Standard Genetic Tests Misread Hybridization Between Human and Livestock Schistosomes

Schistosomiasis remains one of the world’s most burdensome parasitic diseases, and for decades researchers have relied on a small handful of genetic markers to untangle the relationships between the parasites that infect humans and those that infect livestock. A new study published in PLOS Pathogens now delivers a sobering warning about that practice. By pairing traditional marker-based genotyping with whole-genome sequencing across a large collection of Nigerian schistosomes, a team led by Egie E. Enabuele, Roy N. Platt II, Grace A. Arya, and Timothy J. C. Anderson has shown that the two most widely used genetic tools in schistosome epidemiology can produce conclusions that are dramatically at odds with the true ancestry of the parasites. The findings call into question a substantial body of published work on hybridization and zoonotic transmission, and they arrive at a moment when control programs across Africa are increasingly concerned about the role of animals in sustaining human infection.

The parasites at the center of the study are blood flukes of the genus Schistosoma. Schistosoma haematobium, the species responsible for urogenital schistosomiasis across much of Africa and the Middle East, shares its geographic range with closely related livestock parasites, notably S. bovis, which infects cattle, and S. curassoni, which infects sheep and goats. Because these species can interbreed, parasitologists have long worried that livestock may act as reservoirs, seeding human populations with hybrid worms or even directly transmitting animal parasites to people. Distinguishing such events, however, requires a reliable way to tell the species apart, and for most field studies that way has been the internal transcribed spacer, or ITS, a region of nuclear ribosomal DNA, together with the mitochondrial cytochrome c oxidase subunit 1 gene, known as cox1.

The logic of the conventional approach is straightforward. Because ITS and cox1 sequences differ between S. haematobium and its livestock relatives, a worm recovered from a human patient that carries a livestock-type allele at either locus has typically been interpreted as a zoonotic infection, meaning an animal parasite that established itself in a person. When a single worm carries two different ITS variants, one from each of two species, researchers have inferred heterozygosity consistent with recent hybridization, often classifying the specimen as a first-generation hybrid, or F1. These inferences rest on a critical and largely untested assumption: that variation at one or two loci faithfully mirrors the ancestry of the rest of the genome. The new study set out to test that assumption directly, and the answer it obtained was emphatically negative.

The team assembled an unusually rich sample collection. Working at fourteen locations across Nigeria, they collected 132 schistosome parasites from the urine of human patients and 37 adult worms from cattle slaughtered at local sites. For every one of these 169 parasites, the researchers genotyped the ITS region and the cox1 gene using the standard methods employed in field studies worldwide. Crucially, they then went a step further and sequenced the whole genome of each individual worm. This dual approach allowed them to compare what the traditional markers seemed to say with what genome-wide data revealed about each parasite’s actual ancestry, providing an empirical benchmark that previous studies of this kind have lacked.

On the surface, the marker-based results looked like a textbook case of rampant hybridization and zoonotic spillover. Among the parasites recovered from humans, 10.1 percent carried both S. curassoni and S. haematobium ITS variants in the same worm, a pattern conventionally read as evidence of F1 or early-generation hybrids. A striking 21 percent of the human-derived parasites carried livestock schistosome markers at both cox1 and ITS, which under the standard framework would be scored as zoonotic infections with animal parasites. Another 13.7 percent carried S. bovis cox1 alongside mixed S. curassoni and S. haematobium ITS patterns, suggesting even more tangled ancestry. Had the analysis stopped there, the conclusion would have been alarming: livestock parasites apparently infecting people at high frequency, with hybrid swarms circulating in human populations.

The genome sequences told a fundamentally different story. When the researchers analyzed ancestry across the entire genome, all of the parasites recovered from humans clustered tightly together, regardless of what their ITS or cox1 genotypes had suggested. The worms from cattle, by contrast, were clearly and strongly differentiated from the human parasites. There was no sign of the F1 hybrids that the ITS data had implied; not a single worm from a human carried the roughly 50 percent livestock ancestry that a genuine first-generation cross would produce. Instead of discrete hybrid classes, the genomic data revealed a gradient of historical gene flow, with regionally varying levels of S. bovis introgression into the human parasite population. In southern Nigeria, human parasites carried a modest average of 4.9 percent S. bovis ancestry, while in northern Nigeria the average was a mere 0.06 percent, essentially negligible.

The discrepancy between the two approaches has a plausible mechanistic explanation rooted in the biology of these markers. The ITS region and cox1 are inherited as single linked blocks, the former from the nuclear genome and the latter from the mitochondria, and both can retain ancestral variation or be shaped by processes such as incomplete lineage sorting, in which ancient polymorphisms persist in descendant species and are shared by chance. When such shared or ancestral alleles exist, a worm that is genomically almost entirely S. haematobium can nonetheless carry an ITS or cox1 sequence that resembles a livestock species. Under the two-locus framework, that single allele is enough to trigger a diagnosis of zoonotic infection or recent hybridization, even though the rest of the genome tells a story of ancient, limited introgression rather than recent cross-species transmission. The markers, in other words, are not wrong about their own sequences; they are wrong about what those sequences imply for genome-wide ancestry.

The implications for the field are considerable. Studies using ITS and cox1 have been published across many African countries, frequently reporting hybridization between S. haematobium and livestock schistosomes and invoking zoonotic transmission to explain unexpected marker patterns. Those reports have influenced thinking about disease ecology, surveillance priorities, and even the design of control interventions, since the presence of animal reservoirs would argue for treating livestock alongside human populations. The new results demonstrate that two-locus genotyping is uninformative for detecting zoonotic infection or recent hybridization between S. haematobium and its livestock relatives, and the authors are explicit that previous data generated with this approach requires reinterpretation. Findings that appeared to document active spillover may instead reflect shared ancestral variation, while genuinely important regional differences in introgression, such as the north-south contrast observed in Nigeria, were invisible to the marker-based method.

For public health planners, the study offers both a caution and a path forward. The genome-wide evidence suggests that recent zoonotic transmission of livestock schistosomes to humans in the sampled Nigerian populations is far less common than marker-based studies had implied, which is reassuring for programs that had feared widespread animal reservoirs. At the same time, the measurable S. bovis ancestry in southern Nigerian parasites confirms that historical hybridization has occurred and that its footprint varies across the landscape, information that only genomic data can reliably provide. Whole-genome sequencing has become dramatically cheaper in recent years, and this study illustrates why it is increasingly viewed as the standard for questions of species boundaries, hybridization, and transmission. Field collections preserved from earlier marker-based surveys could, in principle, be resequenced, allowing the research community to revisit and correct the epidemiological record.

The work also carries a broader lesson that extends beyond schistosomes. Many parasitology and epidemiology studies continue to rely on one or two barcoding loci to draw inferences about species identity, hybridization, and cross-species transmission, often because of cost and logistical constraints. The Nigerian schistosome data provide a vivid demonstration of how such inferences can mislead when markers are shared through ancestry rather than exchange through recent interbreeding. As genomic resources become accessible to laboratories in endemic countries, the expectation is that marker-based claims of hybridization and zoonosis will increasingly be tested against genome-wide evidence, and that the resulting picture of parasite epidemiology, though perhaps less dramatic, will be far more trustworthy. For a disease that affects hundreds of millions of people, getting that picture right is not an academic nicety; it determines where scarce treatment and surveillance resources are directed, and whether the animals grazing beside human communities are truly the threat that older genetic tools made them appear to be.

Subject of Research: Genomic reassessment of ITS and mitochondrial DNA markers used to infer hybridization and zoonotic transmission between Schistosoma haematobium and livestock schistosomes in Nigeria

Article Title: Misleading inference of schistosome epidemiology from ribosomal internal transcribed spacer (ITS) and mitochondrial DNA

Article References: Enabuele, E. E., Platt II, R. N., Adeyemi, E. E., Aisien, M. S. O., Ajakaye, O. G., Ali, M. U., Amaechi, E. C., Atalabi, T. E., Auta, T., Awosolu, O. B., Dagona, A. G., Edo-Taiwo, O., Ejikeugwu, C. P., Igbeneghu, C., Njom, V. S., Onwude-Agbugui, M., Orji, M.-K. N., Oyinloye, F. O. P., Oyemade, E., … Anderson, T. J. C. (2026). Misleading inference of schistosome epidemiology from ribosomal internal transcribed spacer (ITS) and mitochondrial DNA. PLOS Pathogens, 22(10), e1014625. https://doi.org/10.1371/journal.ppat.1014625

Image Credits: AI Generated

DOI: 10.1371/journal.ppat.1014625

Keywords: schistosomiasis, Schistosoma haematobium, S. bovis, S. curassoni, hybridization, zoonotic transmission, ITS marker, cox1, whole-genome sequencing, introgression, Nigeria, PLOS Pathogens