Two-Dose Melarsomine Protocol Reached 95.3% Efficacy in Shelter Dog Trial

A compressed two-dose melarsomine protocol achieved 95.3 percent efficacy against heartworm disease in a randomized clinical trial of 168 dogs, compared with 98.8 percent for the standard three-dose regimen. The study, published in the Journal of the American Veterinary Medical Association (JAVMA), ran from April 2024 to December 2025 and enrolled dogs with Class 1 or Class 2 heartworm disease.

For practitioners and shelter veterinarians, the headline number is the trade-off: a roughly 3.5-point drop in efficacy buys a shorter timeline and fewer injections. The trial evaluated both protocols alongside doxycycline, moxidectin, and prednisone, so the melarsomine schedule is the only variable that changed.

How did the two protocols differ?

The dosing schedules diverged after the initial stabilization period.

• Two-dose group: melarsomine injections on consecutive days 14 and 15.
• Three-dose group: injections on days 14, 44, and 45.
• Both groups: exercise restriction for four weeks following treatment.

The compressed schedule removes the roughly one-month gap that defines the standard protocol, cutting kennel days, handling events, and per-dog costs.

What did safety data show?

All 168 dogs survived treatment. Complications were uncommon across both arms.

Seven dogs required thoracic radiographs, with or without additional treatment, but none required intensive management. That safety profile matters most in shelter settings, where post-treatment monitoring capacity is limited and every complication consumes staff time.

The authors concluded that the compressed two-dose protocol, when combined with doxycycline, moxidectin, and prednisone, was safe and effective.

What changes at the shelter level?

The trial's practical significance lies in resource economics. Shelters managing overcrowding and limited budgets could shorten the treatment timeline, reduce resource demands, treat more heartworm-positive dogs, improve adoption opportunities, and reserve resources for other animals in need.

In plain terms: a regimen that frees kennel space weeks earlier lets a facility move more dogs through treatment on the same budget.

The research brought together Lucky Dog Animal Rescue, the University of Florida Shelter Medicine Program, Toronto Humane Society, and PetSmart Charities — a collaboration spanning rescue, academic shelter medicine, and philanthropic funding.

How should clinicians weigh the efficacy gap?

The 95.3 percent figure is not equivalent to the 98.8 percent achieved by the three-dose standard, and the study does not claim it is. What the data show is that a meaningful efficacy floor holds when the protocol is compressed, with an acceptable safety margin, in Class 1 and Class 2 disease specifically.

The study population bounds the extrapolation. These results apply to dogs with mild to moderate heartworm disease; the trial did not test the compressed schedule in Class 3 patients, and clinicians should not assume transferability to advanced cases.

The study changes the options on the table for shelter medicine, and future work will determine whether the compressed protocol earns a place in owned-patient practice as well.