Macroparasite Infections

For macroparasite infections, disease tends to be associated with past or present intensity of infection, that is, the numbers of parasites infecting the human host. Intensity of exposure and host genetics are components contributing to intensity of infection. However, as macroparasites may live for many years in the human host, the duration of exposure to infection is also a determinant of infection intensity and, in general, duration of exposure correlates with age and with behaviors associated with particular age and sex factors (e.g., being exposed while working in fields, fishing or watering cattle), so that older individuals may be expected to have higher burdens of disease from macroparasite infections and perhaps also higher mortality. An additional factor to consider is the degree to which immune tolerance develops towards the macroparasite infection, so that a high parasite worm burden may be acquired with little accompanying disease, although when such immu-notolerance breaks down the host's immune reaction may exacerbate pathology—disease is a result of the immune response to macroparasite infection rather than due to the infection itself. (In some circumstances there may be a reduction in level of disease with increasing age, although some changes may be irreversible such as the corneal opacities giving rise to river blindness which are generated by the immune response to certain parasite stages dying in the eye (Maizels and Lawrence, 1991).) Because intensity of infection is the main determinant of disease rather than infection itself (low-intensity infections causing little if any disease), the modeling of these infections concentrates on the distribution of intensity of infection in the exposed population rather than simply prevalence of infection, and age-structured models are necessary to capture observed age profiles of infection and disease. Often a vector or other secondary host may play an essential role in the transmission of such infections, such as simulium flies as vectors of onchocer-ciasis, or snails as intermediate hosts in schisto-somasis and these, in addition to direct treatment of the human host, may also be subjected to measures aimed at controlling infection spread; in these circumstances it becomes most important to consider how to incorporate these stages into the modeling framework. There is insufficient space here to do more than mention the importance of such considerations in relation to modeling, but there are good examples in the literature devoted to this topic. Useful and instructive examples of modeling macroparasite infections and the associated complexities may be found in Anderson and May (1991), Anderson and Medley (1985), Basanez and Ricardez-Esquinca (2001), Brown and Grenfell (2001), Chan et al. (1995), and Medley and Bundy (1996).

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