Fig. 1.

Equilibria of Macdonald’s (1965) homogeneous model (1). The DFE is stable if r₀ < 1 (gray-shaded parameter combinations), while the EE is feasible and stable if r₀ > 1. The black solid line indicates the transcritical bifurcation through which the two equilibria collide and exchange stability (endemicity boundary, $r_0=1$). The gray dashed curves are contour lines for the prevalence of infected snails, while the gray dash-dotted curves refer to the average worm burden in human hosts. Parameter values: $\gamma =5.5·{10}^{-4},$$\mu =1.7·{10}^{-2}$. All rates are expressed in [day${}^{-1}$].