Schistosomiasis, or ‘Bilharzia’, is a major parasitic disease caused by several species of schistosomes. Infecting many warm-blooded hosts, including humans, livestock and wildlife, these parasites require developmental stages within freshwater intermediate host snails to complete their lifecycle. Applying several molecular tools, recent studies in Malawi and other sub-Saharan countries are revealing complex transmission dynamics, elucidating actual and potential zoonotic pathways. These molecular tools are uncovering hidden schistosome hybrids alongside surprising patterns of cross-species infection previously missed by traditional diagnostics. In reshaping the understanding of schistosomiasis transmission, molecular diagnostics highlight our need for integrated, cross-species control efforts based on a ‘One Health’ approach.
First clues of unusual schistosomes – atypical eggs under the microscope
Several years ago, during project work in Malawi, graciously hosted by our Malawian colleagues, Professor Janelisa Musaya, Dr Seke Kayuni and Mr Peter Makaula, we undertook several lakeside community diagnostic surveys for schistosomiasis, providing our Master’s students from the Liverpool School of Tropical Medicine (LSTM), with valuable hands-on public health experiences. Schistosomiasis, or ‘Bilharzia’, is caused by infection with trematode flatworms, also known as blood flukes, within the genus Schistosoma. Depending on species, paired adult worms typically reside around the bladder (Schistosoma haematobium) or intestines (Schistosoma mansoni), laying hundreds of toxic eggs that perforate and impair venous blood vessels. In so doing, worms cause anaemia, multi-organ damage and even trigger urinary cancers. Our Master’s students, expecting to observe the classic ‘oval-shaped’ eggs of S. haematobium in patients’ urine by microscopy, began to notice occasional strangely shaped, atypical eggs. These were more ‘elongated diamond-shaped’, sometimes with an ‘equatorial bulge’, standing out from others when viewed via microscopy (Figure 1).
Eggs of different species of schistosome viewed under microscope. The eggs of schistosomes viewed under a microscope (400 x magnification) can be seen here as ‘elongated ellipsoid’ shapes, against a nylon mesh filter background. The eggs are approximately 150 µm in length and were passed in human urine from infected patients. Note the different morphologies of eggs, later determined via molecular diagnostics to be from different species of schistosome. Several other similar eggs from other patients were later determined to be of cross-species parentage, giving rise to hybrids. The photograph shows three eggs, with species later determined as S. mansoni (left), S. haematobium (bottom) and S. mattheei (right).
Eggs of different species of schistosome viewed under microscope. The eggs of schistosomes viewed under a microscope (400 x magnification) can be seen here as ‘elongated ellipsoid’ shapes, against a nylon mesh filter background. The eggs are approximately 150 µm in length and were passed in human urine from infected patients. Note the different morphologies of eggs, later determined via molecular diagnostics to be from different species of schistosome. Several other similar eggs from other patients were later determined to be of cross-species parentage, giving rise to hybrids. The photograph shows three eggs, with species later determined as S. mansoni (left), S. haematobium (bottom) and S. mattheei (right).
Having significant experience with veterinary parasitology, we were immediately struck by the similarity of these differently shaped eggs with those of other schistosome species Schistosoma bovis, Schistosoma curassoni or Schistosoma mattheei, typically infecting livestock. Whilst we were aware of the few previous reports of cross-species, or zoonotic host infections, and even hybrids between livestock- and human-infecting worms, this had never before been observed in Malawi. Could these strange eggs also be cross-species zoonotic infections, or hybrid forms in Malawi perhaps?
We had many times observed local livestock entering the lake to drink and cool off during the hot days, and often these places of entry were close to where people enjoyed swimming, bathing, washing clothes or fishing (Figure 2). However, livestock species of schistosomes would normally be observed in faecal samples, due to those schistosomes living around the intestines of livestock, and not the bladder. It is generally accepted as much less common for ‘intestinal’ species’ eggs to be present in urine samples. Unfortunately, morphology, sample type, location and our observations alone were insufficient to provide definitive answers to this epidemiological puzzle. Further studies were certainly needed.
A typical scene on the banks of the beautiful Lake Malawi. The Lake is an essential shared resource for the daily lives of communities, tourism, agriculture, industry and wildlife. The health of the lake, its environment and that of the animals and humans who depend on it is intricately linked.
A typical scene on the banks of the beautiful Lake Malawi. The Lake is an essential shared resource for the daily lives of communities, tourism, agriculture, industry and wildlife. The health of the lake, its environment and that of the animals and humans who depend on it is intricately linked.
To address our epidemiological puzzle, we supported our Master’s students in using these new opportunities to develop their project ideas to consider the health of humans alongside that of animals and the environment they share: a ‘One Health’ approach towards improved surveillance of human and animal schistosomiasis in Malawi. Most importantly, could we employ molecular methods to identify these oddly-shaped eggs and to investigate whether humans, livestock and snails carried cross-species zoonotic and/or hybrid schistosomes? Our minds filled with longer term questions too. Do cross-species and/or hybrid infections ‘behave’ the same as ‘usual’ infections; Are they less or more infectious, virulent or susceptible to existing drugs; Might they migrate to different parts of the body, causing unusual pathologies; Do they prefer different age groups of people or livestock; Do they infect snails more easily? What we did know, however, was that if we didn’t use the right tools to look for them, we wouldn’t find them!
Probing the depth and breadth of African schistosomiasis
Schistosomiasis or Bilharzia (sometimes called “snail fever”) affects over 240 million people globally, predominantly in sub-Saharan Africa. The disease was discovered by Theodor Bilharzia in 1851 upon noticing unusual white flatworms within the veins of deceased Egyptian canal cleaners during autopsy. Incidentally, whilst 2025 marks the 200th celebration of his birth, his seminal discovery also carried a longstanding taxonomic fumble and key epidemiological error by conflating S. haematobium and S. mansoni. At that time, Bilharz failed to grasp the occurrence of two separate schistosome species and their coinfection(s) potentials. Similarly, he did not know, nor would he, that these two schistosome species caused urogenital and intestinal schistosomiasis, respectively, and had entirely different and specific intermediate molluscan hosts, freshwater snails of the genus Bulinus and Biomphalaria. Some twenty years later, and again in Egypt, the first schistosome species in livestock, Schistosoma bovis, was described by Prospero Sonsino in 1876.
Even today, whilst many surveys and reports highlight the regions, prevalence and intensity of human-infecting schistosomes, there is much less knowledge of the incidence and locations of livestock infection. Indeed, the epidemiology of schistosomiasis in African livestock remains overlooked, despite the lifecycles and disease pathology for both human-infecting and livestock-infecting worms, of which there are several different species, being broadly similar (Figure 3). In brief, schistosomes are released from infected freshwater snails as microscopic short-lived larval forms, called cercariae, which then burrow through the skin of humans and other warm-blooded animals when they enter infested freshwater lakes, ponds and rivers for bathing. Developing to adult worms in the host body, they take up residence at their preferred egg-laying sites – in blood vessels around the intestines for livestock-infecting worms, or in blood vessels around the bladder or intestines for human-infecting worms depending on the species. Here, worms release their eggs, some of which exit the body through the hosts’ urine and/or faeces. Wherever this egg-infected urine or faeces re-enter freshwater, the eggs hatch, releasing short-lived larval schistosome forms, called miracidia, which then infect freshwater snails, multiplying asexually in their thousands within snails, to be released as a steady stream of separate-sexed infective cercariae forms back into the freshwater... awaiting the next unsuspecting victim. And the infection cycle starts again (Figure 3).
The lifecycle of selected species of schistosomes of humans and livestock. The lifecycle of certain schistosomes infective to humans is broadly similar, as can be seen in this outline of transmission for Schistosoma haematobium and S. mattheei, typically infective to humans and livestock, respectively. However, cross-infections, where S. haematobium infects livestock and S. mattheei infect humans, as well as infections with hybrid worms, resulting from cross-breeding between these different species, have been detected in Africa, where hosts may commonly share the same freshwater locations. Eggs of human-infecting species, with their ‘oval’ shape, and schistosome eggs of livestock-infecting species, with their more ‘diamond-shape’ and ‘equatorial bulge’, are passed in urine and faeces of their infected hosts, respectively. On contaminating freshwater, they hatch to infect freshwater snails, where asexual reproduction generates thousands of larval worms with bifurcated tails, called cercariae. Released from the snail into freshwater, the cercariae seek out and penetrate the skin of humans and livestock, to mature to adult worms, continue releasing eggs and perpetuating the lifecycle. Male and female worms of differing species can sometimes pair-up, interbreeding to produce hybrid offspring. Created in BioRender. LaCourse, J. (2025) https://BioRender.com/v74xbk1.
The lifecycle of selected species of schistosomes of humans and livestock. The lifecycle of certain schistosomes infective to humans is broadly similar, as can be seen in this outline of transmission for Schistosoma haematobium and S. mattheei, typically infective to humans and livestock, respectively. However, cross-infections, where S. haematobium infects livestock and S. mattheei infect humans, as well as infections with hybrid worms, resulting from cross-breeding between these different species, have been detected in Africa, where hosts may commonly share the same freshwater locations. Eggs of human-infecting species, with their ‘oval’ shape, and schistosome eggs of livestock-infecting species, with their more ‘diamond-shape’ and ‘equatorial bulge’, are passed in urine and faeces of their infected hosts, respectively. On contaminating freshwater, they hatch to infect freshwater snails, where asexual reproduction generates thousands of larval worms with bifurcated tails, called cercariae. Released from the snail into freshwater, the cercariae seek out and penetrate the skin of humans and livestock, to mature to adult worms, continue releasing eggs and perpetuating the lifecycle. Male and female worms of differing species can sometimes pair-up, interbreeding to produce hybrid offspring. Created in BioRender. LaCourse, J. (2025) https://BioRender.com/v74xbk1.
When schistosomes break human rules
Following the pioneering work of Robert Leiper in 1915, the understanding of African schistosomiasis was relatively neat: humans are mainly infected by S. haematobium (causing ‘urinary schistosomiasis’, a term later revised to ‘urogenital schistosomiasis’ upon WHO recommendations in 2009, reflecting that the worms reside equally around the blood vessels of the bladder and internal genitalia) or S. mansoni (causing intestinal schistosomiasis, as worms live around the intestines), while local livestock were infected by closely related Schistosoma species S. bovis, S. curassoni or S. mattheei, which reside around the intestinal blood vessels. Other animals, such as rodents and various other wildlife, and even hippopotamuses, have their own Schistosoma species too. Traditional diagnosis typically relies on observing and distinguishing species-specific morphological characteristics of parasites’ eggs in hosts’ faeces or urine, viewed under microscope. Today, antischistosomal treatment with the antihelminthic praziquantel, is firmly focused on humans, particularly schoolchildren, while little attention is given to livestock (or wildlife) diagnosis or treatment. The latter exposes a shortfall in availability of praziquantel and, even if present, would be prohibitively expensive and impractical for expanded use in livestock, notwithstanding administration in wildlife!
Molecular tools reveal ‘One Health’ needs
Essential in our untangling of this complicated epidemiological human–animal–snail dynamic is that modern molecular DNA-based tools, coupled with a more inclusive ‘One Health’ approach, which considers the integrated health of humans, animals and the environment, are revealing just how much we’ve been missing. Those strangely-shaped eggs spotted by our students in patient samples were the first clue that cross-species infection and zoonosis, and, most worryingly, hybrid parasites – worms with DNA from both human-infecting and livestock-infecting schistosome species – might be present in Malawi. Indeed, through developing and then using DNA-based diagnostic tools, we confirmed what the microscope alone couldn’t: that these oddly- shaped eggs in human samples were indeed the schistosome species of livestock, S. mattheei, and in some cases, hybrid Schistosoma species between S. mattheei and S. haematobium and also S. bovis and S. haematobium. This changed everything about our approach, starting with who our diagnostic surveys focused upon, and importantly, how we now have to embed modern molecular diagnostics into our surveys.
Molecular nerds following the roving herds
First, we set about contacting local farmers who allowed us to collect faeces from their livestock. Could we detect Schistosoma species parasite eggs in their samples, as we had done in human samples? Might we find typically human-infecting schistosomes in livestock? Would we discover hybrid schistosomes present in livestock too? Secondly, we visited abattoirs to inspect carcasses of processed livestock. What Schistosoma species and/or hybrid worms might we find? We also, we followed livestock, using collars with Global Positioning System (GPS) dataloggers, during their daily routine, tracking where and when they entered lakes, and whether they were close to where people swam, bathed, washed clothes or fished. In all these places, we collected freshwater snails, the intermediate hosts of Schistosoma worms. Could we detect the various species of worms, or even hybrid worms, in their tissues?
Molecular DNA tools reshape schistosome epidemiology
And finally, we developed molecular DNA-based methods to detect and distinguish between species of Schistosoma worms, and to determine which ones were hybrids. To do this, we developed our own high-throughput surveillance test that was able to pick out samples of interest from thousands of potentially interesting specimens. This method relied on comparing genetic markers from the nuclear and mitochondrial genomes, which allowed us to identify species parentage from each schistosome. Individual samples that had a mix-species parentage was confirmed with further DNA analysis through sequencing regions of the nuclear and mitochondrial genes. Our new assay is able to identify six different species of schistosome based on specific melt-profiles of the Polymerase Chain Reaction (PCR) products in real-time (Figure 4).
The melting temperature peaks for the six species of schistosome generated by the high-resolution melt (HRM) real-time PCR. Plots show the melting temperature peaks corresponding to the horizontal x-axis values, when double-stranded DNA separates into two individual single-strands, or ‘melts’. Plot ‘A’ displays DNA temperature separation peaks for our selected DNA strand from the mitochondrial genome inherited from the female schistosome parent only. Plot ‘B’ shows the temperature separation peaks for our selected DNA strand from the nuclear genome, chromosomes of which would be inherited from both schistosome parents. Schistosomes with parents of a single species would display matching-species temperature peaks for both the single-copy mitochondrial peaks, and for both nuclear copies from parents. Hybrid worms would display temperature peaks matching different species, where one of the nuclear peaks would differ from the other in temperature, but match species for the mitochondrial temperature peak.
The melting temperature peaks for the six species of schistosome generated by the high-resolution melt (HRM) real-time PCR. Plots show the melting temperature peaks corresponding to the horizontal x-axis values, when double-stranded DNA separates into two individual single-strands, or ‘melts’. Plot ‘A’ displays DNA temperature separation peaks for our selected DNA strand from the mitochondrial genome inherited from the female schistosome parent only. Plot ‘B’ shows the temperature separation peaks for our selected DNA strand from the nuclear genome, chromosomes of which would be inherited from both schistosome parents. Schistosomes with parents of a single species would display matching-species temperature peaks for both the single-copy mitochondrial peaks, and for both nuclear copies from parents. Hybrid worms would display temperature peaks matching different species, where one of the nuclear peaks would differ from the other in temperature, but match species for the mitochondrial temperature peak.
So who is infected with what and from whom?
Our initial surveys were revealing and concerning. In the southern region around Lake Malawi, nearly half of cattle we surveyed showed signs of schistosomiasis following faecal diagnosis combining traditional microscopy to look for eggs, with our new molecular approaches. Some 2–3% of infected animals were observed to be shedding eggs of S. haematobium, the usually human-infecting species, and 3–5% were found to be shedding hybrid forms between S. haematobium and S. mattheei, the species specific to livestock. Limited post-mortem inspections at abattoirs revealed at least 10% of cattle harboured schistosome worms, with those obtained to date, all S. mattheei and surprisingly not S. bovis. Goats too were investigated, with up to a quarter of goats found to shed S. mattheei eggs and hybrid S. mattheei/S. haematobium eggs. In the lake areas frequented by both livestock and humans, snails were also found to be infected with S. mattheei, S. haematobium and hybrids. Furthermore, we treated infections and demonstrated short-term cure with praziquantel, for a small number of cattle and goats, with later re-checking of their faeces up to 12 weeks afterwards. Reinfection was shown to start from 6 weeks onwards, and by 12 weeks, all animals were reinfected.
Why molecular DNA tools matter
The key factor in these findings has been the application of molecular diagnostics at scale to sample larval stages of schistosomes from definitive and intermediate hosts. Without DNA analysis of these parasites, hybrids and cross-species host infections would have remained unconfirmed or invisible to traditional microscopy approaches. Molecular techniques have given us a much clearer picture of what is really happening in the field (or rather, in the lakes and bodies of the schistosomes’ hosts). If schistosomiasis control strategies overlook animal reservoirs or fail to identify hybrid parasites, they risk being incomplete and ineffective. Hybrid worms may respond differently to treatment or have altered infectivity or transmission dynamics. Understanding these factors will be essential for designing robust, sustainable control programmes.
‘One Health’: A holistic framework for future control
These findings in Malawi, and other reports emerging over the last few years in other parts of Africa, have major implications for how we approach controlling schistosomiasis. Traditionally, control programmes have focused almost exclusively on surveying and treating people. This siloed approach however, ignores the complex interactions between people, livestock, wildlife, snails and their shared environment.
The ‘One Health’ framework offers a more integrated approach, recognising that the health of people is deeply connected to the health of animals and ecosystems. In the case of schistosomiasis, this means including livestock, and even wildlife, in surveillance and treatment plans, managing water sources shared between animals and people, and monitoring snail populations as key vectors. To do this, we must bring together veterinarians, medics, ecologists, parasitologists, health ministries and public health officials. Molecular diagnostics too should become a routine aspect of surveillance and intervention planning, not just for their precision, but for their ability to uncover the hidden complexity of parasitic transmission in real-world settings. Looking ahead to WHO 2030 targets, a key public health challenge today and tomorrow is to devise and apply appropriate interventions to mitigate zoonosis.
For our Master’s students at Liverpool, their studies with our colleagues in Malawi were just the beginning in exploring the world of ‘One Health’, which is a vivid reminder of how interconnected life can be, and how the smallest worm can bridge species, hosts, habitats and health systems in ways we are only just beginning to understand.
Further reading
Buonfrate D, Ferrari TCA, Akim Adegnika A, Stothard JR, Gobbi FG. Human schistosomiasis. Lancet. 2025 Feb 22;405(10479):658-670. doi: 10.1016/S0140-6736(24)02814-9. PMID: 39986748.
Juhász A, Makaula P, Cunningham LJ, Jones S, Archer J, Lally D, Namacha G, Kapira D, Chammudzi P, LaCourse EJ, Seto E, Kayuni SA, Musaya J, Stothard JR. 2024. Revealing bovine schistosomiasis in Malawi: Connecting human and hybrid schistosomes within cattle. One Health. 14;19:100761. doi: 10.1016/j.onehlt.2024.100761. PMID: 39021560; PMCID: PMC11253675.
Juhász A, Makaula P, Cunningham LJ, Field L, Jones S, Archer J, Mainga B, Lally D, Namacha G, Kapira D, Chammudzi P, LaCourse EJ, Nkolokosa C, Seto E, Kayuni SA, Musaya J, Stothard JR. 2024. Revealing caprine schistosomiasis and its One Health importance in Malawi: A molecular epidemiological investigation augmented with a praziquantel treatment and GPS animal tracking pilot sub-study. One Health. 19;19:100918. doi: 10.1016/j.onehlt.2024.100918. PMID: 39507305; PMCID: PMC11539161.
LaCourse, J. (2025) Figure created using BioRender. Available at: https://BioRender.com/v74xbk1 (Accessed: 19 May 2025).
Liang S, Ponpetch K, Zhou YB, Guo J, Erko B, Stothard JR, Murad MH, Zhou XN, Satrija F, Webster JP, Remais JV, Utzinger J, Garba A. 2022. Diagnosis of Schistosoma infection in non-human animal hosts: A systematic review and meta-analysis. PLoS Negl Trop Dis. 6;16(5):e0010389. doi: 10.1371/journal.pntd.0010389. PMID: 35522699; PMCID: PMC9116658.
Ame S, Juma O, Juhász A, Ali M, Suleiman TS, Gobert GN, Cunningham LJ, Cawley A, Atkins L, Jones S, LaCourse EJ, Kabole F, Stothard JR. A pilot investigation of bovine schistosomiasis on Unguja Island, Zanzibar, raises a new concern for elimination of urogenital schistosomiasis. Parasit Vectors. 2025 Mar 5;18(1):89. doi: 10.1186/s13071-025-06698-y. PMID: 40045425; PMCID: PMC11883948.
Lo NC, Bezerra FSM, Colley DG, Fleming FM, Homeida M, Kabatereine N, Kabole FM, King CH, Mafe MA, Midzi N, Mutapi F, Mwanga JR, Ramzy RMR, Satrija F, Stothard JR, Traoré MS, Webster JP, Utzinger J, Zhou XN, Danso-Appiah A, Eusebi P, Loker ES, Obonyo CO, Quansah R, Liang S, Vaillant M, Murad MH, Hagan P, Garba A. 2022. Review of 2022 WHO guidelines on the control and elimination of schistosomiasis. Lancet Infect Dis. 22(11):e327-e335. doi: 10.1016/S1473-3099(22)00221-3. Epub 2022 May 17. PMID: 35594896.
Stothard JR, Juhász A, Musaya J. 2025. Schistosoma mattheei and zoonotic schistosomiasis. Trends Parasitol. 41(2):87-90. doi: 10.1016/j.pt.2024.12.008. Epub 2025 Jan 6. PMID: 39765449.
World Health Organization; 2022. Ending the neglect to attain the sustainable development goals. One Health: approach for action against neglected tropical diseases 2021-2030. Geneva: Licence: CC BY-NC-SA 3.0 IGO. ISBN 978-92-4-004241-4 (electronic version)
Acknowledgements
In writing this article, we wish to acknowledge the enormous contribution to all those named and acknowledged within the studies described and referenced in the further reading below. Without their participation, generosity, dedication, expertise, funding, collaboration and friendship, this important work contributing to the further understanding of schistosomiasis and its control could not have happened. Thank you all. We are especially grateful for funding from the Wellcome Trust, and Liverpool School of Tropical Medicine, Faculty of Education, Master's Research Dissertation Projects.
Author information
Dr James LaCourse is a Reader/Associate Professor in Education at Liverpool School of Tropical Medicine (LSTM). His research interests encompass both field and laboratory approaches to the study of several parasites. Focusing upon helminths, he undertakes field work in sub-Saharan Africa and the UK, to study helminth epidemiology, as well as employing molecular, biochemical and proteomic technologies to investigate helminth epidemiology, detoxification, drug resistance, and establishment and maintenance of parasitic infection. He has over 75 publications in top journals and conferences. James is Director of Studies for LSTM’s MSc Tropical Disease Biology programme. Email: [email protected].
Dr Alexandra Juhász is a veterinarian and researcher at Liverpool School of Tropical Medicine (LSTM), whose work focused on zoonotic diseases and parasite transmission within a One Health framework. Her research combines molecular epidemiology and fieldwork across Africa, notably through projects on schistosome hybridisation in both humans and cattle, including the HUGS and SHIS-CAM studies, and investigations of fusidic acid as a treatment for bovine onchocerciasis. Alongside her research, she is Director of Studies for LSTM’s MSc One Health in Tropical Disease, and contributes to a range of teaching in parasitology and disease management, supervising students and developing course content, integrating research and teaching to train future global health professionals. Email: [email protected].
Dr Lucas Cunningham is a parasitologist at Liverpool School of Tropical Medicine (LSTM), whose research focuses on the development and application of molecular diagnostics for parasitic diseases. His work spans schistosomiasis, African trypanosomiasis, strongyloidiasis, and hookworm infections, with an emphasis on zoonotic and hybrid species, particularly in Malawi and Uganda. He has led field-based diagnostic innovation, including mobile labs and capacity-building initiatives. Lucas completed his PhD at LSTM on sleeping sickness vectors, and later joined the HUGS study on schistosome hybridisation. In addition to his research, he has contributed to teaching and training in parasitology and molecular diagnostics across several countries. Email: [email protected].
Professor Russell Stothard is a medical parasitologist at Liverpool School of Tropical Medicine (LSTM), specialising in neglected tropical diseases, with a particular focus on the epidemiology and control of schistosomiasis. His multidisciplinary research spans molecular diagnostics, social science, and One Health approaches, addressing the impact of schistosomiasis on co-infections with malaria and HIV, as well as its links to stigma and menstrual hygiene. Lately, his work examines the emergence of zoonotic schistosome hybrids and their public health implications. His research has influenced multiple WHO policies. Alongside his research, he contributes to teaching and global health training through expert committees and guideline development. Russ is co-Director of Studies for LSTM’s MSc One Health in Tropical Disease. Email: [email protected].
