A retrospective comparison of two-dose and three-dose melarsomine, with doxycycline and a macrocyclic lactone, in 1246 shelter dogs

BACKGROUND: Doxycycline and a macrocyclic lactone (ML/doxy) are now standard additions to melarsomine treatment protocols. The safety and efficacy of the two-dose melarsomine protocol have not been revisited since these adjuncts were added to treatment guidelines. METHODS: This was a retrospective cohort study comparing two-dose and three-dose melarsomine in dogs with class 1 and 2 heartworm disease, treated at Miami-Dade Animal Services from 2014 to 2024. All dogs received ML/doxy. Outcome measures were efficacy 5-10 months after the last melarsomine injection, survival to 8 weeks post-melarsomine, and post-treatment complications. Efficacy was assessed for different MLs administered at intake. RESULTS: Denominators for analyses varied on the basis of available data. There were 889 dogs in the two-dose melarsomine group and 357 in the three-dose group. Follow-up antigen tests were negative in 242/252 dogs (96.0%; confidence interval [CI] 92.9-97.8%) in the two-dose group and 24/24 (100%; CI 86.2-100%) in the three-dose group (P > 0.9999; not significant [NS]). All-cause mortality was 4/491 (0.8%) in the two-dose group and 4/154 (2.6%) in the three-dose group (P = 0.097; NS). Heartworm-related mortality was 1/491 (0.2%) in the two-dose group and remained at 4/154 (2.6%) in the three-dose group (P = 0.013). The deaths in the three-dose group were not considered to be related to the specific treatment protocol. Mild post-melarsomine complications occurred in 66/889 (7.4%) dogs in the two-dose group and 13/357 (3.6%) in the three-dose group (P = 0.014). Severe complications occurred in 3/889 (0.3%) dogs in the two-dose group and 5/357 (1.4%) in the three-dose group (P = 0.047). Two dogs that initially received ivermectin (2/65; 3.1%) and two that received milbemycin (2/35; 5/7%) had a positive follow-up antigen test, compared with none (0/33) for sustained-release moxidectin. CONCLUSIONS: The findings suggest that two-dose melarsomine, with ML/doxy, is a potential alternative to three-dose melarsomine for class 1 and 2 heartworm disease. Moxidectin may be the preferred ML in the two-dose protocol.