Working within a One Health framework, the overarching aim of our ZELS projects is to elucidate the complex dynamics of zoonotic hybrid schistosomiasis transmission, with the ultimate goals of improving the health of affected people and their livestock.

Thiago cattle Senegal (credit Elsa Leger)


Hybridisation amongst parasitic agents, particularly concerning those with zoonotic potential, is a major emerging public and veterinary health concern at the interface of evolution, epidemiology, ecology, and control. Co-infections, where individual hosts are infected by more than one infectious agent at the same time, are the norm within humans and animals. Increasing levels of anthropogenic changes are shifting the opportunities for encountering new infections of both human and animal origin, and thereby also the occurrence of co-infections with multiple agent species and strains.

Co-infection can have a significant impact on the pathogens involved, often as a result of synergistic or antagonistic interactions, where changes in establishment, growth, maturation, reproductive success, and/or drug efficacy have all been documented. Furthermore, co-infections between parasites can allow for heterospecific (between-species or between-lineage) mate pairings, resulting in either infertility or parthenogenesis (asexual reproduction where eggs occur without fertilization), introgression (the introduction of alleles of one species into the gene pool of another through repeated backcrossing of an inter-specific hybrid with one of its parent species), or whole genome admixture.

One group of infectious agents where opportunities for, and subsequent evidence of, hybridisation between parasites of humans with those from animals is rapidly emerging are those of the neglected tropical diseases (NTDs)—highly debilitating diseases infecting more than a fifth of the world’s human population, and their livestock, with devastating consequences. One such major NTD is schistosomiasis, caused by Schistosoma spp. trematodes, is a freshwater-snail-borne Neglected Tropical Disease (NTD) of global medical and veterinary importance, with over 220 million people currently infected and untold millions of livestock. The disease causes abdominal pain, bloody urine and stools and can damage the liver, spleen, intestines, lungs and bladder, ultimately leading to death to many cases.

Whilst in endemic regions of Asia, animal hosts are acknowledged as important zoonotic reservoirs, within SSA, in contrast, the zoonotic component of schistosomiasis transmission and the implications of the multi-host aspects of schistosomiasis for disease control and reaching the elimination targets has, until recently, been largely ignored. This was particularly the case for S. haematobium, the causative agent of urogenital schistosomiasis in humans, which was assumed to be an exclusively human infection – and thus amenable to elimination by targeting treatment at humans alone.

In 2013, the World Health Organization (WHO) set ambitious goals for controlling schistosomiasis morbidity by 2020, eliminating it as a public health problem (EPHP) in all endemic countries by 2025, and complete interruption of transmission in selected regions by 2025, with the revised 2021 WHO-NTD Roadmap aiming for EPHP by 2030. However, despite decades of mass administration of the anthelmintic praziquantel to, predominantly, school-aged children, reports of schistosomiasis being more prevalent than previously thought, its persistence and/or re-emergence in previously cleared areas, together with potential reduced drug efficacy in populations under high drug pressure, all serve to highlight that additional control strategies must be introduced if this major disease is to ever reach elimination.

Mothers and children in Senegal (Credit Poppy Berdoy-Webster)


The objective of the RVC research is a better understanding of the evolution, ecology, transmission dynamics and morbidity impact of this potentially emerging disease threat, of paramount important to the health of humans and their livestock, particularly amongst the poorest of the poor. More generally, this research enhances our understanding of a wide spectrum of multi-host parasitic diseases of humans and animals, and in particular the role of evolution of host ranges and introgressions within major taxonomic groups, in our rapidly changing world.

The RVC project informs and contributes to sustainable control and development strategies for these people. Extensive parasite sampling in humans, livestock, wildlife and snail intermediate hosts and the application of novel molecular and diagnostic tools, as well as mathematical modelling and socio-economic surveys, are being used to elucidate the epidemiology of novel zoonotic hybrid schistosomes and its impact on host spectrum, PZQ efficacy, host morbidity and ultimately transmission potential.

Thiago cattle in Senegal (Credit Elsa Leger)


The threat to public health presented by zoonotic spillover of pathogens from animal reservoirs is predicted to increase with rapid anthropogenic changes and global trends such as migration and changing land use. This research highlights the importance of recognising the multi-host multi-parasite aspects of disease systems under evolutionary pressure. The work to date has demonstrated how zoonotic spillover and complex interactions between pathogen species, such as parasite hybridisation, may have implications such as resilience to current disease control strategies, as well as facilitating the spread of tropical diseases such schistosomiasis beyond their original geographical boundaries.

The outcomes of this study have already substantially contributed to a change in international policy on disease control and knowledge, modifying attitudes and practice of those inflicted with the threat of this disease. Pressure will now be placed for the implementation of modified praziquantel treatment regimes and diagnostic tools for both people and animals living in zoonotic high transmission zones with the application of a One Health framework for schistosomiasis control.

The research contributes to the major push to control and eliminate schistosomiasis as a public health problem, as set by the WHO NTD roadmap and the London Declaration of the NTD coalition, which pledge to contribute towards the control or elimination of schistosomiasis by the end of the decade.

Funders and Partners  


Title Publication Year
Spillover, hybridization and persistence in schistosome transmission dynamics at the human-animal interface Proceedings of the National Academy of Sciences of the United States of America 2021
Estimating the financial impact of livestock schistosomiasis on traditional subsistence and transhumance farmers keeping cattle, sheep and goats in Northern Senegal Parasites and Vectors 2021
Hybridized zoonotic Schistosoma infections result in hybridized morbidity profiles: a clinical morbidity study amongst co-infected human populations of Senegal Microorganisms 2021
Diagnosis of Schistosoma Infection in Non-Human Animal Hosts: A Systematic Review and Meta-Analysis Preprints 2021
Schistosomiasis and the Global Goals The New England Journal of Medicine 2020
Multi-host transmission of Schistosoma mansoni in Senegal Emerging Infectious Diseases 2020
Divergence across mitochondrial genomes of sympatric members of the Schistosoma indicum group and clues into the evolution of Schistosoma spindale Scientific Reports (Nature Group) 2020
Meta-analyses of Schistosoma japonicum infections in wild rodents across China over time indicates a potential challenge to the 2030 elimination targets  PLoS Neglected Tropical Diseases 2020
Prevalence and distribution of schistosomiasis in human, livestock and snail populations in northern Senegal: A One Health epidemiological study of a multi-host system The Lancet Planetary Health 2020
Schistosomiasis – assessing progress towards the 2020 and 2025 goals The New England Journal of Medicine 2019
Precision mapping of snail habitat provides a powerful indicator of human schistosomiasis transmission Proceedings of the National Academy of Sciences 2019
Ancient hybridization and introgression of an invadolysin gene in schistosome parasites Molecular Biology and Evolution 2019
Evaluation of DNA extraction methods on individual helminth egg and larval stages for whole-genome sequencing Frontiers in Genetics 2019
Plagorchis sp, in small mammals of Senegal and the potential emergence of a zoonotic trematodiasis International Journal for Parasitology: Parasites and Wildlife 2019
Rodents of West Africa and their role as intermediate hosts of Hydatigera spp. (Cestoda: Taeniidae) Parasitology 2019
Rodents as natural hosts of zoonotic Schistosoma species and hybrids: an epidemiological and evolutionary perspective from West Africa The Journal of Infectious Diseases 2018
Opportunities and challenges for modelling evolving multi-host, multi-parasite transmission dynamics: zoonotic hybrid schistosomiasis in West Africa Evolutionary Applications 2018
Single-sex schistosome infections of definitive hosts: implications and applications for schistosome transmission and control in a changing world  PLoS Pathogens 2018
Whole genome amplification and exome sequencing of archived schistosome miracidia Parasitology 2018
Who acquires infection from whom and how? - disentangling multi-host and multi-mode transmission dynamics in the ‘elimination’ era Philosophical Transactions of the Royal Society London Series B (Biological Sciences) 2017
Hybridizations within the Genus Schistosoma: implications for evolution, epidemiology and control Parasitology 2017
Should we be treating animal schistosomiasis in Africa? The need for a One-Health economic evaluation of schistosomiasis burden and control in sub-Saharan Africa Transactions of the Royal Society of Tropical Medicine and Hygiene 2017
Introgressed animal schistosomes schistosoma curassoni and S. bovis naturally infecting humans Emerging Infectious Diseases 2016 
One Health – an Ecological and Evolutionary Framework for tackling Neglected Zoonotic Diseases Evolutionary Applications 2016
Hybridization in parasites: consequences for adaptive evolution, pathogenesis and public health in a changing world PLoS Pathogens 2015

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