Understanding the Heartworm Treatment Process for Infected Pets

A clear, step-by-step guide to what happens when a pet is diagnosed with heartworms, from diagnosis to recovery.

By Garret Merkley · Explainer · Jun 19, 2026

Branched from Understanding Heartworm Risk in Humid Climates

Quick take

Heartworm treatment is a lengthy, multi-stage process requiring strict veterinary oversight.
It primarily involves injections to kill adult worms, preceded and followed by other medications.
Strict activity restriction is crucial during and after treatment to prevent serious complications.
Prevention is significantly safer, simpler, and less expensive than treating an established infection.

Heartworm treatment is the medical process designed to eliminate adult heartworms (*Dirofilaria immitis*) and their offspring from an infected pet's body, primarily dogs. This serious and complex procedure aims to save the pet's life, alleviate symptoms, and prevent further damage to the heart, lungs, and other organs.

Initial Assessment and Pre-Treatment Preparation

Before treatment begins, your veterinarian will confirm the diagnosis and assess the severity of the heartworm infection through blood tests and potentially X-rays or ultrasounds. This helps determine the pet's overall health and the extent of heart and lung damage. Often, a course of antibiotics, typically doxycycline, is prescribed for several weeks. This targets *Wolbachia*, bacteria that live symbiotically within heartworms. Killing these bacteria weakens the heartworms and reduces the inflammatory reaction when the worms die, making the subsequent adulticide treatment safer.

The Adulticide Phase: Eliminating the Worms

The core of heartworm treatment involves administering an adulticide medication, melarsomine dihydrochloride (Immiticide), which is an arsenic-based drug. This is given by deep intramuscular injection into the lumbar muscles. The most common protocol involves a series of injections: one initial injection, followed by two more injections 24 hours apart, approximately 30 days later. Sometimes, a two-injection protocol is used for less severe cases. These injections are painful, and pets often receive pain management and anti-inflammatory medications during this period.

Critical Post-Treatment Care and Monitoring

After the adulticide injections, the dead and dying heartworms begin to break down in the pet's pulmonary arteries. This is the most critical period, as these fragments can cause blockages (pulmonary emboli) or severe inflammation. Strict activity restriction, often requiring cage rest for 6-8 weeks, is absolutely essential. Any strenuous activity can increase blood flow and dislodge worm fragments, leading to life-threatening complications. Your veterinarian will monitor your pet closely and may prescribe additional medications to manage inflammation or pain. Around 4-6 weeks after the final adulticide injection, a microfilaricide (to kill larval heartworms) is usually given, and the pet will then be started on lifelong heartworm prevention.

Approximately six months after the final adulticide injection, a follow-up heartworm antigen test is performed to confirm that all adult worms have been successfully eliminated. If the test is still positive, further treatment may be necessary.

What Pet Owners Must Do

Strictly enforce activity restriction as prescribed by your vet.
Administer all medications on schedule, including antibiotics and pain relief.
Attend all follow-up appointments and testing.
Commit to lifelong heartworm prevention once treatment is complete.
Report any signs of lethargy, coughing, difficulty breathing, or loss of appetite immediately.

Treating heartworm disease is vital because, left untreated, the infection progresses, leading to severe and irreversible damage to the heart, lungs, and other organs. This can result in chronic illness, heart failure, and ultimately, death. While the treatment is intensive, risky, and costly, it offers the best chance for a pet to recover and live a healthy life. It applies whenever a pet tests positive for heartworm infection, regardless of whether they are showing symptoms yet.

Is heartworm treatment painful for my pet?

The melarsomine injections themselves can be painful, and pets may experience soreness, swelling, and discomfort at the injection site. Your veterinarian will provide pain management to help keep your pet comfortable during and after the injections.

How long does recovery take?

The entire treatment process, from initial diagnosis to confirmation of negative status, can take 6-9 months. The most critical recovery period for activity restriction is typically 6-8 weeks after the final adulticide injection.

Can cats get heartworms, and is their treatment different?

Yes, cats can get heartworms, but they are considered atypical hosts. The worms don't live as long, and fewer typically mature. There is no approved adulticide treatment for cats, so feline heartworm management focuses on supportive care for symptoms and preventing new infections.

How much does heartworm treatment cost?

Heartworm treatment is significantly more expensive than prevention, often ranging from hundreds to several thousands of dollars, depending on the pet's size, the severity of the infection, and geographic location. This cost includes diagnostics, medications, injections, follow-up tests, and supportive care.

What happens if I don't treat my pet?

If left untreated, heartworm disease will progress, leading to increasing damage to the heart, lungs, and other organs. This can cause severe coughing, difficulty breathing, lethargy, weight loss, heart failure, and eventually, death. Untreated heartworm disease is ultimately fatal.

Sources

American Heartworm Society (AHS) Treatment Guidelines