PREVALENCE OF INFECTION, MEAN WORM BURDEN AND DEGREE OF WORM AGGREGATION AS DETERMINANTS OF PREVALENCE OF DISEASE DUE TO INTESTINAL HELMINTHS

Theoretical predictions of disease prevalence due to helminth infectio ns based upon a simple probabilistic model which considers the infecti on prevalence, the mean worm burden and the degree of worm aggregation are presented. A numerical reappraisal of the likely estimates of the degree of aggregation based upon maximum likelihood estimates of the negative binomial distribution is presented. The prevalence of disease shows a positive relationship with the mean worm burden. This associa tion is hyperbolic when helminth parasites are severely aggregated but is s-shaped when helminth parasites tend to be overdispersed. The pre valence of disease decreases with the degree of worm aggregation when the values of the mean intensity are low; as the mean intensity increa ses this association becomes positive. The relationship between preval ence of disease and prevalence of infection is hyperbolic for severe d egrees of parasite aggregation and is s-shaped for intermediate degree s of aggregation. However, if the mean intensity is low and the degree of aggregation is high there could be a negative relationship between the prevalence of disease and the prevalence of infection. The presen ce of disease due to helminth parasites is feasible for determined ran ges of values of the infection prevalence, mean intensity and the degr ee of worm clumping.