Long-term Evaluation of Viabilitity of Microfilariae and Iv Transplanted Adult Dirofilaria Immitis in Microfilaremic Dogs Treated With Low-dose, Short- and Long-treatment Regimens of Doxycycline and Ivermectin

Background: Microfilarial (mf) counts were monitored over 21.3 months for any rebound that might occur in counts and adulticidal efficacy was assessed following administration of low dosage with short- and long-treatment regimens of doxycycline and ivermectin to heartworm microfilaremic dogs. Methods: : Twelve heartworm-naïve Beagles infected with 10 pairs of adult Dirofilaria immitis by IV transplantation were randomly allocated to 3 groups of 4 dogs. All treatments started on Day 0. On Day 0, Group 1 (short-treatment regimen) received doxycycline orally at 10 mg/kg once daily for 30 days plus ivermectin orally (min., 6 mcg/kg) on Days 0 and 30. Group 2 (long-treatment regimen) received doxycycline orally at 10 mg/kg once daily until individual dogs became mf negative (72-98 days) and ivermectin every other week until individual dogs became mf negative (6-7 doses). Group 3 was the untreated control. Mf counts and antigen (Ag) tests were conducted. Dogs were necropsied for recovery and enumeration of heartworms on Day 647. Results: : Day -1 mean mf counts were 15,613 , 23,950 and 15,513 mf/mL for Groups 1, 2 and 3, respectively. Mean counts for Group 1 and 2 declined until days 239 and 97, respectively, when all were negative. Group 3 had high mf counts throughout the study. There was not a rebound in mf counts in any of the treated dogs after they became amicrofilaremic. All dogs in Group 1 and Group 3 were Ag positive throughout the study and had at least 1 live female worm at necropsy. All dogs in treated Group 2 were positive for Ag through Day 154, but were antigen-negative on days 644 and 647, as all had only male worms. Mean live adult worm recoveries for Groups 1, 2 and 3 were 6.8 (range, 5-8), 3.3 (range, 1-6) and 16.0 (range, 14-17), respectively, with a percent reduction in adult worm counts of 57.5% for Group 1 and 79.3% for Group 2. Conclusions: : This data lends support to the use of the American Heartworm Society Canine Guidelines for adulticide therapy recommendation of initiating doxycycline plus an ML treatment at the time of the heartworm-positive diagnosis.