USE OF MELARSOMINE DIHYDROCHLORIDE (RM-340) FOR ADULTICIDAL TREATMENTOF DOGS WITH NATURALLY ACQUIRED INFECTIONS OF DIROFILARIA-IMMITIS ANDFOR CLINICAL PROPHYLAXIS DURING REEXPOSURE FOR 1 YEAR

Heartworm-infected dogs were treated therapeutically with a new heartw orm adulticide (melarsomine dihydrochloride, RM 340) and then put on a Strategic Program with treatment every 4 months for clinical prophyla xis to take advantage of the drug's potent activity against 4-month-ol d immature as well as adult Dirofilaria immitis. Ten random-source dog s with naturally acquired heartworm infections (microfilariae- and ant igen-positive) were given melarsomine (2.2 mg kg(-1) twice 3 h apart) by i.m. injection in the lumbar muscles to clear their existing infect ions. They were then placed outdoors (August 1988) in a high-risk area in Georgia (USA) for heartworm transmission and given melarsomine at the same posology every 4 months (Strategic Program) for 12 months as a clinical prophylactic measure. Five nontreated heartworm-naive beagl es placed at this site during the same period served as 'controls' to monitor heartworm transmission, After exposure for 12 months, the ten treated and five 'control' dogs were taken indoors and held for 5 mont hs. Microfilaremia and antigenemia levels were monitored in both group s by testing at 4-5 month intervals throughout the study and the inten sity of infection was determined at necropsy. Microfilaremia levels in treated dogs dropped dramatically following the initial therapeutic t reatment and remained either negative or low. Only two of the five 'co ntrol' dogs became microfilaremic, and this occurred near the end of t he study. Nine of the ten treated dogs were antigen-negative 4 months after the initial therapeutic treatment, and all of them were antigen- negative at all bleedings thereafter. Four of the five 'control' dogs were antigen-positive at necropsy, and only one of these was positive 4 months earlier. Based on these antigen data, the initial treatment c leared 90% of the dogs of worms, and no worms were detected in any of the treated dogs thereafter. However, it is possible that undetectable immature heartworms were present. Although all of the treated dogs we re antigen-negative at necropsy, three of them had a total of eight he artworms, seven of which were clearly immature, as determined by worm length measurements, and the remaining worm was a young adult female t hat was probably too young to be detected. All of the five 'control' d ogs had heartworms (average 7.4; range 1-16), and about half of these worms were clearly immatures. Therapeutic treatment followed by strate gic treatment with melarsomine every 4 months during reexposure was at least 89.2% effective overall. The drug appeared to be safe and fully effective against heartworms of about 4 months of age and older but a ppeared to be less effective against larvae which were only a few week s old.