Novel, sensitive and low-cost diagnostic tests for 'river blindness' - Detection of specific antigens in tears, urine and dermal fluid

Sensitive, specific and low-cost diagnostic tests for onchocerciasis are indispensable for monitoring the efficacy of control programs, as well as for preventing blindness (when the tests are combined with efficacious chemotherapy Three new tests to detect Onchocerca-specific antigens in tears, dermal fluid and urine employ antibodies to O. volvulus-specific recombinant proteins, Oncho-C27 and OvD3B, encoded by genes within the immunodominant Onchocerca OV 33-3 gene family, and expressed in yeast and in E. coli, respectively. In these assays, Onchocerca-specific antigens in test samples are bound onto a solid surface and revealed using appropriate enzyme-labelled antibodies. Proteins in the samples are first transferred to Hybond-N + membrane disks or nitrocellulose paper using either a transblot or a dotblot machine, and then reacted with specific O. volvulus antibodies. Bound antibodies are revealed with species-specific peroxidase-labelled antibodies and peroxidase substrate. Positive tests give a brown colour. In one of the two assays developed to detect Onchocerca antigens in tears, the sensitivity was enhanced by first adsorbing the specific antibodies onto the membrane surface in order to immobilize and concentrate the Onchocerca-specific antigen molecules on the membrane. The specificity of the recombinant proteins for Onchocerca volvulus had been verified by ELISA, classical Western blot and modified DSIA. The tests are a dipstick immunobinding assay for ocular microfilariae (DSIA), a transblot immunobinding assay for the detection of skin microfilariae (TADA) and a dot-blot immunobinding assay for detecting urinary microfilariae and their antigens (DIA). Their specificity and sensitivity were evaluated in the field on 110 subjects with proven ocular microfilariae, 130 subjects with clinical and parasitological evidence of onchocerciasis, 25 subjects infected with other helminths and 120 normal controls. The minimal detection limits of Oncho-C27 protein by DSIA, TADA and DIA were 500 ng/ml, 154 ng/ml and 508 ng/ml, respectively. By contrast, their sensitivities were: 100% for DSIA and 82.5% for TADA employed on samples of tears; 97% for TADA skin test and 96% for DIA used on urine samples.