Background: Dirofilaria immitis is a life-threatening nematode spreading globally. Arsenical treatment is currently recommended for removal of adult worms. However, arsenical treatment is not available in some countries, and there are dogs that cannot tolerate the rapid kill of adult worms; therefore, alternative adulticide slow-kill treatments are needed. Criticisms against the use of these alternative protocols include the potential for allowing disease to progress and for the development of ML-resistant worms.
Methods: The efficacy of a protocol that includes semi-annual doses (i.e. every 6 months) of commercially available extended-release injectable moxidectin suspension (ProHeart® SR-12) with 30-day oral administration of doxycycline was studied in 20 dogs with naturally occurring D. immitis infections. Each dog received treatment with ProHeart® SR-12 (0.5 mg moxidectin/kg) by subcutaneous injection and oral doxycycline (10 mg/kg/bid × 30 days) every 6 months until two consecutive negative antigen test results were obtained. Pulmonary and cardiac evaluations were performed by radiographic and echocardiographic parameters. Physical examinations, complete blood counts, clinical chemistry profiles, microfilariae and antigen tests were performed periodically.
Results: At enrollment, all dogs were positive for D. immitis antigen and 18 were microfilaremic. On day 30, microfilaremia counts decreased, and all dogs became amicrofilaremic by day 150. On day 180, 11 dogs were antigen-negative, and 7 more became negative by day 360. The two remaining antigen-positive dogs converted to negative by day 540 or 810. All antigen tests performed 180 days after the first negative test were negative. There was no decline in cardiac performance of the dogs throughout the study. Overall, pulmonary clinical conditions, presence of worms by echocardiography, and enlargement of caudal and main pulmonary arteries improved after treatment. Physical examinations, complete blood count results, and clinical chemistry profiles were within normal reference values. Respiratory conditions were improved, no damage to the heart was observed, and the treatment protocol was well tolerated by the animals.
Conclusions: This alternative adulticide treatment was efficacious and well tolerated in naturally infected dogs. The injectable formulation provides the advantage of having veterinarians able to administer, monitor, and assess the efficacy and condition of the dog throughout the treatment and post-treatment periods.
Keywords: Adulticide treatment; Dirofilaria immitis; Doxycycline; Heartworm; Macrocyclic lactone; Moxidectin.