The effects of drugs on Onchocerca volvulus. 4. Trials of melarsonyl potassium.

The effects of the arsenical drug melarsonyl potassium on Onchocerca volvulus were investigated in patients in Cameroon infected with the Cameroon forest and Sudan savanna strains of the parasite. Two intramuscular dosage schedules were tested: the first comprised 4 consecutive daily doses of 200 mg repeated once after a 10-14 day interval, i.e., 2 (4x200 mg). The second was a single dose schedule at 7.1 mg/kg-10 mg/kg, with a maximum of 500 mg.In most trials the drug had no immediate action on microfilarial concentrations. Only after the 2(4x200 mg) melarsonyl course against the Sudan savanna strain was a slight microfilaricidal action detected.The 2(4x200 mg) course of melarsonyl apparently killed or sterilized most or all of the adult female worms in the patients tested, leaving the residual population of microfilariae to decline gradually, from natural mortality, over the ensuing 2 years. These residual microfilariae could be killed with diethylcarbamazine.Single doses of melarsonyl at 7.1 mg/kg-10 mg/kg were somewhat less effective in killing or sterilizing adult worms, but it is suggested that if doses at the higher end of this range were to be repeated annually patients could be rendered free from microfilariae by the end of 3 years.It is emphasized that the risks of arsenical encephalopathy should at present preclude the use of melarsonyl potassium in the treatment of onchocerciasis, but that if this danger could be avoided the drug might prove to be of considerable use for mass therapy in control campaigns.