K. Polman et al., Relating serum circulating anodic antigens to faecal egg counts in Schistosoma mansoni infections: a modelling approach, PARASITOL, 121, 2000, pp. 601-610
Circulating anodic antigen (CAA) levels in serum and faecal egg counts are
both quantitative measures of Schistosoma mansoni worm burdens. In this stu
dy, we have tested whether circulating anodic antigens can be included into
an established egg count model. A data set with 3 repeated faecal egg coun
t and serum CAA measurements of 50 individuals from a community in Burundi
with moderate endemicity was used. By means of Monte Carlo simulation, both
antigens and egg counts were related to an underlying worm pair distributi
on, taking into account the variation in repeated measurements (within indi
viduals) and the variation in worm burdens (between individuals). Models wi
th various assumptions (e.g. presence or absence of density-dependent egg p
roduction) were tested. Whereas observed and predicted egg counts agreed fa
irly well, the circulating antigen data could not be described satisfactori
ly. In particular, the predicted number of negative antigen concentrations
was much lower than observed, while the number of light positives was overe
stimated. There seems to be a mechanism that causes a shift of expected (lo
w) positive CAA concentrations towards zeros, which the proposed models do
not provide for. Possible biological as well as assay-related mechanisms th
at may account for this shift are discussed. The assumption that serum CAA
concentrations are a simple direct reflection of worm (pair) burdens could
not be corroborated by this modelling exercise. Apparently, the relationshi
p between (measured) CAA concentrations, egg counts and worm burdens in hum
an S. mansoni infections is more complex than assumed.