Ecological solutions to human disease problems

Parasitic diseases remain a major cause of death and disability in the developing world. Most of these “neglected tropical diseases” (NTDs) are worms with complex life cycles that connect them – and their human hosts – intimately with the environment. Modern medical tools like drugs and vaccines can fall short of achieving desired health outcomes for these diseases. Research in the Wood Lab focuses on using ecological techniques to develop solutions for these otherwise intractable public health problems.

We work on schistosomiasis, a water-borne NTD that affects more than 200 million people worldwide. Our research focuses on the spatial scale of schistosomiasis transmission and aims to inform practical, ecologically based approaches to disease control. This work is conducted in collaboration with an interdisciplinary team – The Upstream Alliance – and together we unravel the complex associations between the parasites, their human, livestock, and snail hosts, the prawns, fishes, and other species with which they interact, and their socio-economic context.

Schistosome worms use freshwater snails as intermediate hosts. Parasite infectious stages are shed from the snails into rivers, lakes, and other freshwater bodies, where they infect people by penetrating their skin. The snails serve as an effective environmental reservoir of disease, always standing at the ready to produce schistosomes, which are infectious even to people who have recently received treatment for schistosomiasis. The WHO recognizes that snail control will be key to schistosomiasis elimination, but little is known of the ecology of these snails, and one thing that makes them difficult to study is that their populations seem to be both patchy in space and ephemeral in time. My team has undertaken spatially and temporally resolved sampling of schistosome-competent snails and their habitat at the site of the world’s largest recorded schistosomiasis epidemic—the Lower Senegal River Basin in Senegal. We aimed to learn:

  1. Do humans contract their schistosome infections from snails that are present in their local environment, suggesting that transmission risk may be amenable to local control interventions?
  2. Do snails cluster in space, and if so, are those clusters sufficiently persistent in time that we can target them using snail control interventions?
  3. Are there easy-to-measure environmental correlates that might effectively predict the abundance of snails and of human infections?

Our sampling revealed positive relationships between intermediate host snails and human urogenital schistosomiasis reinfection. However, we also found that snail distributions were so patchy in space and time that obtaining useful data required effort that exceeds what is feasible in standard monitoring and control campaigns (Figure 1A). Instead, we identified several environmental proxies (Figure 1B) that were more effective than snail variables for predicting human infection (Figure 1C): the area covered by suitable snail habitat (i.e., floating, non-emergent vegetation), the percent cover by suitable snail habitat, and size of the water contact area. Unlike snail surveys, which require hundreds of person-hours per site to conduct, habitat coverage and site area can be quickly estimated with drone or satellite imagery. This, in turn, makes possible large-scale, high-resolution estimation of human urogenital schistosomiasis risk to support targeting of both mass drug administration and snail control efforts. You can read more about this project in this recent paper and the associated press materials.

wood_et_al_2019_figure_1

Figure 1. Conceptual diagram. (A) Snail density/abundance may not be correlated with human schistosomiasis cases if snails are ephemeral and patchy and therefore difficult to quantify without sampling that is intensive in space and time. (B) If the presence of suitable snail habitat is both stable in space and time and an effective predictor of the presence of snails, (C) it might provide a better indicator of human schistosomiasis cases than would direct counts of snails. Some environmental predictors of the presence of suitable snail habitat are easily observable by satellite or drones; for example, see (D) aerial images taken by drone for representative small-area (Merina Gewel 1) and large-area (Syer 1) sites.

 

We hope that this innovation will help public health agencies in developing countries to efficiently find the villages that need their help the most. We also hope that this insight will inspire additional research into vegetation removal as an intervention for schistsomiasis control. Ecology is the bottleneck on progress toward schistosomiasis control and elimination — and now ecologists are stepping in and filling that gap. Our team is excited to continue using our ecology toolkit to help address one of the world’s most burdensome parasitic diseases.

Learn more about our recent work on the spatial ecology of schistosomiasis here:

 

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