29/06/2025
Availability, an inevitable challenge
When Immiticide Isn’t an Option: What Vets Can Do to Kill Heartworms Without Melarsomine
By Dr. Geoff Carullo, DVM, Dip., PCCP, Dip., PCVS
For decades, melarsomine dihydrochloride (Immiticide or Diroban) has been the gold standard for adult heartworm treatment in dogs.
But what happens when that gold standard disappears?
Across the globe, melarsomine has either been phased out, become increasingly difficult to procure, or is entirely unavailable. Shortages and withdrawals have left many veterinary practitioners—especially in developing regions—searching for effective, safe, and practical alternatives.
Here’s what every vet needs to know.
🧬 Why Melarsomine Was Phased Out
Before we talk about alternatives, let’s understand why melarsomine has been pulled off shelves in many countries:
Production Challenges:
Manufacturing melarsomine is complex. It’s arsenic-based, expensive, and requires strict handling protocols. When Merial’s original supplier stopped production, global availability plummeted.
Low Profitability:
Despite being effective, it’s not profitable enough for pharmaceutical companies to keep investing in production.
Clinical Risk:
Melarsomine treatment carries risks—painful injections, pulmonary embolism, and even treatment-related death. Not all clinics have the capacity to monitor these complications.
Regulatory Withdrawal:
In some countries like Japan and parts of Europe, it has simply been discontinued due to low demand and safety concerns.
So if you can't use melarsomine—what now?
✅ Option 1: The “Slow-Kill” Protocol (Moxi–Doxy Method)
The most researched and widely accepted alternative is a combination of monthly moxidectin and 30 days of doxycycline.
🔬 How It Works:
Doxycycline eliminates Wolbachia, an endosymbiotic bacterium inside heartworms. This weakens the worms, limits pathology, and reduces reproduction.
Moxidectin (via monthly oral or topical preventives like Advantage Multi or Simparica Trio) slowly kills the adult worms over 10–18 months.
📊 Efficacy:
Up to 93% achieve no-antigen-detected (NAD) status within 12 months.
Lower risk of thromboembolism compared to rapid adulticidal treatment.
Endorsed as a salvage or secondary option by the American Heartworm Society when melarsomine is inaccessible.
⚠️ Caution:
Requires strict exercise restriction for several months.
Longer treatment course (vs. melarsomine’s 2–3 months).
May not be suitable for severe or late-stage infections.
✅ Option 2: Extended Macrocyclic Lactone Alone
When doxycycline isn’t an option, some clinicians use macrocyclic lactone preventives alone (e.g., ivermectin, moxidectin).
Slower and less effective than Moxi–Doxy.
Still helps reduce microfilariae and prevents new infections.
Not a recommended sole therapy unless no other treatment is available.
✅ Option 3: Surgical Removal (For Very High Worm Burdens)
In extremely severe cases with caval syndrome or vena cava involvement, adult worms can be surgically removed from the heart and pulmonary arteries.
Requires advanced facilities and skilled surgeons.
Best used for emergency decompression in dying patients.
Usually followed by slow-kill or melarsomine (if available) for residual worms.
✅ Option 4: Custom Protocols with Local Resources
In some low-resource settings, veterinarians have adopted hybrid protocols:
Topical moxidectin + oral doxycycline as a compromise for clients who cannot afford full injectable protocols.
Incorporation of short-term corticosteroids to manage inflammation.
Use of antigen and microfilariae monitoring every 6 months to guide progress.
Each protocol should be tailored based on:
Dog’s age, stage, and comorbidities.
Owner compliance.
Access to diagnostics and medications.
🧠 Final Thoughts: What This Means for Global Veterinary Practice
The absence of melarsomine isn’t the end of heartworm treatment—it’s a call for innovation, adaptation, and informed care. With the right strategy, dogs can still live long, heartworm-free lives.
📌 Key Takeaways:
Melarsomine is no longer reliably available globally due to supply, cost, and risk issues.
The Moxi–Doxy protocol is the most validated alternative and should be the first-line option where melarsomine is unavailable.
Other treatments (macrocyclic lactones alone, surgery, or custom protocols) are viable based on case severity and context.
Education, strict exercise restriction, and long-term monitoring are crucial regardless of the chosen path.
As veterinarians, we adapt—not because we want to, but because our patients deserve nothing less.
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