Dirofilaria Immitis Transmission by Treatment of Heartworm Microfilaremic Dogs with Doxycycline

Background This study was conducted to determine whether heartworm infective larvae (L ₃ ) collected from mosquitoes fed on blood from dogs during the daily administration of doxycycline at various dosages and treatment schedules and a single prophylactic dose of ivermectin could develop normally in dogs. Methods Twenty heartworm microfilaremic Beagles allocated to four groups of five dogs served as blood donors. These donor dogs in Groups 1, 2 and 3 received doxycycline orally at 10.0, 7.5 or 5.0 mg/kg BID x 28 days, respectively, and ivermectin orally (min., 6 mcg/kg) on Days 0 and 30. Group 4 served as the untreated control. For Study A, four groups of Aedes aegypti mosquitoes were allowed to feed on blood from these four groups of dogs, respectively, on Day 14, while another set of four groups of mosquitoes in Study B were allowed to feed on Day 21. Fourteen days after the mosquitoes were infected, L ₃ were collected and 50 L ₃ /dog were inoculated SC into 16 dogs: 8 dogs (4 groups of 2 dogs each, A-1 to A-4) for Study A on Day 14 and 8 dogs (4 groups of 2 dogs each, B-1 to B-4) for Study B on Day 21. All 16 dogs were processed for necropsy on the same day for recovery and enumeration of adult heartworms at 252 (Study A) and 245 (Study B) days PI. Results Eleven of the 12 dogs which had received L ₃ from mosquitoes fed on blood from treated dogs had no adult heartworms at necropsy, while 1 dog administered doxycycline orally at the lowest dosage and the shorter treatment period of 5.0 mg/kg daily for 14 days allowed 1stunted adult female heartworm to reach the heart and pulmonary arteries. All control dogs had male and female heartworms at necropsy. Conclusions Treatment of microfilaremic dogs with doxycycline administered daily at 10.0 or 7.5 mg/kg for 14 days and one monthly dose of a heartworm preventive (ML) renders the L ₃ incapable of normal development in dogs and widens the scope of a multimodal approach to heartworm prevention in reducing the spread of heartworm disease.