Modeling schistosomiasis transmission: the importance of snail population structure

Abstract

Background Schistosomiasis is a neglected tropical disease endemic in 54 countries. A major Schistosoma species, Schistosoma mansoni, is sustained via a life cycle that includes both human and snail hosts. Mathematical models of S. mansoni transmission, used to elucidate the complexities of the transmission cycle and estimate the impact of intervention efforts, often focus primarily on the human host. However, S. mansoni incurs physiological costs in snails that vary with the age of the snail when first infected. Snail demography and the age of snail infection could thus affect the force of infection experienced by humans, which is frequently used to predict the impact of various control strategies. Methods To address how these snail host and parasite interactions influence model predictions, we developed deterministic models of schistosomiasis transmission that include varying complexity in the snail population age structure. Specifically, we examined how model outputs, such as schistosome prevalence in human and snail populations, respond to the inclusion of snail age structure. Results Our models suggest that snail population age structure modifies the force of infection experienced by humans and the relationship between snail infection prevalence and corresponding human infection prevalence. There are significant differences in estimated snail infection, cercarial density and mean worm burden between models without snail population dynamics and those with snail populations, and between models with a homogeneous snail population and those with age stratification. The variation between finely age-stratified snail populations and those grouped into only juvenile and adult life stages is, however, minimal. Conclusions These results indicate that including snails and snail age structure in a schistosomiasis transmission model alters the relationship between snail and human infection prevalence. This highlights the importance of accounting for a heterogeneous intermediate host population in models of schistosomiasis transmission where the impact of proposed control measures is being considered.

Variability of biological traits of Bulinus truncatus and Biomphalaria pfeifferi, the intermediate host snails of schistosomiasis, from three climatic zones of Côte d’Ivoire

“…Heterogeneity in snail life history and demographic traits may affect parasite transmission in multiple ways. For instance, snail susceptibility to infection decreases with age (Anderson et al, 2021) and, while cercarial shedding rate increases with snail body size, the lifetime production of cercariae increases with snail's survival rate (Niemann and Lewis, 1990). Therefore, variations in snail life history traits may have implications in the transmission of both human and livestock Schistosoma parasite (Tchuenté and N'Goran, 2009;Webster et al, 2007;Moné et al, 2012;Boon et al, 2018).…”

First evidence of a causal link between genetic variation and thermal adaptation in a schistosome host snail

“…Similarly, the higher available energy budget (in terms of sugar and fat content) of the cold-origin snails might result in more and bigger parasites because more energy is available for schistosome development (Mas-Coma et al, 2009). Additionally, the high reproductive capacities of the warm-origin snail populations found in our study generate highly age-stratified populations that can result in an elevated parasite output (Anderson et al, 2021). The pronounced age-structuring of Senegalese B. truncatus snails and the associated high parasite output provides a partial explanation for the sustained prevalence of urogenital schistosomiasis in the Senegal River Basin, despite sustained control efforts (Kokaliaris et al, 2021).…”

A Place-Based Conceptual Model (PBCM) of Neotricula aperta/Schistosoma mekongi habitat before and after dam construction in the Lower Mekong River

“…The schistosomiasis exposure sBN could be used by local disease control teams to identify areas of exposure and improve the efficiency of mass drug administration [71]. A study to address how snail hosts and their interaction with S. mansoni influence model predictions indicates that model outputs, such as schistosome prevalence in human and snail populations, respond to the inclusion of snail age structure [72]. Another study linked quantitatively hydrological drivers to distinct population dynamics through specific density feedbacks and shows that model averaging based on statistical methods yields reliable projections of snail abundance [73].…”