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1. Schistosome infection in Senegal is associated with different spatial extents of risk and ecological drivers for Schistosoma haematobium and S. mansoni

   https://doi.org/10.1371/journal.pntd.0009712  

   Jones, Isabel J. ; Sokolow, Susanne H. ; Chamberlin, Andrew J. ; Lund, Andrea J. ; Jouanard, Nicolas ; Bandagny, Lydie ; Ndione, Raphaël ; Senghor, Simon ; Schacht, Anne-Marie ; Riveau, Gilles ; et al ( September 2021 , PLOS Neglected Tropical Diseases)

   Secor, W. Evan (Ed.)

   Schistosome parasites infect more than 200 million people annually, mostly in sub-Saharan Africa, where people may be co-infected with more than one species of the parasite. Infection risk for any single species is determined, in part, by the distribution of its obligate intermediate host snail. As the World Health Organization reprioritizes snail control to reduce the global burden of schistosomiasis, there is renewed importance in knowing when and where to target those efforts, which could vary by schistosome species. This study estimates factors associated with schistosomiasis risk in 16 villages located in the Senegal River Basin, a region hyperendemic for Schistosoma haematobium and S . mansoni . We first analyzed the spatial distributions of the two schistosomes’ intermediate host snails ( Bulinus spp. and Biomphalaria pfeifferi , respectively) at village water access sites. Then, we separately evaluated the relationships between human S . haematobium and S . mansoni infections and (i) the area of remotely-sensed snail habitat across spatial extents ranging from 1 to 120 m from shorelines, and (ii) water access site size and shape characteristics. We compared the influence of snail habitat across spatial extents because, while snail sampling is traditionally done near shorelines, we hypothesized that snails further from shore also contribute to infection risk. We found that, controlling for demographic variables, human risk for S . haematobium infection was positively correlated with snail habitat when snail habitat was measured over a much greater radius from shore (45 m to 120 m) than usual. S . haematobium risk was also associated with large, open water access sites. However, S . mansoni infection risk was associated with small, sheltered water access sites, and was not positively correlated with snail habitat at any spatial sampling radius. Our findings highlight the need to consider different ecological and environmental factors driving the transmission of each schistosome species in co-endemic landscapes. 

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2. Reducing disease and producing food: Effects of 13 agrochemicals on snail biomass and human schistosomes

   https://doi.org/10.1111/1365-2664.14087  

   Haggerty, Christopher J. E. ; Halstead, Neal T. ; Civitello, David J. ; Rohr, Jason R. ( December 2021 , Journal of Applied Ecology)

   Agricultural expansion is predicted to increase agrochemical use two to fivefold by 2050 to meet food demand. Experimental evidence suggests that agrochemical pollution could increase snails that transmit schistosomiasis, a disease impacting 250 million people, yet most agrochemicals remain unexamined.

   Here we experimentally created >100 natural wetland communities to quantify the relative effects of fertilizer, six insecticides (chlorpyrifos, terbufos, malathion, λ‐cyhalothrin, permethrin and esfenvalerate), and six herbicides (acetochlor, alachlor, metolachlor, atrazine, propazine and simazine) on two snail genera responsible for 90% of global schistosomiasis cases.

   We identified four of six insecticides (terbufos, permethrin, chlorpyrifos and esfenvalerate) as high risk for increasing snail biomass by reducing snail predators. Hence, malathion and λ‐cyhalothrin might be useful for improving food production without increasing schistosomiasis. This top‐down effect of insecticides on predators was so strong that the effects of herbicides on schistosomiasis risk were masked in the presence of predators because there were so few snails. In the absence of snail predators, herbicide effects on snails were generally negative by reducing submerged vegetationHydrilla verticillata. The exception was that atrazine and acetochlor significantly increased the biomass of infected snails and total snails respectively.

   Like insecticides, fertilizer had strong positive effects on snail populations. Fertilizer increased both snail habitat (submerged vegetation) and snail food (periphyton), but of these two pathways, the increases in snail habitat resulted in greater snail population growth. Total snail biomass was positively associated with both infected snail biomass and parasite production and thus human infection risk.

   Synthesis and applications. Our findings suggest that fertilizers and insecticides generally have consistently higher chances of increasing human schistosomiasis than herbicides in natural communities. Furthermore, our results highlight the need to identify other low risk insecticides, which might help reduce crop pests without increasing snails and thus risk of schistosomiasis.

    

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3. Deep Learning Segmentation of Satellite Imagery Identifies Aquatic Vegetation Associated with Snail Intermediate Hosts of Schistosomiasis in Senegal, Africa

   https://doi.org/10.3390/rs14061345  

   Liu, Zac Yung-Chun ; Chamberlin, Andrew J. ; Tallam, Krti ; Jones, Isabel J. ; Lamore, Lance L. ; Bauer, John ; Bresciani, Mariano ; Wolfe, Caitlin M. ; Casagrandi, Renato ; Mari, Lorenzo ; et al ( March 2022 , Remote Sensing)

   Schistosomiasis is a debilitating parasitic disease of poverty that affects more than 200 million people worldwide, mostly in sub-Saharan Africa, and is clearly associated with the construction of dams and water resource management infrastructure in tropical and subtropical areas. Changes to hydrology and salinity linked to water infrastructure development may create conditions favorable to the aquatic vegetation that is suitable habitat for the intermediate snail hosts of schistosome parasites. With thousands of small and large water reservoirs, irrigation canals, and dams developed or under construction in Africa, it is crucial to accurately assess the spatial distribution of high-risk environments that are habitat for freshwater snail intermediate hosts of schistosomiasis in rapidly changing ecosystems. Yet, standard techniques for monitoring snails are labor-intensive, time-consuming, and provide information limited to the small areas that can be manually sampled. Consequently, in low-income countries where schistosomiasis control is most needed, there are formidable challenges to identifying potential transmission hotspots for targeted medical and environmental interventions. In this study, we developed a new framework to map the spatial distribution of suitable snail habitat across large spatial scales in the Senegal River Basin by integrating satellite data, high-definition, low-cost drone imagery, and an artificial intelligence (AI)-powered computer vision technique called semantic segmentation. A deep learning model (U-Net) was built to automatically analyze high-resolution satellite imagery to produce segmentation maps of aquatic vegetation, with a fast and robust generalized prediction that proved more accurate than a more commonly used random forest approach. Accurate and up-to-date knowledge of areas at highest risk for disease transmission can increase the effectiveness of control interventions by targeting habitat of disease-carrying snails. With the deployment of this new framework, local governments or health actors might better target environmental interventions to where and when they are most needed in an integrated effort to reach the goal of schistosomiasis elimination. 

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4. Modeling how and why aquatic vegetation removal can free rural households from poverty-disease traps

   https://doi.org/10.1073/pnas.2411838121  

   Doruska, Molly_J ; Barrett, Christopher_B ; Rohr, Jason_R ( December 2024 , Proceedings of the National Academy of Sciences)

   Infectious disease can reduce labor productivity and incomes, trapping subpopulations in a vicious cycle of ill health and poverty. Efforts to boost African farmers’ agricultural production through fertilizer use can inadvertently promote the growth of aquatic vegetation that hosts disease vectors. Recent trials established that removing aquatic vegetation habitat for snail intermediate hosts reduces schistosomiasis infection rates in children, while converting the harvested vegetation into compost boosts agricultural productivity and incomes. We develop a bioeconomic model that interacts an analytical microeconomic model of agricultural households’ behavior, health status, and incomes over time with a dynamic model of schistosomiasis disease ecology. We calibrate the model with field data from northern Senegal. We show analytically and via simulation that local conversion of invasive aquatic vegetation to compost changes the feedback among interlinked disease, aquatic, and agricultural systems, reducing schistosomiasis infection and increasing incomes relative to the current status quo, in which villagers rarely remove aquatic vegetation. Aquatic vegetation removal disrupts the poverty-disease trap by reducing habitat for snails that vector the infectious helminth and by promoting the production of compost that returns to agricultural soils nutrients that currently leach into surface water from on-farm fertilizer applications. The result is healthier people, more productive labor, cleaner water, more productive agriculture, and higher incomes. Our model illustrates how this ecological intervention changes the feedback between the human and natural systems, potentially freeing rural households from poverty-disease traps.

    

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5. Re-assessing thermal response of schistosomiasis transmission risk: Evidence for a higher thermal optimum than previously predicted

   Aslan, IH ; Pourtois, JD ; Chamberlin, AJ ; Mitchell, KR ; Mari, L ; Lwiza, KM ; Wood, CL ; Mordecai, EA ; Yu, A ; Tuan, R ; et al ( June 2024 , PLOS NEGLECTED TROPICAL DISEASES)

   None (Ed.)

   The geographical range of schistosomiasis is affected by the ecology of schistosome parasites and their obligate host snails, including their response to temperature. Previous models predicted schistosomiasis’ thermal optimum at 21.7°C, which is not compatible with the temperature in sub-Saharan Africa (SSA) regions where schistosomiasis is hyperendemic. We performed an extensive literature search for empirical data on the effect of temperature on physiological and epidemiological parameters regulating the free-living stages of S. mansoni and S. haematobium and their obligate host snails, i.e., Biomphalaria spp. and Bulinus spp., respectively. We derived nonlinear thermal responses fitted on these data to parameterize a mechanistic, process-based model of schistosomiasis. We then re-cast the basic reproduction number and the prevalence of schistosome infection as functions of temperature. We found that the thermal optima for transmission of S. mansoni and S. haematobium range between 23.1–27.3°C and 23.6–27.9°C (95% CI) respectively. We also found that the thermal optimum shifts toward higher temperatures as the human water contact rate increases with temperature. Our findings align with an extensive dataset of schistosomiasis prevalence in SSA. The refined nonlinear thermal-response model developed here suggests a more suitable current climate and a greater risk of increased transmission with future warming for more than half of the schistosomiasis suitable regions with mean annual temperature below the thermal optimum. 

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