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.