New research led by Liverpool School of Tropical Medicine (LSTM) warns that hybrid forms of the parasites that cause schistosomiasis are undermining existing disease control strategies and could accelerate the spread of infection in Africa and beyond.
Schistosomiasis is a disease caused by water borne schistosome parasites that affects more than 200 million people worldwide, causing chronic illness, disability and, in severe cases, life threatening organ damage. Despite decades of large-scale control programmes based on mass drug administration, transmission remains stubbornly persistent in many settings, particularly where people depend on freshwater for daily activities.
A major new Special Issue of Philosophical Transactions B of the Royal Society brings together 12 multidisciplinary studies showing that schistosome parasites are hybridising far more frequently than previously recognised. These hybrids emerge when different parasite species, including those infecting humans and livestock, mix and reproduce in shared freshwater environments.
Researchers found that some hybrid parasites can display altered biological traits, including changes linked to virulence, host range and transmission potential. These changes complicate diagnosis, as hybrid eggs can be harder to identify using standard methods, and raise concerns that current control strategies, which largely focus on treating human infection alone, may be missing key reservoirs of disease.
Several researchers also warn that hybrids are already reshaping patterns of schistosomiasis across Africa and could enable the disease to establish in new regions. Previous outbreaks of urogenital schistosomiasis in southern Europe have shown that transmission outside Africa is possible when animal and human parasites overlap, highlighting the need for stronger surveillance.
Professor Russell Stothard from LSTM is the senior author on several papers and the co-editor of the Special Issue. He said: "Schistosomes are demonstrating parasite evolution in action with remarkable speed. This new body of work reveals a level of biological complexity and flexibility that changes how we appreciate African schistosomes, classify them and hope to mitigate their detrimental impact".
"Hybrid schistosomes are not unusual biological outliers, they are becoming a central part of the epidemiological landscape. This raises important questions for future disease surveillance, particularly genital schistosomiasis, and associated treatment strategies that should better embrace One Health approaches."
The studies show that hybridisation is driving the emergence of new variants in both northern and southern Africa, each responding differently to ecological and agricultural pressures. One hybrid variant, Schistosoma haematobium x S. mattheei, which links a human-infecting species with a livestock species, is now a common cause of genital tract disease in men and women in parts of Malawi, underscoring the blurred boundaries between human and animal health.
Together, the authors argue that without improved surveillance, including genetic monitoring of parasites, and greater integration of human, animal and environmental health strategies, hybrid schistosomes could slow or reverse progress towards elimination targets.
Professor Janelisa Musaya, Associate Director at Malawi Liverpool Wellcome Programme (MLW), said: "In Malawi, we have observed a disturbing trend of persistent infection, particularly in areas where people and livestock coexist, despite over a decade of annual mass drug administration. These landmark studies finally help us unravel the mystery behind this resilience, revealing how hybridisation between human and animal parasites complicates the path to elimination.
"This evidence is a call to action for our national control programmes to adopt a 'One Health' approach that addresses the blurred boundaries between human and animal health."
The Special Issue, Parasite evolution and impact in action: Exploring the importance and control of hybrid schistosomes in Africa and beyond, is published by the Royal Society and will be available from 8 January.
Five of the papers are led by LSTM scientists, with several arising from the Wellcome funded HUGS project, a major collaboration between Liverpool School of Tropical Medicine and the Malawi Liverpool Wellcome Programme.
