The Schistosome Individual-Based Dynamic Energy Budget (SIDEB) model
We built an individual-based epidemiology model for the human schistosome, S. mansoni , infecting a size-structured B. glabrata host population (Malishev and Civitello 2019). The overall model is a complete within-host dynamic energy budget (DEB) model (Kooijman 2010) for B. glabrata host biomass and S. mansoni parasite biomass integrated with a resource production model and a between-host parasite transmission model of infection contact between hosts and schistosome miracidia, the free-living life stage excreted by humans into freshwater environments.
Both susceptible and infected hosts follow a DEB model for growth, reproduction, survival, and production of parasite cercariae (Civitello and others 2018). The model uses coupled differential equations to track changes in resource inputs (either logistically growing algae or a detritus subsidy), host growth, somatic maintenance, maturity/reproduction, reserve density, parasite biomass and reproduction, and complementary traits of the infection pathway, such as damage density to hosts. A detailed breakdown of state variables and parameters for the sub-models and resource growth equations are found in (Malishev and Civitello 2019).
The infection rate of susceptible snail hosts is based on a constant daily introduction rate of free-living miracidia and host and parasite DEB parameters are from experiments on manipulated resource supply rates and periodic starvation periods (Civitello et al. 2020). We simulated a 150-day season of host and parasite infection rates and densities resembling the typical schistosomiasis transmission season. Simulations were seeded with an initial host population of 50 individuals and either logistically growing algae or a detritus resource subsidy. Resource supply levels determine infected host density and parasite output rates as host competition for resources weakens feeding pressure and increases overall mortality and per-capita intake rate, allowing remaining hosts to become more infectious by capitalizing on available food. This means for logistically growing algae, host density reduces resource growth rates, whereas a resource subsidy such as detritus continues to increase irrespective of host density. The complete model, including equations for the within-host energetics, between-host transmission, and population level dynamics are provided in Appendix 1.