Evaluation of a single injection of a sustained-release formulation of moxidectin for prevention of experimental heartworm infection after 12 months in dogs

OBJECTIVE: To evaluate the efficacy of a single injection of a sustained-release formulation of moxidectin in preventing heartworm (Dirofilaria immitis) infection for 12 months in dogs. ANIMALS: 14 healthy dogs. PROCEDURE: Group A (nontreated control dogs; n = 6) received sterile vehicle administered SC, and group B (treated dogs; n = 6) received a sustained-release formulation of moxidectin administered SC. All dogs were housed in a heartworm-endemic area for 11.5 months, and heartworm antigen and modified Knott tests were performed monthly. All dogs (including 2 additional control dogs group C]) were then inoculated with infective-stage larvae (L3) of D. immitis, and 4.5 months later, all dogs were euthanatized and post-mortem examinations were performed. Adult D. immitis were counted and measured, and their age was estimated. RESULTS: All dogs in groups A and C were infected with young (4- to 4.5-month old) adult male and female D. immitis. No dogs in group B were infected with heartworms. CONCLUSIONS AND. CLINICAL RELEVANCE: The age of heartworms recovered suggests that infection was the result of experimental inoculation and not natural exposure to mosquitoes during the 11.5-month period the dogs resided in a heartworm-endemic area. A single SC injection of a sustained-release formulation of moxidectin was effective in providing protection against heartworm infection after 12 months in dogs. This formulation is a convenient method of heartworm prophylaxis that could eliminate the problem of poor owner compliance.