A heartworm diagnosis usually means three injections over more than a month, weeks of strict rest, and a bill many shelters cannot carry for every dog they pull. A randomized trial of 168 shelter dogs suggests two injections do nearly the same job.
Researchers compared standard three-dose melarsomine with a compressed two-dose version in dogs with Class 1 and Class 2 disease. Six months after the last injection, 95.3 percent of two-dose dogs tested antigen negative, against 98.8 percent of three-dose dogs. All 168 survived and none needed hospitalization. The trial ran from April 2024 to December 2025 at Lucky Dog Animal Rescue in Florence, South Carolina, with the University of Florida Shelter Medicine Program, Toronto Humane Society and EveryPet, funded by PetSmart Charities.
Both groups got sustained-release moxidectin, 28 days of doxycycline and prednisone. The two-dose group had melarsomine on days 14 and 15. The three-dose group had it on days 14, 44 and 45. Both schedules start earlier than the American Heartworm Society’s day 60, so the trial compares two compressed timelines rather than the current standard. Stomach upset and low energy were more common on three doses, 48.2 percent against 31.8 percent.
What this means for your dog: the short schedule cut melarsomine costs by about a third, days of exercise restriction by 61 percent, and estimated cost of care by around 60 percent. It was tested only in early and moderate disease, so safety in a dog with a heavy worm burden is unknown, and the “no worse than” statistical test came out inconclusive. If you adopt a heartworm-positive dog, ask which protocol your vet is using and why, and do not skip the rest. Parasites are on the move elsewhere too, including screwworm in Texas wildlife. Veterinary Practice News has the trial summary.