Efficacy of an injectable, sustained-release formulation of moxidectin in preventing experimental heartworm infection in mongrel dogs challenged 12 months after administration.

The objective of this study was to ascertain the ability of a single subcutaneous injection of a sustained-release (SR) formulation of moxidectin to protect dogs against challenge inoculation with infective Dirofilaria immitis larvae 364 days after administration. Twenty four purpose-bred adult mixed-breed dogs were grouped into three blocks of eight based on weight and sex. Saline solution (0.9% NaCl) or a moxidectin SR formulation at volumes designed to deliver 0.17 or 0.27 mg moxidectin/kg b.w. was injected subcutaneously on day 0. Throughout the post-treatment period, injection sites of all dogs were periodically examined visually and by palpation. Palpable swellings were characterized as to size, consistency and the presence of associated pain or erythema. On day 364, each dog was inoculated subcutaneously with 50 D. immitis L3. On days 510 and 511, dogs were euthanatized, and their hearts, lungs and thoracic cavities were inspected for the presence of adult heartworms. number, sex and viability of recovered heartworms were determined. The mean number of heartworms recovered from dogs that had received the saline control injection was 35.7. No heartworms were recovered from any dog treated with either 0.17 or 0.27 mg moxidectin/kg b.w. For variable periods of time following treatment, small (1-4 mm diameter), firm, subcutaneous swellings could be palpated at the injection sites of dogs treated with 0.17 or 0.27 mg moxidectin/kg b.w. These swellings contracted progressively and eventually disappeared except for the case of one animal treated with 0.27 mg/kg, in which the swelling persisted for the entire study period. At no time during the study was pain or erythema noted at the injection site of any dog, and no dog exhibited any adverse systemic reaction related to treatment. We conclude that under conditions pertaining in this study, a single subcutaneous injection of a moxidectin SR formulation at dosing rates of either 0.17 or 0.27 mg/kg b.w. can safely protect adult dogs against experimental challenge inoculation with infective heartworm larvae for a period of 12 months.