MICROFILARIAE RECURRENCE IN POLYNESIAN WUCHERERIA-BANCROFTI CARRIERS TREATED WITH REPEATED SINGLE DOSES OF 100 MU-G KG OF IVERMECTIN

Forty-six Polynesian carriers of Wuchereria bancrofti were treated wit h 3 successive single doses of ivermectin, 100 mug/kg, given every 6 m onths. Immediate microfilaricidal activity of ivermectin was excellent in all carriers, since residual mean microfilaraemia levels, 2 d afte r each of the 3 treatments, were less than 1% of pretreatment levels. Before initial treatment, geometric mean microfilaraemia was 500 micro filaria (mf)/ml for the whole group (range 21-6398 mf/ml); 6 months af ter each successive treatment it was 197, 108 and 87 mg/ml, respective ly, 39.4, 21.6 and 17.4% of the pre-initial treatment level. By consid ering the mean percentage recurrent level at 6 months after the 3rd tr eatment (36.8%) as a threshold, it was possible to classify the carrie rs into 2 groups: 17 in whom the percentage recurrent level was > 36.8 % and who were considered as 'fast repopulating' individuals, and the remaining 29 who were considered as 'slow repopulating' individuals. I n the latter group, 6 months after each of the 3 treatments, the recur rent microfilaraemia levels were 22.7%, 8.0% and 4.9% of the pre-initi al treatment level, respectively, while they were 95.1%, > 100% and > 100% in the former. The constant recurrence of mf suggests that iverme ctin, at a dosage of 100 mug/kg, had no effect on adult worms in 'fast repopulating' individuals, whereas the progressive lessening in recur rence of mf suggests some activity (sterilizing or killing) of ivermec tin on W. bancrofti macrofilariae in 'slow repopulating' individuals. The reason for repeated 'repopulating' in only certain treated individ uals remain unclear; further studies are planned to assess whether a h igher dose (400 mug/kg) of ivermectin could result in lower long-term recurrence of mf in 'fast repopulating' carriers.