for bathroom breaks, and no off-leash time, no stairs if avoidable, no roughhousing with other pets. This level of restriction lasts from around day 60 through at least eight weeks after the final injection, roughly three to four months total of very limited activity.
What Treatment Costs
The total bill depends on your dog’s size, the severity of the infection, and your geographic area. Here’s a rough breakdown of the individual components:
- Initial antigen test: $35 to $75
- Confirmatory testing: $20 to $40
- Chest X-rays: $125 to $200
- Echocardiogram (if needed): $300 to $1,000
- Doxycycline (28-day course): $30 to $150
- Melarsomine injections and hospital care: $500 to $1,500
- Steroids: $10 to $40 per round
- Monthly heartworm preventive: $6 to $18 per dose
- Microfilaria follow-up test: $20 to $40
All told, most owners spend between $600 and $3,000. Larger dogs cost more because the injection drug is dosed by weight. Dogs with severe disease need more diagnostics and stabilization, pushing costs toward the higher end. If the one-year follow-up test is still positive, a second round of injections adds another $500 to $1,000 plus medications.
Why “Slow Kill” Isn’t Recommended
Some owners hear about a cheaper alternative: just keeping the dog on monthly preventive (and sometimes an antibiotic) without the injections, letting the worms die off on their own over one to two years. This is sometimes called “slow kill” therapy.
It does work to some degree. Long-term preventive use kills larvae and shortens the lifespan of adult females. But the approach has serious drawbacks. While the worms linger, they continue damaging the lungs and heart. The slow die-off of worms can still trigger blood clots in the lungs, and the effects of this gradual process on cardiac damage and clot risk are not well documented. The dog remains infectious to mosquitoes (and therefore other dogs) for much longer. The American Heartworm Society does not endorse this approach when the standard protocol is an option.
That said, some dogs with other serious health conditions may not be safe candidates for the standard injections. In those cases, your vet may choose a modified approach as the lesser risk.
What Recovery Looks Like
Most dogs with mild to moderate infections tolerate treatment well. The injection site in the back muscles can be sore and swollen for several days, and some dogs are lethargic or have a reduced appetite for a day or two after each injection. The steroid taper helps manage inflammation but can make your dog hungrier and thirstier than usual, and they may need more bathroom breaks.
The hardest stretch is the weeks after the final two injections, when your dog may feel perfectly fine and want to run and play while you’re tasked with enforcing strict crate rest. Puzzle feeders, frozen treats, and calm chew toys help pass the time. Some owners find that a mild vet-prescribed sedative makes the confinement period more manageable for anxious or high-energy dogs.
By about five to six months after treatment starts, most dogs can gradually return to normal activity. The follow-up antigen test at the one-year mark confirms whether the infection has been fully cleared. Once that test comes back negative and your dog is on year-round monthly preventive, the chapter is closed.