Dog & Cat HospitalCalabasas

Patient Education

Heartworm Treatment

A carefully staged treatment protocol to clear an active heartworm infection as safely as possible — followed by strict rest, monitoring, and a long-term prevention plan.

Heartworm disease is caused by Dirofilaria immitis, a parasitic worm transmitted by mosquitoes that takes up residence in a dog's heart and pulmonary arteries, where it can grow up to a foot long and cause serious cardiopulmonary damage. Treatment is available, but it is a staged, weeks-long process that carries real risk if rushed or performed without proper monitoring. At Dog & Cat Hospital Calabasas, Dr. Brian Norman's internal medicine expertise and our on-site diagnostics allow us to evaluate disease severity accurately and guide dogs through the full treatment protocol — from initial staging to the final clearance test months later. Because cats cannot be treated with the same drugs used in dogs, heartworm prevention is the only reliable protection for felines.

How It Works

The American Heartworm Society's recommended protocol uses melarsomine dihydrochloride (an injectable medication) to kill adult heartworms. Because dead worms are absorbed by the body and can cause dangerous pulmonary emboli (blockages) if too many die at once, the process is deliberately staged:

  • Staging and stabilization — chest radiographs and bloodwork characterize disease severity; dogs with significant inflammation receive doxycycline (an antibiotic that weakens the bacteria heartworms depend on) and monthly heartworm prevention to eliminate microfilariae (baby worms) before adulticidal therapy begins
  • First injection — a single deep intramuscular injection of melarsomine starts the kill; strict exercise restriction begins immediately and must be maintained throughout treatment
  • 30-day rest period — the dog must be kept calm and confined, with no running, jumping, or off-leash activity, to minimize the risk of pulmonary embolism as worms die and are reabsorbed
  • Two-injection series — 30 days after the first injection, two additional injections are given 24 hours apart to clear remaining adults
  • Post-treatment monitoring — activity restriction continues for another 6–8 weeks; a follow-up antigen test at 6 months confirms clearance

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What to Expect

Heartworm treatment is a multi-month commitment for the owner as much as the pet. The injection sites may be sore for several days, and dogs often feel fatigued and reluctant to move during the first week after each melarsomine dose — which is actually a good sign, since rest is protective. Coughing or mild exercise intolerance can persist during the die-off period; these are monitored at scheduled rechecks. The most critical owner responsibility is enforcing strict rest — even a short burst of excitement or running can be dangerous when dead worm material is in the pulmonary circulation. Crate rest or calm leash-only bathroom trips only, for weeks, is the standard expectation. Monthly prevention is continued during and after treatment to prevent reinfection and control any remaining microfilariae.

Good Candidates

  • Dogs who test positive on a heartworm antigen test, regardless of whether they are showing symptoms yet
  • Dogs with mild to moderate heartworm disease (Class I or II) — the best candidates for the full adulticidal protocol with the lowest risk of complications
  • Dogs with more severe disease (Class III) who have been medically stabilized first — treatment is still pursued with additional precautions
  • Dogs whose owners can commit to strict exercise restriction for the full 8–10 weeks of the protocol
  • Note: cats cannot be treated with melarsomine and are managed supportively; prevention is the only proven protection for felines

Frequently Asked Questions

1What happens if heartworm disease is left untreated in a dog?
Without treatment, heartworms continue to live, grow, and reproduce inside the heart and pulmonary arteries, causing progressive cardiopulmonary damage, exercise intolerance, right-sided heart failure, and eventually death. Early-stage disease causes minimal symptoms, which is why annual testing matters even when a dog appears healthy.
2How long does heartworm treatment take for a dog?
The full protocol spans approximately four to five months from the first injection to the final clearance test at six months post-treatment. The most intensive exercise-restriction period is the 8–10 weeks surrounding the melarsomine injections.
3Is heartworm treatment dangerous for dogs?
The treatment carries real risk — primarily from pulmonary embolism as dead worms are absorbed — but the staged protocol and strict rest period are specifically designed to minimize that risk. Leaving heartworm disease untreated is considerably more dangerous than treating it carefully.
4Why does my dog have to rest so much during heartworm treatment?
When adult worms die, fragments travel to the pulmonary (lung) circulation and are gradually absorbed. Exercise increases blood flow through the lungs, raising the risk of those fragments causing dangerous blockages. Strict rest is one of the most important things an owner can do to keep the dog safe during treatment.
5Can a dog get heartworms again after being treated?
Yes — treatment clears an existing infection but provides no future immunity. Year-round heartworm prevention must be started during treatment and continued for the life of the pet to prevent reinfection.
6Can my cat get heartworm disease?
Cats can contract heartworm disease from mosquito bites, but they are atypical hosts — worms rarely reach adulthood, and there are far fewer of them. Unfortunately, there is no safe adulticidal treatment for cats, so prevention is the only reliable protection; an infected cat is managed supportively.
7How do I know if my dog has heartworms?
Many dogs show no symptoms in early infection. Diagnosis requires a blood antigen test, which detects proteins released by adult female heartworms; a positive result is confirmed and staged with bloodwork and chest radiographs to plan the safest treatment approach.
8What is doxycycline used for in heartworm treatment?
Doxycycline targets Wolbachia, a bacteria that lives inside heartworms and on which the worms partly depend. Eliminating Wolbachia weakens the worms before adulticidal treatment, reduces inflammation, and helps sterilize female worms to limit microfilarial spread — it is a standard part of the pre-treatment phase.

Related

ConditionsHeartworm Disease

Your dog tested positive — let's talk through next steps

A heartworm diagnosis is serious but treatable. Our team will stage the disease, explain the full protocol, and guide you through every step of the process.

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