A simple computer-assisted method to identify schistosome cercariae

Several methods have been suggested to identify schistosome cercariae. In the present work, a new method is proposed, based on the analysis of the distribution of sensory endings (sensillae) on the body of the cercariae, revealed by an impregnation with silver nitrate. We determined the mutual distances between the sensillae and calculated the mean values, standard deviations, coefficients of asymmetry, and of kurtosis of the distribution of these mutual distances. Applied to two species, Schistosoma mansoni from Brazil and S. intercalatum from Cameroon, these mutual distances had the same mean value and the same standard deviation but quite different coefficients of symmetry (0.34 ± 0.11 versus 0.73 ± 0.08; P < 0.0001) and of kurtosis (-0.82 ± 0.27 versus -0.58 ± 0.31; P < 0.0001). The latter two indices were therefore very effective for discriminating the two species. The present method can be applied to other species and to hybrids in the field.