Diagnosis of schistosomiasis by reagent strip test for detection of circulating cathodic antigen

Abstract

A newly developed reagent strip assay for the diagnosis of schistosomiasis based on parasite antigen detection in urine of infected individuals was evaluated. The test uses the principle of lateral flow through a nitrocellulose strip of the sample mixed with a colloidal carbon conjugate of a monoclonal antibody specific for Schistosoma circulating cathodic antigen (CCA). The strip assay to diagnose a group of highly infected schoolchildren in Mwanza, Tanzania, demonstrated a high sensitivity and association with the intensity of infection as measured both by egg counts, and by circulating anodic antigen and CCA levels determined by enzyme-linked immunosorbent assay. A specificity of ca. 90% was shown in a group of schistosome-negative schoolchildren from Tarime, Tanzania, an area where schistosomiasis is not endemic. The test is easy to perform and requires no technical equipment or special training. The stability of the strips and the conjugate in the dry format lasts for at least 3 months at ambient temperature in sealed packages, making it suitable for transport and use in areas where schistosomiasis is endemic. This assay can easily be developed to an end-user format.

FIG. 1.

Schematic diagram of the test principle of a lateral flow test.

FIG. 2.

Examples of CCA strips run in wet format. (A) Set of standards to allow semiquantitative scoring of the samples: Neg. and 0 are scored as 0, 0.3 is scored as 1, 3 is scored as 2, 30 is scored as 3, and an even more intense line is scored as 4. (B) Five random samples that are scored against the standard series. The respective scores for samples 1 to 5 are 3, 0, 4, 3, and 2, respectively. It should be noted that the faint line at 0.3 ng/ml may not be well reproduced in print.

FIG. 3.

Association of CCA strip score with the intensity of schistosome infection as determined by egg counts in stool (A), CAA concentration in serum (B), and CCA concentration in urine (C). The strips were run in the wet format.