Evaluation of density-dependent fecundity in human Schistosoma mansoni infections by relating egg counts to circulating antigens through Deming regression

Regression analysis of the relationship of serum circulating anodic and cat hodic antigens (CAA and CCA), as a possible direct measure of worm burden, and fecal egg counts allows the study of phenomena like density-dependent f ecundity in human Schistosoma mansoni infections. For a reliable analysis, variations in egg count measurements as well as in circulating antigen leve ls have to be taken into account, and an accurate estimation of these varia tions (represented by parameter lambda in the so-called Deming regression) is of great importance. From a new, extensive data set of repeated measurem ents of fecal egg counts and CAA and CCA concentrations we determined the r espective values for parameter lambda, and (re)analysed the relationship be tween circulating antigens and egg counts in 3 data sets from Burundi, Sene gal and Zaire by Deming regression. For comparison, ordinary linear regress ion was performed as well, which considerably biased the regression lines f or CCA, but not for CAA. The analyses resulted in a clearly non-proportiona l relationship between egg counts and CAA, and, to a lesser extent, CCA. As suming that egg counts and antigen measurements directly reflect egg produc tion and worm burdens, respectively, our findings reinforce the indication of density-dependent fecundity in schistosomiasis mansoni, as suggested by others.