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 postmortem 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.