Placental schistosomiasis, the presence of Schistosoma haematobium eggs in placental tissue, may be more common in endemic regions than a recent diagnostic study from Gabon suggests, according to a letter to the editor published in PLOS Neglected Tropical Diseases. The letter, written by Carlotta Helbig, Tina Bohnes, Katja Schneider and Michael Baier of Jena University Hospital and the University Hospital Tübingen, responds to a cross-sectional, diagnostic proof-of-concept study conducted in Lambaréné, Gabon, and argues that the diagnostic approach used in that study likely underestimated the true prevalence of the condition.
Schistosomiasis remains a neglected tropical disease that imposes a substantial burden in endemic countries, and it is primarily endemic in sub-Saharan Africa, including Gabon, where the predominant species is S. haematobium. Pregnant women represent a significant at-risk population, because untreated infection can lead to adverse birth outcomes. It is against this background that the letter authors say they read the original Gabonese study with great interest.
The original study screened pregnant women in Lambaréné for S. haematobium infection using light microscopy of a single 10 millilitre urine sample. From this screening, the researchers established a Schistosoma-positive group of 28 women and a Schistosoma-negative group of 20 women. Placental tissue samples were obtained from all participants and examined for the presence of S. haematobium eggs by light microscopy of macerated tissue, and positive findings were confirmed by quantitative polymerase chain reaction, or qPCR.
The results of that screening were striking. Among the women with Schistosoma infection, four women, or 14 percent (4 of 28), tested positive for the presence of S. haematobium eggs in the macerated placenta sample. In the non-infected group, one woman, or 5 percent (1 of 20), also tested positive for placental schistosomiasis. All five positive results were confirmed by qPCR. By applying these prevalences to the entire cohort, the authors of the original study estimated a hypothetical placental schistosomiasis prevalence of 6.3 percent. By contrast, much higher prevalences of 22.6 percent and 19 percent have been described in the literature, a discrepancy that the letter authors say raises the question of why the prevalence in this cohort was estimated to be so much lower than in previously published data.
A central concern in the letter concerns the sensitivity of the diagnostic pathway. The letter authors acknowledge that microscopic examination of 10 millilitres of urine for S. haematobium is the diagnostic approach used in endemic regions and has been recommended by the World Health Organization. However, they point out that urine screening on three consecutive days shows higher diagnostic sensitivity and would lead to more robust and reliable results. Urine microscopy is also limited by the fact that newly acquired infections in their early stages cannot yet be detected, because of the prepatent period of 10 to 12 weeks during which eggs are not yet shed.
The letter goes on to describe alternative or complementary diagnostic approaches that may provide improved diagnostic accuracy, including antigen detection tests and molecular methods. One novel option for diagnosing S. haematobium infection described in the literature is the Schistoscope, an automated digital microscope that uses artificial intelligence. This device has been field-validated in Lambaréné, Gabon, where it showed overall non-inferior performance compared with conventional microscopy. Although specificity varied across different studies, tools like this could represent a promising option, particularly in low-resource settings, because they are less reliant on trained microscopists.
Molecular diagnostics feature prominently in the letter’s argument. The authors highlight an evaluation by Hamway and colleagues of a novel cell-free DNA qPCR assay based on 20 microlitres of plasma, which was found to have higher sensitivity than conventional diagnostic methods such as urine filtration microscopy. Such approaches, potentially even from fingerprick blood in the future, should be assessed in further studies, the letter argues. These techniques could, in principle, detect infections that single-day urine microscopy misses, thereby refining estimates of how many pregnant women in endemic areas carry the parasite.
The letter also scrutinises the interpretation of one of the study’s most intriguing findings: that 5 percent of the Schistosoma-uninfected group had placental S. haematobium eggs. The letter authors urge that this result be interpreted with caution, particularly given the very small sample size involved, since it rests on a single woman. They suggest it may reflect limitations in diagnostic sensitivity rather than true maternal infection status. In other words, some women classified as uninfected on the basis of a single urine microscopy may in fact have been infected, and the placental egg finding could be a consequence of that missed infection rather than a biological anomaly. The letter authors also suggest that it would be helpful to express placental egg counts relative to tissue weight, for example per 50 or 100 grams, in addition to the number of eggs per 10 millilitres of urine and the total number of eggs found in the placenta that the original study reported.
A further methodological point concerns the confirmatory testing strategy. Confirmatory qPCR was performed for all microscopically positive macerated placenta samples, and the results were concordant, with all samples also testing positive by qPCR. The qPCR target comprised the Dra1 motif, a highly repetitive sequence of the S. haematobium complex that also covers S. intercalatum. What remains unclear to the letter authors is why qPCR testing was not performed on placenta samples that were negative for S. haematobium eggs by microscopy. Including these samples, they argue, could have provided a more diagnostically robust and comprehensive assessment, because molecular methods likely offer generally higher sensitivity than light microscopy and would additionally allow the detection of egg debris generated through maceration that would otherwise remain unnoticed. Consequently, the letter concludes, relying solely on egg detection by light microscopy likely underestimates the prevalence of placental S. haematobium in this cohort, since microscopy-negative samples were not subsequently assessed by the more sensitive qPCR method.
Beyond diagnostics, the letter looks ahead to the clinical significance of placental schistosomiasis. In the context of adverse birth outcomes associated with the condition, the authors say it would be interesting to gather more information on key parameters such as birth weight and gestational age. If these two parameters were available retrospectively, variables like small-for-gestational-age status could be assessed or included in future meta-analyses, allowing birth outcomes to be compared between women with placental schistosomiasis and those without. Because placental tissue was examined in the study, the letter authors also suggest it would be interesting to state whether data on placental weight were obtained, since placental weight has been associated with fetal growth. Taken together, the letter frames the Gabonese proof-of-concept study as a valuable but conservative first estimate, and argues that more sensitive, repeated and molecular screening, together with richer birth outcome data, will be needed to establish the true burden of placental schistosomiasis in endemic regions.
Subject of Research: Placental schistosomiasis diagnostics and prevalence estimation in pregnant women in Gabon
Article Title: Letter to the editor: “The underestimated burden of placental schistosomiasis in endemic regions: Findings from a cross-sectional, diagnostic proof-of-concept study on placental schistosomiasis in Gabon”
Article References: Helbig, C., Bohnes, T., Schneider, K., & Baier, M. (2026). Letter to the editor: “The underestimated burden of placental schistosomiasis in endemic regions: Findings from a cross-sectional, diagnostic proof-of-concept study on placental schistosomiasis in Gabon”. PLOS Neglected Tropical Diseases, 20(10), e0014819. https://doi.org/10.1371/journal.pntd.0014819
Image Credits: AI Generated
DOI: 10.1371/journal.pntd.0014819
Keywords: placental schistosomiasis, Schistosoma haematobium, Gabon, pregnant women, neglected tropical diseases, urine microscopy, qPCR, diagnostic sensitivity, adverse birth outcomes, Lambaréné, Schistoscope, Dra1 motif
News Source: Ophelia Keating. (October 29, 2026). Placental Schistosomiasis in Gabon May Be Undercounted, Letter Warns. Scienmag.
