Research Article

A model for variations in single and repeated egg counts in schistosoma mansoni infections

S. J. De Vlasa1, B. Gryseelsa2, G. J. Van Oortmarssena1, A. M. Poldermana2 and J. D. F. Habbemaa1

a1 Department of Public Health and Social Medicine, Medical Faculty, Erasmus University Rotterdam, P.O. Box 1738, 3000 DR Rotterdam, The Netherlands

a2 Laboratory of Parasitology, Faculty of Medicine, University of Leiden, P.O. Box 9605, 2300 RC Leiden, The Netherlands

Faecal egg counts are often used to measure Schistosoma mansoni infection, but the considerable variation between successive counts complicates their interpretation. The stochastic model described in this paper gives a description of observed egg counts in a population and can be used as a tool to gain an insight into the underlying worm load distribution. The model distinguishes between two sources of variation in egg counts: (1) variation caused by the difference in worm load between individuals, and (2) the variability of egg counts for an individual with a given worm load. Empirical data, single and repeated measurements, from surveys in five villages in Burundi and Zaire have been used to fit and validate the model. We have discussed possible mechanisms that explain the differences in estimated values between the villages. The model indicates that the expected number of eggs in a stool sample per S. mansoni worm pair is lower than suggested by autopsy data and that, possibly as a consequence of immunity, the inter-individual variation in worm loads decreases with age.

(Received August 15 1991)

(Revised November 02 1991)

(Accepted November 04 1991)