Suspected melarsomine myositis highlights heartworm treatment risks

A new Today’s Veterinary Nurse case report is drawing attention to a severe suspected myositis event after melarsomine injections, underscoring a familiar tension in heartworm care: melarsomine is still the cornerstone of adulticide treatment, but it carries meaningful local and systemic risks that require careful administration and monitoring. The American Heartworm Society continues to recommend melarsomine because the standard three-dose protocol eliminates more than 98% of adult worms in a known time frame, making it the fastest, most effective way to stop disease progression in infected dogs. (heartwormsociety.org)

That context matters because melarsomine complications are not new, even if severe muscle injury is uncommon. FDA labeling for Diroban, a melarsomine product, says significant irritation at the intramuscular injection site was observed in field trials, and about 30% of treated dogs experienced some kind of injection-site reaction. The most commonly reported adverse events included injection-site reactions, coughing or gagging, lethargy, inappetence, fever, lung congestion, and vomiting. (dailymed.nlm.nih.gov)

What makes this case notable is the apparent severity of the suspected muscle injury. While the full Today’s Veterinary Nurse article was not directly retrievable in search results, the broader literature shows that melarsomine-associated tissue injury can, in rare cases, extend well beyond expected post-injection soreness. A 2020 case series from the University of Georgia described two dogs that developed acute neurologic deficits after melarsomine administration, with MRI and pathology findings consistent with extensive epaxial myositis and epidural steatitis after injections given according to manufacturer guidance. Earlier reports in JAVMA similarly described neurologic complications thought to reflect drug migration from the injection site along fascial planes. (thieme-connect.com)

The administration details are central to the risk discussion. The American Heartworm Society says melarsomine should be delivered into the epaxial muscle, which it describes as a “meaty muscle” with good blood supply. The group warns that injections into a smaller muscle can cause significant and protracted lameness, subcutaneous administration can lead to sterile abscesses, and intravenous administration is fatal. That is consistent with product labeling and pharmacology references that describe melarsomine as a drug with a narrow therapeutic window and a well-recognized rate of local injection-site reactions. (heartwormsociety.org)

There are also signs the profession is looking for ways to reduce technique-related complications. One recent study evaluating ultrasound-aided melarsomine injection reported that injection depth was measured before administration, suggesting some clinicians see value in adding precision, especially in dogs where body condition or muscle mass may make standard landmarking less reliable. In a separate 2024 outpatient heartworm treatment study, local injection-site reactions were again the most common adverse event, reported in 40% of treated dogs, although only a small minority of pet parents sought additional medical care. (journals.sagepub.com)

Expert and industry guidance still lands in the same place: melarsomine remains the recommended adulticide because untreated heartworm disease continues to damage the pulmonary vasculature and heart, and slower non-arsenical alternatives have important limitations. But the nursing workflow around treatment is just as important as the injection itself. Today’s Veterinary Nurse has previously noted that veterinary nurses are often responsible for preparing the epaxial lumbar site, monitoring for reactions after treatment, documenting changes, and helping pet parents understand what is expected versus what warrants urgent reassessment. (heartwormsociety.org)

Why it matters: For veterinary teams, this case is less about changing the standard of care than about reinforcing execution. Severe suspected myositis after melarsomine appears to be rare, but common injection-site pain and swelling are not, and uncommon complications can escalate quickly if they are mistaken for routine soreness. That puts a premium on precise injection technique, consistent pain scoring, clear discharge instructions, and fast recheck pathways when a dog shows persistent pain, swelling, reluctance to move, neurologic changes, or systemic decline after treatment. (dailymed.nlm.nih.gov)

What to watch: The next question is whether this case prompts more discussion around standardized post-melarsomine monitoring, stronger pet parent counseling on what “normal” recovery looks like, and broader use of tools or training that help clinicians confirm proper epaxial placement before injection. (journals.sagepub.com)