Efficacy of ivermectin against Dirofilaria immitis microfilariae in naturally infected dogs

Ivermectin, given in a single subcutaneous dose of 200 micrograms/kg, was evaluated for its efficacy against Dirofilaria immitis microfilariae in a trial with naturally infected dogs. Sodium thiacetarsamide (2.2 mg/kg) was administered twice daily for 2 days to 2 groups of 10 dogs. Two weeks later, 1 group was treated with ivermectin, and the other group was given the vehicle. Microfilariae were not detected (by blood sample concentration techniques) in 90% of the dogs 21 days after they were treated with ivermectin. Microfilariae were not detected in samples of any dog at days 28 and 35. However, 1 dog (not the one positive at day 21) had a detectable microfilaria at day 42. One dog with pretreatment ascites, exercise-induced cough, and high microfilaremial count was anorectic and depressed 2 days after treatment with ivermectin; the clinical signs were resolved 4 days later.