Flubendazole (FLBZ) is a potent and efficacious macrofilaricide after parenteral administration. Studies in animal models and one trial in patients infected with revealed that FLBZ elicits minimal effects on microfilariae (mf). Severe complications after ivermectin (IVM) treatment of patients with high microfilaraemia are of great concern. We examined the potential of FLBZ to rapidly kill mf, the phenomenon proposed to underlie the complications. Mf of were exposed to FLBZ, its reduced metabolite, albendazole, or IVM in vitro. Viability of mf was unaffected by FLBZ (10 μM, 72 hours); similar results were obtained with mf of We also measured the effects of FLBZ on transmission of mf. were fed FLBZ-exposed mf and dissected 24 hours or 14 days postfeeding to count mf that crossed the midgut and developed to infective L. FLBZ impaired the ability of mf to cross the midgut, regardless of duration of exposure (≥ 2 hours). FLBZ also prevented the development of mf to Ls, irrespective of duration of exposure or concentration. FLBZ is not microfilaricidal under these conditions; however, it blocks transmission. These results support the possibility that FLBZ may be a useful macrofilaricide in loiasis regions and may limit transmission from treated individuals.

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