Building the molecular tools to monitor the success of biological control for schistosomiasis
Schistosomiasis is a water-borne parasitic disease, caused by trematode blood flukes of the genus Schistosoma. As an important and impactful neglected tropical disease of global importance, schistosomiasis has been subject to large-scale attempts to reduce the disease burden, with the key tool being school-based or community-wide preventative chemotherapy (mass drug administration; MDA) with the anthelminthic praziquantel. Over 1.5 billion doses of praziquantel have been donated by the manufacturer, Merck, for deworming in sub-Saharan Africa over the last 15 years. MDA has been enormously successful in reducing schistosomiasis morbidity, and this success has led to an increasing focus of using MDA to interrupt transmission in many regions, and an ultimate goal of the elimination of the disease. There is widespread agreement that successful elimination will require additional interventions to supplement MDA (Rollinson et al, 2013).
In the pre-praziquantel era, snail control was the major contributor to successful campaigns to control schistosomiasis in many regions (Sokolow et al, 2018). While snail control continues to be a major focus of research in schistosomiasis control (e.g. Allan et al. 2020) and could be very cost-effective (Lo et al. 2018) , it is no longer used at a large scale in control programmes. Snail control approaches include molluscicides, engineering and biological habitat modification and biological control. There has been concern over the broader environmental impacts of all of these approaches, potentially limiting their large-scale applicability, but biological control with native species seems likely to be the most acceptable, particularly if predators with fisheries value could be used (Ozretich et al. 2022). A number of snail predators have been proposed as likely candidates for biological control of snail numbers, including prawns, a number of bird species and fish. While the impact of snail biological control will ultimately be assessed by impact on schistosomiasis transmission, there is also a need to monitor the impact of snail control directly on the target schistosome-transmitting snails. Traditionally this involves manual searching for snails in areas of appropriate habitat, which is slow (and thus expensive) and difficult to replicate (Sokolow et al, 2018). Furthermore, a number of biological control programmes have been plagued by off-target impact on the biological community, so careful and holistic monitoring of such programmes is critical.
The Brierley lab is soon (planned for late 2022) to introduce 100,000 captive-bred tagged individuals of the native African sharptooth catfish (Clarias gariepinus) to Lake Victoria near Mwanza, Tanzania to augment the local population. In this project, we will leverage Brierley’s work to test novel molecular approaches to monitoring both the impact of the catfish release on both species of Schistosoma-transmitting snails (Biomphalaria sudanica and B. choanomphala) and other snails, but also more widely on the fauna of Lake Victoria in the Mwanza area. The Brierley team is already monitoring schistosome transmission and collecting baseline material that will become available for this project, and extensive baseline data on trematode diversity is available from the area thanks to previous work by the NHM group. The precise course of the project will depend on the interests of the student, but the student will have the opportunity to develop molecular tools to monitor snail communities and fish diets via molecular barcoding (e.g. Routtu et al. 2014) of snail and trematode diversity in environmental DNA (e.g. Douchet et al. 2022) and to monitor the populations of target snail species using amplicon panels with next-generation sequencing, as well as developing interdisciplinary fieldwork skills in malacology, fisheries biology and parasitology and expertise in bioinformatics and data analysis.