Background:The most important risk encountered in distributing Mectizan® for the control of onchocerciasis in areas where Loa loa is co-endemic is the development of an encephalopathic syndrome in people with very high levels of L. loa [> 30,000 microfilariae/milliliter blood (mf/ml)] following treatment with Mectizan®. This syndrome, which occurs rarely, is characterized by symptoms such as confusion, lethargy, coma, and urinary incontinence.The reason this syndrome develops is that Mectizan®, in addition to being an effective drug against the microfilariae of Onchocerca volvulus (the causative agent of onchocerciasis), is also effective against L. loa microfilariae, the rapid killing of which has been associated with this encephalopathy. Like all serious illnesses, this encephalopathy requires prompt medical and nursing care to provide supportive treatment and to prevent nosocomial infections. With competent and timely medical care, patients usually recover fully. The pathogenesis of this encephalopathy remains unknown.L. loa is known, or suspected, to be endemic in humid forest areas of Central and East Africa. The precise distribution of L. loa is still being defined and mapped. Methodologies for mapping include parasitologic surveys, rapid assessment of L. loa based on the restricted definition of eye worm passage (RAPLOA), and predicted prevalence based on environmental factors conducive to the breeding of the vector of L. loa, Chrysops spp.Recent epidemiologic studies have shown that in areas where the prevalence of L. loa microfilaremia in adults exceeds 20%, the percentage of adults with L. loa microfilaremia greater than 30,000 mf/ml is about 1%.1 The threshold for increased community risk of L. loa encephalopathy following Mectizan® treatment has been defined as 20% microfilaremia prevalence which corresponds to a 40% prevalence of history of eye worm passage as measured by RAPLOA.

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