With reference to the gold standard diagnosis of FECT, performance of the urine OV-ES assay on field samples yielded a high diagnostic sensitivity and diagnostic specificity of 81% and 70%, respectively. The prevalence of OV by antigen detection (64%) was higher than that determined by FECT (53%). The main difference was due to the finding of a considerable num- ber of antigen positive cases (28 out of 63 cases, 44%) that were deemed the egg-negative using FECT. Autopsy data have shown that a considerable number of egg-negative cases (70%) can occur in individuals with worm loads