Fig. 2.

Analysis of a two-group community with access to a single water source (model (2) with $G=2$ and $S=1$). The two groups differ for their relative abundance (h₁ and h₂, with $h_1+h_2=1$) and intrinsic transmission risk (ϵ₁ and ϵ₂). A) Endemicity boundaries: parasite establishment is possible (${\mathcal{R}}_0^{21}>1$) for parameter combinations lying above the bifurcation curves, which correspond to ${\mathcal{R}}_0^{21}=1$ and are obtained for different values of the basic reproduction number r₀ of an equivalent homogeneous community (labels). B) State variables at the stable equilibrium: the dashed curves are contour lines for the prevalence of infected snails, while the dash-dotted curves refer to the average worm burden in human hosts (dark gray: high-risk group; light gray: low-risk group). The DFE is stable if ${\mathcal{R}}_0^{21}<1$ (gray-shaded parameter combinations), while the EE is feasible and stable if ${\mathcal{R}}_0^{21}>1$. The black solid line indicates the endemicity boundary ${\mathcal{R}}_0^{21}=1$. In panel B, the overall exposure (βN) and contamination (χH) rates are assumed to be equal, and their value is set so as to match that of a homogeneous community endowed with a basic reproduction number $r_0=0.9$ (thus, $\beta N=\chi H=\sqrt{r_0\gamma \mu }$). Other parameters as in Fig. 1.