Predisposition of individuals and families in Mexico to heavy infection with Ascaris lumbricoides and Trichuris trichiura

An analysis is presented of the population dynamics of the major helminth parasites of man with the aim of understanding observed patterns in the age-specific prevalence and intensity of infection. Mathematical models are used to investigate the possibility of controlling helminth diseases by mass chemotherapy, and to explore the advantages of selective treatment of the most heavily infected individuals in a community.

The design of mass and targetted community-based chemotherapy programmes for the control of the major helminth infections of man is discussed in relation to the population and transmission dynamics of the parasites. Rapid reinfection following a single mass or targetted anthelmintic application is shown to be a universal feature of helminth transmission, as a consequence of the regulatory or feed-back mechanisms controlling population abundance within both individuals and the community as a whole. Control of reinfection requires repeated community treatment where the intensity of application and the interval between treatments are dependent on the reproductive life-expectancy of the adult worm, the net force of transmission prior to control (the basic reproductive rate) and the factors which create aggregation in the distribution of parasite loads within the population. Selective or targetted treatment is shown to be most effective for the control of morbidity as opposed to the control of transmission. The impact of targetted treatment depends critically on the factors that generate heterogeneity in parasite burdens and on whether or not selectivity is based on a single or repeated identification of the 'wormy' fraction of the community. Monte Carlo simulation studies are employed to assess the likely impacts of different control strategies on average parasite abundance/person and the distributions of parasite loads within populations. Future epidemiological research needs are discussed in relation to theoretical work and recent field studies of predisposition to heavy infection.

Frequency distributions of parasitic helminths within human communities are invariably highly aggregated, the majority of worms occurring in relatively small fractions of the host populations. It has been suggested that the heavily infected individuals are predisposed to this state, not by chance, but by as yet undefined genetic, ecological, behavioral, or social factors. Analyses of individual post-treatment patterns of hookworm reinfection among 112 villagers in an endemic area of West Bengal provide quantitative evidence of predisposition to heavy infection. This observation has implications for the design of control programs based on chemotherapy because of the potential economic advantage of selective or targeted treatment as opposed to mass or blanket treatment.