Prevalence of infection, mean worm burden and degree of worn aggregation as determinants of prevalence of disease due to intestinal helminths

RESUMO

Theoretical predictions of disease prevalence due to helminth infections based upon a simple probabilistica model which considers the infection prevalence, the mean worm urden and the degree of worm aggregation are presented. A numerical reappraisal of the likely estimates of the degree of aggregation based upon maximun likelihood estimates of the negative binomial distribution is presented. The prevalence of disease shows a positive relationship with the mean worm burden. This association is hyperbolic when helminth parasites are severely aggregated but is s-shaped when helminth parasites tend to be oversidpersed. The prevalence of disease decreases with the degree of worm aggregation whem the values of the mean intensity are low; as the mean intensity increases this association becomes positive. The relationship between prevalence of disease and prevalence of infection is hyperbolic for severe degrees of parasite aggregation and is s-shaped for intermediate degrees of aggregation. However, if the mean intesnsity is low and the degree of aggreation is high there could be a negative raltionship between the prevalence of disease due to helminth parasites is feasible for determined ranges of values of the infection prevalence, mena intensity and the degree of worm clumping