The cause of NS is unknown. Various factors and exposures have been postulated to cause NS, including exposure to munitions, prior history of measles infection, consumption of baboon meat and sorghum, and others. Two recent case-control investigations, one conducted among children with NS in Uganda during 2009 - 2010 and another conducted in S. Sudan in 2011 (below) have not supported these associations. One risk factor that appears to consistently demonstrate an association with NS is evidence of infection, past or present, with the microfilarial parasite Onchocerca volvulus, or a similar microfilarial parasite that is serologically cross-reactive with Onchocerca. O. volvulus is transmitted by black flies. Clinical manifestation of onchocerciasis is mainly ocular, with resultant vision loss due to inflammatory keratitis ("River Blindness"). A significant association of NS with detection of O. volvulus microfilariae in skin snips was observed in a case-control investigation in Sudan in 2001 - 2002, and in an assessment in Tanzania (84% of 62 children with NS had microfilariae or O. volvulus nucleic acid in skin snips). Various studies and reviews have identified an association between onchocerciasis and epilepsy in general (5, 10, 11, 12, 13, 14, 15, 16, 17, 44, 46). A high prevalence of seizure disorders in onchocerciasis endemic areas was noted in Western Equatoria in 1946.