Long-term evaluation of viability of microfilariae and intravenously transplanted adult Dirofilaria immitis in microfilaremic dogs treated with low-dose, short- and long-treatment regimens of doxycycline and ivermectin

Abstract

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 intravenous transplantation were randomly allocated to three groups of four 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 (minimum, 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 Groups 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 one 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 These data lend support to the use of the American Heartworm Society Canine Guidelines for adulticide therapy recommending the initiation of doxycycline plus a macrocyclic lactone (ML) at the time of the heartworm-positive diagnosis. Graphical Abstract

Inability of Dirofilaria immitis infective larvae from mosquitoes fed on blood from microfilaremic dogs during low-dose and short-treatment regimens of doxycycline and ivermectin to complete normal development in heartworm naïve dogs

“…For heartworm adulticidal therapy, the American Heartworm Society currently recommends the initiation of daily, oral doses of doxycycline administered at 10 mg/ kg bid for 28 days along with a ML heartworm preventive 2 months before the first injection of the three-injection protocol for melarsomine are given [17,18]. While the doxycycline plus ML treatment will eventually clear microfilariae from the blood, some microfilariae may persist for up to 3 weeks following administration of high-dose moxidectin plus doxycycline [19] and up to 5.6 months following initiation of twice daily doses (bid) of 10 mg/kg of doxycycline for 30 days and 6 monthly prophylactic doses of ivermectin [20] or up to 8 months following single daily doses (sid) of 10 mg/kg of doxycycline for 72-98 days and 6-7 bi-weekly prophylactic doses of ivermectin [4] (in this issue). Considering that circulating mf are present for weeks to months after doxycycline plus ML treatment is started, the question arises as to whether these lingering mf are capable of infecting susceptible mosquito vectors and, if so, whether the L 3 from these mosquitoes will develop normally and migrate to the heart and pulmonary arteries of dogs after transmission.…”

“…These treated (8) and untreated control (4) dogs served as microfilaremic blood donors for this study. For further details on these dogs, see data published elsewhere in this issue of the journal [4].…”

“…Wolbachia endosymbionts (Wolbachia) bacteria are present in every life cycle stage of Dirofilaria immitis and every other filarial species that harbors this bacteria, and they are needed for normal growth and development of these filarial parasites [1][2][3]. Tetracycline drugs have been shown to have adverse effects, ranging from sublethal to lethal, on various filarial life cycle stages (see [4] in this issue for further details), and there is general agreement that these antifilarial effects can be attributed to death of the Wolbachia because these antibiotics have no effect on Wolbachia-negative filarial species, such as Acanthocheilonema viteae [5,6], and the antibacterial effects precede the antifilarial effects [3,7,8].…”