Togetherness among Schistosomes: its effects on the dynamics of the infection

Abstract

This paper deals with the overall transmission dynamics of schistosomiasis, and other similar helminthic infections, in which the parasitic worms have a sexual stage in the primary (human) host. Most previous studies have assumed the worms to be independently randomly distributed among hosts; we explore the effects of various kinds of aggregation in the worm distribution, and their implications for medically interesting quantities such as prevalence of worms, prevalence of pairs, and mean egg output per host. The effects of worm aggregation upon the transmission dynamics are discussed, with particular regard to the “threshold” and “breakpoint” phenomena. It is shown that the analysis can be extended, simply but generally, to include the effects of a latent period before infected snails become infective. Most of the presentation is for a monogamous worm mating system, but the opposite extreme of promiscuous worms is also treated (in Appendix C). Although mainly concerned with these new results, the paper also attempts a limited amount of review and synthesis. This includes: relationships among earlier models; the threshold's dependence on the transmission parameters (man-to-snail versus snail-to-man), for arbitrary worm aggregation; the relation between Rosenfield's [30] empirical work and more abstract models.