Heartworm treatment in dogs is a multi-step process that takes roughly six to nine months from diagnosis to confirmed cure. The standard protocol uses a combination of an antibiotic, monthly preventive medication, a steroid, and a series of three injections of an arsenic-based drug called melarsomine that kills the adult worms living in your dog’s heart and lungs. It’s effective, but it requires strict rest and careful timing.
How the Treatment Works, Step by Step
The American Heartworm Society recommends a specific timeline that most veterinarians follow. It begins the day your dog tests positive and unfolds over several months, with each phase serving a distinct purpose.
Day 0: Your dog is diagnosed through a blood test that detects heartworm proteins. Exercise restriction starts immediately. If your dog is showing symptoms like coughing or fatigue, your vet will stabilize them first and may start a tapering course of steroids to reduce inflammation.
Days 1 through 28: Your dog begins a four-week course of the antibiotic doxycycline, taken twice daily. This targets a bacterium called Wolbachia that lives inside heartworms. Killing off Wolbachia weakens the worms and, more importantly, reduces the lung damage that occurs when worms die later in treatment. Your dog also starts on a monthly heartworm preventive at this point, which kills any young larvae circulating in the bloodstream and prevents new infections from mosquito bites.
Day 60: The first melarsomine injection is given deep into the muscles of the lower back. This initial single dose kills a portion of the adult worms, roughly 82% in studies. Your vet will prescribe another round of tapering steroids, and exercise restriction becomes even stricter: crate rest with leash-only bathroom breaks.
Days 90 and 91: The second and third injections are given 24 hours apart. This two-dose punch is far more potent, achieving 96% to 100% kill rates in clinical trials. Combined with the first injection a month earlier, the three-dose protocol is 100% effective against male heartworms and 98% effective against females. Another round of steroids follows, and strict rest continues for six to eight more weeks.
Day 120: Your vet checks for baby heartworms (microfilariae) still circulating in the blood. If any are found, a treatment is given to clear them.
Nine months after the injections: A follow-up antigen test confirms the treatment worked. The American Heartworm Society updated this timeline in 2018, extending it from six months to nine. More sensitive modern tests can pick up residual heartworm proteins that haven’t fully cleared yet, so waiting the full nine months avoids a misleading positive result.
Why Exercise Restriction Matters So Much
This is the hardest part of treatment for most dog owners, and it’s also the most important. When melarsomine kills adult heartworms, the dead worms break apart and travel through the bloodstream into the smaller vessels of the lungs, where the body gradually absorbs them. Physical activity increases blood flow and heart rate, which can push larger worm fragments into the lungs all at once, causing a blockage called a pulmonary thromboembolism.
Signs of this complication include rapid or labored breathing, coughing (sometimes with blood), blue-tinged gums, collapse, and in severe cases, sudden death. The risk is highest in the weeks following the melarsomine injections, which is why your vet will insist on crate rest and leash-only outdoor time. Even dogs that seem perfectly fine and energetic need to be kept quiet. The total period of restricted activity spans from diagnosis all the way through six to eight weeks after the final injection, often four months or more of limited movement.
What the Medications Do
The treatment uses four types of medication working together, each with a specific role:
- Doxycycline (antibiotic): Eliminates Wolbachia bacteria inside the heartworms. This reduces the inflammatory damage to lung tissue that occurs when worms die and break down. It also weakens the worms themselves and disrupts their ability to reproduce.
- Monthly heartworm preventive: Kills immature larvae that are less than two months old. Starting this early ensures no new worms are maturing while you’re trying to kill the existing adults. Two months of preventive before the injections closes a gap in the worms’ life cycle where they’d otherwise be too old for preventives but too young for melarsomine to reach.
- Melarsomine (injection): The only FDA-approved drug that kills adult heartworms. It’s an arsenic-based compound given by deep intramuscular injection into the lower back. The three-dose schedule, spread over a month, kills worms gradually rather than all at once, reducing the risk of a dangerous mass die-off clogging the lungs.
- Steroids: Prescribed in a tapering dose after injections to control inflammation as dead worms are absorbed. This helps protect the lungs and airways during the most vulnerable phase of treatment.
What Treatment Costs
The total cost varies depending on your dog’s size, the severity of infection, and your location. Diagnostic workup, including blood tests and imaging, typically runs $200 to $300. The treatment itself, covering the injections, medications, and follow-up visits, generally falls between $500 and $1,200. Dogs with severe infections that need hospitalization or additional stabilization will be on the higher end. By comparison, a year of monthly heartworm prevention costs roughly $50 to $150, which is why prevention is so heavily emphasized.
The “Slow Kill” Alternative
For dogs that can’t safely tolerate melarsomine, either because they’re too sick, too old, or have other health conditions that make the injections risky, there’s an alternative approach. This combines monthly preventive medication with doxycycline to gradually weaken and kill adult worms over time. It’s sometimes called the “slow kill” or “soft kill” method.
In one study, this approach produced a negative heartworm test by day 180 of treatment, only about 60 days longer than the standard protocol. However, it comes with a significant trade-off: the worms survive longer inside the heart and lungs, which means ongoing damage to those organs for additional months. The standard melarsomine protocol remains the recommended first choice whenever a dog can handle it, because it stops that damage sooner.
What Recovery Looks Like
Most dogs tolerate treatment well, though the injection site can be sore for a few days. You may notice swelling or tenderness in the lower back where the shots were given. Some dogs are lethargic or have a reduced appetite for the first week or two after injections. Mild coughing in the weeks following treatment is common as the body breaks down dead worms in the lungs, but heavy coughing, difficulty breathing, or lethargy that worsens rather than improves warrants an immediate call to your vet.
The weeks of crate rest can take a toll on both you and your dog. Puzzle toys, frozen treats, and short, calm leash walks for bathroom breaks help manage boredom. Many owners find this phase more challenging than the medical side of treatment. But dogs that complete the full protocol and rest period generally make a full recovery, especially when the infection is caught before significant heart or lung damage has occurred. Once your dog tests negative at the nine-month mark, year-round monthly prevention keeps them protected going forward.