Modelling host cell availability and the crowding effect in Eimeria infections

Within-host mathematical models of Eimeria maxima and Eimeria praecox infec tions of the chicken are presented and used to investigate the role of host cell availability as a possible determinant of the so-called 'crowding eff ect', whereby the fecundity of the parasites decreases as infectious dose i ncreases. Assumptions about the number of available host cells, the average lifespan of these cells and the age structure within the host-cell populat ion were made and mathematical models were constructed and combined with ex perimental data to test whether these conditions could reproduce the crowdi ng effect in the two species. Experimental data demonstrated that crowding during in vivo infections was apparent following very low infectious doses, but none of the models could adequately reproduce crowding at the same dos es while maintaining realistic estimates of the dynamics of the enterocyte pool. However, both the size and lifespan of the enterocyte pool were demon strated to have substantial effects on the fecundity of the infections, par ticularly at higher doses. These data indicate that host cell availability cannot be solely responsible for the crowding effect. Alternative factors S uch as the influence of the primary immune response to the parasite may als o be explored using within-host models and other applications of these mode ls are discussed. (C) 2001 Australian Society for Parasitology Inc. Publish ed by Elsevier Science Ltd. All rights reserved.