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.