The 2024 Oropouche epidemic attracted worldwide attention for several reasons. The virus expanded geographically into regions where it had never before been detected, the number of reported cases rose substantially, and the first globally documented Oropouche-associated fatalities occurred. In parallel, several studies provided evidence of vertical transmission and raised concerns about the potential impact of maternal infection on fetal development and survival. Notably, pregnancy losses among Oropouche-infected women were reported in Brazil as far back as the 1980s, predating the current wave of warnings by decades. What has been missing, the authors argue, is a systematic strategy to move from suggestive case reports to robust causal inference.

The evidence that currently fuels concern comes largely from individual case reports. Researchers have described Oropouche virus RNA or antigens in placental and fetal tissues, and anti-Oropouche IgM antibodies have been detected in microcephalic newborns. These findings support the plausibility of vertical transmission, but they fall short of demonstrating that the virus actively replicates in fetal tissue, which would substantially strengthen the biological case for fetal injury. The authors emphasize that viral infections can damage tissue indirectly, for example through immune imbalance, yet direct evidence of replication remains the gold standard for establishing pathogenicity.

To close that gap, the framework proposes molecular techniques capable of detecting positive-sense viral RNA, the form of viral genetic material produced during active replication. Strand-specific reverse transcription PCR assays, designed to target complementary RNA and viral messenger RNA rather than the negative-sense genomic RNA packaged in virions, could provide evidence consistent with active Oropouche replication in placental or fetal samples. Complementing this, signal-amplified in situ hybridization methods such as RNAscope coupled with confocal microscopy, or PrimeFlow RNA assays combined with flow cytometry, could localize viral replication within tissues and identify which specific cell populations harbor replicating virus. Histopathological and immunohistochemical analyses using antibodies against non-structural viral proteins expressed only during replication could further link viral activity to tissue damage.

Animal models form the second pillar of the proposed framework. Previous studies have demonstrated vertical transmission of Oropouche virus and fetal impairment in mice, including infection with an ancestral viral strain. However, these results were consistently observed only in animals with impaired type I interferon signaling, suggesting that vertical transmission in this setting is context-dependent and limiting extrapolation to natural human infection. The authors point instead to golden hamsters, which have been proposed as a model for Oropouche pathogenesis and may represent a promising in vivo system for investigating fetal effects without genetic or immunological manipulation. They also suggest examining SJL mice, a strain likely more susceptible to viral infection, which gained attention during the Zika epidemic for their permissiveness to viral teratogenicity studies.

Non-human primates offer a third experimental avenue. These animals have been used successfully to investigate virus-associated fetal outcomes for other pathogens, including Zika virus and congenital cytomegalovirus, and could prove suitable for Oropouche studies. Intrauterine infection of immunocompetent animals would provide a proof-of-concept approach to evaluate the direct fetal consequences of Oropouche infection, a strategy previously applied to teratogenic viruses including Zika and Cache Valley virus, an orthobunyavirus in the same genus as Oropouche. Intracranial inoculation in neonatal animal models and embryonated chicken eggs could also be employed, as these approaches were previously used to characterize the teratogenic potential of other orthobunyaviruses such as Aino, Akabane and Cache Valley viruses.

The authors stress that any such experiments should use both historical and recently circulating Oropouche strains to detect possible differences in fetal effects between lineages, and should employ minimally passaged viruses to avoid adaptive mutations that would distort the wild-type phenotype. Testing at different stages of gestation would further clarify whether particular windows of pregnancy carry heightened risk, a question of direct clinical relevance for counseling pregnant women in endemic areas.

Viral reverse genetics represents a particularly promising tool within the framework, especially for probing the role of the type I interferon response. Because in vivo studies have shown possible vertical transmission only in animals lacking an adequate interferon response, and because interferon antagonism is an established pathogenicity mechanism of orthobunyaviruses, including the teratogenic Schmallenberg virus, the Oropouche NSs gene, which encodes a known type I interferon antagonist, becomes a prime suspect. Reverse genetics could be used to manipulate this gene, identify the residues and domains involved in interferon antagonism, and test whether these functions contribute to adverse fetal outcomes. Such findings could also inform genomic surveillance by identifying molecular signatures in circulating strains that are potentially associated with fetal injury.

On the epidemiological side, the framework calls for well-designed case-control studies, modeled on those conducted during the Zika epidemic, to estimate the likelihood of miscarriage or developmental abnormalities among Oropouche-infected pregnant women and to identify other risk factors, such as the most vulnerable gestational period. Prospective postnatal follow-up of exposed newborns would be timely for investigating potential later neurodevelopmental effects, including those occurring in the absence of microcephaly, a pattern documented in congenital Zika infection. Close immunological monitoring of infected pregnant women, focusing on type I and type III interferons and interferon-stimulated genes such as IFIT1, could reveal host factors that modulate vertical transmission. The authors acknowledge real obstacles: infections outside outbreak periods are rarely reported, outbreaks have historically been concentrated in remote areas, and the recent decline in transmission across several affected regions may limit participant recruitment. Multicenter studies and prospective surveillance strategies, they argue, are the practical answer.

The overarching message is one of scientific caution paired with methodological ambition. Despite recent advances in understanding Oropouche virus, its potential for vertical transmission and adverse fetal outcomes remains unproven, and establishing causality will require the deliberate integration of molecular virology, animal modeling, reverse genetics and epidemiology. The authors contend that such efforts are essential not only for resolving a pressing scientific uncertainty but also for improving surveillance strategies, guiding clinical management of pregnancies in endemic regions, and mitigating whatever impact Oropouche infection may ultimately prove to have on fetal health. As the virus continues to expand its footprint in the Americas, the framework offers a roadmap for converting alarming anecdotes into actionable evidence.

Subject of Research: Assessment of potential Oropouche virus-associated adverse fetal outcomes through experimental and epidemiological approaches

Article Title: A framework for assessing potential Oropouche virus-associated adverse fetal outcomes

Article References: Tanaka, L. F., Dutra, N. B. D. M., Lopes, T. R. R., Carmo, R. F., Oliveira-Filho, E. F. D., Gil, L. H. V. G., & Silva Júnior, J. V. J. (2026). A framework for assessing potential Oropouche virus-associated adverse fetal outcomes. BMC Infectious Diseases, 26(1), Article 1739. https://doi.org/10.1186/s12879-026-14420-1

Image Credits: AI Generated

DOI: 10.1186/s12879-026-14420-1

Keywords: Oropouche virus, vertical transmission, adverse fetal outcomes, microcephaly, pregnancy, orthobunyavirus, type I interferon, reverse genetics, animal models, case-control studies, placental infection, neurodevelopment

The study, published in Virology Journal, focused on Culex pipiens sensu lato and Culiseta annulata, two species that dominate the UK mosquito fauna and are considered among the most plausible vectors should arboviruses such as West Nile or Usutu virus establish local transmission cycles. Between 2021 and 2022, the team collected adult mosquitoes across the grounds of the two zoological collections, sites chosen deliberately because zoos concentrate large numbers of animals in outdoor enclosures where mosquitoes feed readily. That combination of abundant vectors, diverse vertebrate hosts and regular human visitors makes zoological collections potential interfaces for virus exchange between mosquitoes, animals and people, and therefore valuable sentinel locations for surveillance.

In total, 4,042 mosquitoes were processed and sequenced across 44 pools. The researchers extracted total nucleic acids and applied a metagenomic sequencing strategy designed to capture both RNA and DNA viruses, allowing them to recover viral genomes without relying on prior assumptions about which viruses might be present. Bioinformatic assembly and taxonomic classification of the resulting reads yielded 26 viral genomes, a striking haul for a country whose mosquito viromes have remained comparatively understudied. Nine of these represented entirely novel species, distributed across both RNA and DNA virus families, underscoring how much basic viral diversity in northern European mosquitoes still awaits description.

A consistent pattern emerged across the sampling sites: several viruses were detected in every pool analysed, indicating substantial overlap in virome composition between the two zoos. This shared core of viruses suggests that certain mosquito-associated viruses are widespread and stable components of the local mosquito fauna rather than isolated curiosities confined to a single habitat. Such ubiquity matters for risk assessment, because a virus that maintains itself efficiently across geographically separated mosquito populations has, by definition, the ecological flexibility that emergence requires, even if most of these viruses ultimately prove harmless to vertebrates.

The headline findings are two novel orthobunyaviruses, named Atherstone virus and Deva virus, belonging to the family Peribunyaviridae. Orthobunyaviruses are a genus with a serious pedigree: it includes recognised human and animal pathogens such as Schmallenberg virus, which devastated European ruminant farming after its emergence in 2011, and Oropouche virus, a growing public health concern in the Americas. The tripartite RNA genomes of orthobunyaviruses encode a viral RNA-dependent RNA polymerase on the large segment, surface glycoproteins on the medium segment, and nucleoprotein plus accessory proteins on the small segment. Finding two previously unknown members of this genus in British mosquitoes immediately elevates them to candidates for closer scrutiny.

Not all orthobunyaviruses pose equal risk, and the study went beyond simple detection to ask which of the two new viruses resembles a genuine arbovirus, capable of replicating in both mosquitoes and vertebrate hosts. The team employed MosViR, a machine-learning classification tool trained on known mosquito viruses to predict whether a given virus is likely to be mosquito-restricted or arthropod-borne. For Atherstone virus, detected at both zoos, the prediction pointed towards arbovirus status, a result that alone would justify follow-up. More compelling still was the genomic evidence: Atherstone virus carries an open reading frame encoding a non-structural protein S, or NSs, a well-characterised virulence factor in orthobunyaviruses that antagonises host antiviral responses, including the interferon system, and is a hallmark of pathogenic members of the genus.

Deva virus tells a different story. Its small segment lacks an NSs open reading frame altogether, and MosViR did not predict it to be an arbovirus. The absence of NSs, combined with the absence of a vertebrate-infecting signature, is consistent with Deva virus being an insect-specific virus, locked into replication within mosquito cells and their close relatives. Insect-specific viruses of this kind are increasingly recognised as common and sometimes abundant residents of mosquito viromes. Although they do not infect vertebrates directly, they are far from biologically irrelevant: they can modulate mosquito immunity, compete with arboviruses for cellular resources, and in some cases reduce the ability of mosquitoes to transmit pathogenic viruses, making them subjects of interest both for basic virology and for potential interference-based control strategies.

The broader viral inventory recovered in the study reinforces this picture of a layered virome. Alongside the orthobunyaviruses, the sequencing identified viruses spanning multiple families, including flavivirus-like and other RNA virus groups as well as DNA viruses, with several classified as novel species. Distinguishing arboviruses from insect-specific viruses is one of the central analytical challenges in mosquito metagenomics, because both can be present in the same sample and their genomes can be difficult to interpret from sequence data alone. Tools such as MosViR, combined with comparative genomics against reference databases and catalogues of known arboviruses, provide a probabilistic framework for triage, flagging which discoveries warrant experimental characterisation in cell culture and vertebrate systems and which can be provisionally set aside as likely insect-restricted.

Why does this matter for the United Kingdom specifically? The country sits at the northern edge of the range of many mosquito species, and climate warming, changes in land use and the movement of animals and goods are all expected to alter the dynamics of mosquito-borne disease in temperate Europe. Usutu virus has already established itself in British birds and mosquitoes, and West Nile virus circulates periodically in neighbouring countries. Risk frameworks built around these known threats are essential, but the new study argues that they should be complemented by surveillance for the unknown: viruses already resident in local vectors whose properties have never been tested. A virus that today appears to be insect-specific could, through reassortment with a related pathogen or through evolutionary change, acquire new capabilities. Orthobunyaviruses are particularly notable in this respect because their segmented genomes can reassort when mosquitoes are co-infected with related viruses, generating novel combinations with unpredictable host ranges.

The authors are careful not to overstate the immediate danger. Detection of a viral genome in a mosquito does not demonstrate that the virus infects or harms vertebrates, and neither Atherstone nor Deva virus has been shown to cause disease. What the study provides is a prioritised list of candidates and a demonstration of method: that large-scale metagenomic surveillance of UK mosquitoes is feasible, productive and capable of revealing both the diversity of local viromes and the specific viruses that merit laboratory follow-up. Isolation of Atherstone virus, characterisation of its host range, and serological surveys of animals and humans near the sampling sites would be the logical next steps. In the meantime, the work adds the United Kingdom to the growing map of regions where the viruses closest to home, rather than those arriving from abroad, are proving to be the first order of business for emerging infection surveillance.

Subject of Research: Metagenomic surveillance of mosquito viromes at two UK zoos reveals novel mosquito-associated orthobunyaviruses

Article Title: Emergence risks from within? Metagenomic analysis of mosquito viromes from two zoos reveals mosquito-associated orthobunyaviruses in the UK

Article References: Pilgrim, J., Seechurn, N., Cunningham-Oakes, E., Gillespie, L., Dobbs, P., Lopez, J., Kohl, A., Hughes, G. L., Blagrove, M. S. C., Baylis, M., & Darby, A. C. (2026). Emergence risks from within? Metagenomic analysis of mosquito viromes from two zoos reveals mosquito-associated orthobunyaviruses in the UK. Virology Journal. https://doi.org/10.1186/s12985-026-03305-z

Image Credits: AI Generated

DOI: 10.1186/s12985-026-03305-z

Keywords: orthobunyavirus, mosquito, virome, metagenomics, arbovirus, Culex pipiens, Culiseta annulata, surveillance, zoonotic risk, insect-specific virus, United Kingdom, Peribunyaviridae