Onchocerciasis diagnosis and the probability of visual loss in patients with skin snips negative for Onchocerca volvulus microfilariae.

Two methods have been used to verify the diagnosis of patients living in the onchocerciasis hyperendemic region of Liberia found to have O.v. negative skin snips. The first was based on a histological and an electronmicroscopic examination of the skin to determine whether the characteristic onchocerciasis dermatitis was absent, and to compare the results with age matched subjects having minimal to densely O.v. mf infested skin. The second method involved the skin response of patients with O.v. mf negative skin snips to topically applied DEC. No O.v. mf were found in skin snips serially sectioned and examined histologically from patients in which the results of incubation studies were completely negative. Skin snips from these seemingly O.v. mf negative patients had a dermatitis, whereas no dermatitis was found in skin snips taken from subjects living in nearby nonendemic areas. Patients with O.v. mf positive skin snips responded to DEC applied topically with increased pruritus and a papular rash whose severity was proportionate to the intensity of infestation. Patients with O.v. mf negative skin snips responded to DEC applied topically with increased pruritus and developed either a localized papular rash or widely dispersed papules; confirmation of onchocerciasis in these patients was found in the identification of O.v. mf in histological sections of the papules.