A Fever Nobody Worried About, and the Questions It Now Raises for Pregnancy

A plain-language guide to four 2026 studies from Brazil, the United States, the United Kingdom and France on Oropouche virus, the placenta and the developing brain

Oropouche virus is an insect-borne infection spreading through South America and the Caribbean that was long considered a mild week-long fever. Four laboratory studies published in 2026 show that it can cross the placenta and damage the cells that build a baby's brain. Here is what that research found, what it does not yet prove, and what it means for families.

A virus that was easy to overlook

Oropouche fever has been known since the 1950s. It is spread not by mosquitoes but by a tiny biting insect called a midge, which lives in and around the Amazon basin. Someone bitten typically develops a fever, headache, aching joints and sometimes a rash, feels unwell for about a week, and recovers. For sixty years that was the whole story. A review gathering every published study of the virus from 1975 through 2025 found almost nothing about its effects on the brain and nothing at all about pregnancy [1] — not because researchers had looked and found nothing, but because nobody had looked.

Part of the reason is that the virus circulated where outbreaks are hard to count, and an illness lasting a week rarely sends anyone to a research laboratory. But part of it is a pattern public health has seen before: a virus is considered harmless right up until the number of people infected becomes large enough for its rarer consequences to become visible. That is what happened with Zika before 2015.

What changed

Starting in late 2023, Oropouche began behaving differently. Scientists studying viruses in the Brazilian Amazon found that a new version had emerged, formed when two related viruses infected the same host and swapped pieces of their genetic material — a process called reassortment [2]. This new version spread out of the Amazon toward Brazil's crowded Atlantic coast, then reached Cuba and the Dominican Republic. By 2024, health authorities in the United States were identifying cases in travellers returning from affected countries [3], and cases also turned up in Canada and Europe.

As infections grew, doctors in Brazil began reporting something troubling. Letters and then a formal case series described pregnancies that ended in miscarriage or stillbirth, and babies born with microcephaly — a head smaller than expected, usually meaning the brain did not grow normally — in whom evidence of Oropouche virus could be found [4], [5]. In one of the saddest cases, a 47-day-old infant died, and examination of the brain showed the virus itself in the tissue. A review pulled these reports together and asked whether Oropouche belonged on the list of infections known to harm a pregnancy [6].

But case reports, however worrying, cannot answer the key question. Finding a virus in a baby who also has microcephaly does not prove the virus caused it. To move from suspicion to understanding, researchers needed to test the biology directly. That is what the four 2026 studies did.

Testing whether the virus can get through

Two of the studies used pregnant mice. In the first, from the University of Campinas in Brazil working with Washington University School of Medicine in St. Louis, researchers infected pregnant mice and then carefully switched off different parts of the immune system to see which one was holding the virus back [7].

The answer was clear and somewhat surprising. What protected the babies was not their own immune defences — it was the mother's. A group of immune signals called type I interferons, an early alarm system against viruses, kept the virus largely confined to the mother. When researchers blocked those signals in the mother, virus in the placenta rose roughly eighty-fold and appeared in the fetus. When they removed the same signals from the fetus but left the mother's intact, nothing changed. In plain terms: a healthy maternal immune response is the main thing standing between the virus and the baby.

The second mouse study, from Indiana University School of Medicine working with the MRC–University of Glasgow Centre for Virus Research, asked a different question: is this a new danger, or an old one that was simply never noticed [8]? They compared the original 1960 strain with a recent strain from the 2024 Cuban outbreak. Both infected the placenta. The older strain reached high levels in placental tissue, concentrated in the layer where the mother's and baby's blood supplies come closest together, and live virus could be recovered from it. The newer strain infected placentas more often in early pregnancy, but the two were similar in human placental cells grown in the laboratory.

This matters more than it may sound. The ability to reach the placenta is not something the virus recently acquired — which suggests that congenital Oropouche infection may have been happening quietly, in places with limited testing, for a very long time.

That same study found something hopeful too. Mouse pups born to mothers infected during pregnancy carried protective antibodies from their mothers. The protection was real but short-lived: strongest in the first week of life and fading by two weeks. If the same is true in humans, a vaccine given to mothers might one day protect newborns during their most vulnerable period.

What the virus does to brain tissue

The other two studies asked what happens if the virus reaches the developing brain. Both grew miniature versions of early human brain tissue in the laboratory from stem cells. These "brain organoids" are pea-sized clusters of real human cells that organise themselves in ways resembling the earliest weeks of brain development. They are not brains — no blood supply, no immune cells, no thoughts — but they contain the actual cell types a fetal brain is built from.

A team at Université Paris Cité and Inserm in France found that human neural progenitor cells — the stem cells that divide over and over to produce every neuron in the cortex — were extremely susceptible [9]. They were infected more readily than a laboratory cell line specifically chosen because it has almost no antiviral defences. In organoids, the virus spread through the progenitor cells, and within four days the delicate rosette-shaped structures that mark healthy early brain tissue had broken apart; within eight days, organised structure was gone entirely. Cell division dropped dramatically — one measure fell nearly fourteen-fold — and markers of cell death rose sharply. Younger organoids were hit harder than older ones, suggesting the earliest weeks of pregnancy carry the greatest risk.

A team at the D'Or Institute for Research and Education and the Federal University of Rio de Janeiro in Brazil reached the same conclusion using a recent Brazilian strain alongside the original [10]. Their organoids grew less, their internal architecture collapsed, and their stem-cell populations were depleted. Looking at which proteins the infected tissue lost, they found two things missing together: collagens, the scaffolding that holds brain tissue in shape, and a control protein called EMX2 that tells the developing cortex how to organise itself. Losing the scaffolding and the blueprint at once is a plausible explanation for why a brain would end up smaller.

This pattern of damage closely resembles what Zika virus does, which is how Zika causes microcephaly [11]. The similarity is part of why researchers are taking Oropouche seriously.

What this does — and does not — mean for families

It is worth being precise about the limits here, because uncertainty cuts both ways.

These studies show that the virus can reach the placenta and can damage developing brain tissue. They do not show how often that happens in a real pregnancy. Notably, in both mouse studies, even when virus reached the fetus, the pups were born looking normal. Infection and harm are not the same thing, and most infections in most pregnancies may well cause no damage at all.

There is also no test result that currently predicts an outcome, no treatment, and no vaccine. Testing for the virus is available mainly through public health or specialist laboratories rather than routine hospital tests, and the blood test can be confused by related viruses.

What is reasonable, and useful, is this. If you are pregnant and have travelled to Brazil, Cuba, Peru, Bolivia, Colombia or the Dominican Republic — or live in an affected area — tell your doctor, and mention any fever you had. Prevention is entirely about avoiding bites: the midges that carry this virus can pass through ordinary mosquito netting, so insect repellent, covering skin and avoiding dawn and dusk outdoors matter more than a bed net. And if a baby is born with an unexplained small head after such an exposure, Oropouche is now a reasonable thing to test for.

Breastfeeding deserves a specific mention, because families ask. Researchers have looked at whether the virus might pass through breast milk, but so far only by reasoning from related viruses rather than by measuring milk directly [12]. Until that measurement exists, this is a conversation to have with your own clinician, weighing the substantial and well-established benefits of breastfeeding on the other side.

What researchers are working on next

The single most important study has not been done yet: following a large group of pregnancies with confirmed Oropouche infection from start to finish, with careful ultrasound scans, testing at birth, and developmental follow-up over the first years of life. That is the study that turned Zika from a frightening series of anecdotes into a real number doctors could give families, and it is what is needed here.

Alongside it, scientists are working on vaccines, on better and more widely available tests, on measuring whether the virus is present in breast milk, and on figuring out which weeks of pregnancy carry the most risk. There is also an open question the current studies disagree about: whether the newer versions of the virus are actually more dangerous to a fetus, or simply better at spreading between people. One study found the newer strain reached placentas more often [8]; the other found it infected brain stem cells less efficiently than the original [10]. The most likely explanation is that today's virus spreads more easily but is not fundamentally more harmful — meaning the rise in reported cases reflects a bigger outbreak, not a crueller virus [13].

That is genuinely reassuring in one respect: outbreaks can be controlled. And it is a reminder of how much has been learned in a very short time about a virus that, two years ago, almost nobody was studying at all.

References

  1. Wang X, Ding Y, Duarte-Neto AN, et al. Comprehensive analysis of epidemiological and clinical features of Oropouche virus infection (1975 to 2025): a systematic review. New Microbes New Infect. 2026;70:101725. doi:10.1016/j.nmni.2026.101725 ↩
  2. Naveca FG, de Almeida TAP, Souza V, et al. Human outbreaks of a novel reassortant Oropouche virus in the Brazilian Amazon region. Nat Med. 2024;30(12):3509–3521. doi:10.1038/s41591-024-03300-3 ↩
  3. Morrison A, White JL, Hughes HR, et al. Oropouche virus disease among U.S. travelers — United States, 2024. MMWR Morb Mortal Wkly Rep. 2024;73(35):769–773. doi:10.15585/mmwr.mm7335e1 ↩
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  5. das Neves Martins FE, Chiang JO, Nunes BTD, et al. Newborns with microcephaly in Brazil and potential vertical transmission of Oropouche virus: a case series. Lancet Infect Dis. 2025;25(2):155–165. doi:10.1016/S1473-3099(24)00617-000617-0) ↩
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  7. Muraro SP, de Souza GF, Alippe Y, et al. Type I interferon controls vertical transmission and fetoplacental infection of Oropouche virus. iScience. 2026;29(2):114647. doi:10.1016/j.isci.2026.114647 ↩
  8. Gunter KB, Bowen JM, Clarke AT, et al. Conserved pathogenesis of ancestral and contemporary Oropouche virus strains in a murine pregnancy model. Nat Commun. 2026;17(1):6542. doi:10.1038/s41467-026-72711-2 ↩
  9. Albert A, Couture L, Piumi F, et al. Oropouche virus infects human neural progenitor cells and alters the growth of brain organoids. iScience. 2026;29(3):114957. doi:10.1016/j.isci.2026.114957 ↩
  10. Brum G, Grizente V, Lima Monteiro FL, et al. Microcephaly-like phenotype triggered by novel reassortant and prototypic Oropouche virus strains in brain organoids. eBioMedicine. 2026;130:106408. doi:10.1016/j.ebiom.2026.106408 ↩
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  13. Scachetti GC, Forato J, Claro IM, et al. Re-emergence of Oropouche virus between 2023 and 2024 in Brazil: an observational epidemiological study. Lancet Infect Dis. 2025;25(2):166–175. doi:10.1016/S1473-3099(24)00619-400619-4) ↩