Asymptomatic Oropouche Virus Spread: Molecular Evidence in Manaus Blood Donors During Recent Outbreak

Understanding the Groundbreaking Discovery in Manaus

The recent publication in the Journal of Infectious Diseases has shed new light on the silent spread of Oropouche virus (OROV), an arbovirus endemic to the Amazon basin. Researchers screened blood donations from Manaus, the capital of Amazonas state in Brazil, during the peak of the 2023-2024 outbreak, uncovering molecular evidence of asymptomatic circulation. This finding underscores the virus's ability to propagate undetected, posing risks to public health and blood supply safety. 0 72

Manaus, a bustling hub in the Brazilian Amazon with over 2 million residents, serves as a critical node for arbovirus transmission due to its dense urban-rural interface and tropical climate. The study highlights how everyday activities like blood donation can reveal hidden epidemics, emphasizing the need for vigilant surveillance in higher education-driven research initiatives at institutions like the Universidade Federal do Amazonas (UFAM).

What is Oropouche Virus and How Does It Spread?

Oropouche virus (OROV), belonging to the Orthobunyavirus genus in the Peribunyaviridae family, causes Oropouche fever—a self-limiting illness mimicking dengue or Zika. First identified in 1955 in Trinidad and Tobago, it primarily transmits via bites from the midge Culicoides paraensis, with secondary roles for Culex quinquefasciatus mosquitoes. Symptoms include abrupt fever (38-40°C), severe headache, chills, myalgia, arthralgia, and retro-orbital pain, lasting 2-7 days. Severe cases, though rare, involve meningitis or encephalitis. 13

Unlike symptomatic cases, asymptomatic infections evade clinical detection, facilitating community spread. Vertical transmission from mother to fetus has been documented, leading to fetal death or microcephaly. In Brazil's Amazon, where midges thrive in peridomestic environments, urban outbreaks amplify risks. 72

The Scale of the 2023-2024 Oropouche Outbreak in Brazil

Brazil witnessed OROV's largest recorded outbreak from late 2023 to 2024, with over 26,000 confirmed cases by December 2025. Amazonas reported thousands, Manaus being the epicenter. By mid-2025, national cases exceeded 11,800, spilling to 18 states. 62 63 A reassortant lineage (genotype III) drove this surge, linked to neurologic issues and congenital anomalies.

• 2023: Initial re-emergence post decades of quiescence.
• 2024: Explosive growth, 81.8% of cases since 2015.
• 2025: Continued transmission, 11,888 cases Jan-Jul. 30

Underreporting is rampant; seroprevalence suggests true burden 10-20x higher.

Study Design: Screening Blood Donors for Silent Infections

Led by Pamela Milani from Vitalant Research Institute and collaborators including Brazilian experts from Fundação Hospitalar Amazonas, the team implemented nucleic acid testing (NAT) on minipools of 18 donations. From November 2023 to May 2024, 661 pools (11,898 donations) were tested via validated RT-qPCR. Reactive pools underwent individual donor tracing, whole-genome sequencing, and phylogenetic analysis. Serosurveys bookended the period to gauge immunity shifts. 72

This prospective surveillance mirrored blood bank protocols, providing real-world viremia data. Ethical approvals ensured donor confidentiality, aligning with Brazil's robust research governance.

Key Molecular and Serological Findings

Of 661 minipools, 43 (6.5%) were OROV RNA-positive, peaking at 20% in January 2024. Half exceeded the quantifiable limit, indicating high-titer viremia suitable for transmission. Sequenced genomes clustered with Brazil's reassortant lineage, confirming outbreak linkage.

Seropositivity surged from 13.0% (Nov 2023) to 29.7% (Jun 2024), estimating ~390,000 infections in Manaus—far exceeding notified cases. The RNA-detectable window averaged 5.4 days (95% CI: 3.8-7.0), informing incidence models. 72

Photo by LOGAN WEAVER | @LGNWVR on Unsplash

• NAT yield: 6.5% pools reactive.
• Seroincidence: ~17% rise.
• Genomic identity: Matches clinical strains.

Implications for Blood Transfusion Safety in Brazil

Asymptomatic viremia raises transfusion-transmitted infection (TTI) concerns, though no cases confirmed yet. High viral loads mirror risks seen in Zika/dengue. Brazil's blood centers, screening for HIV/hepatitis, now face emerging arbovirus threats. Enhanced NAT or pathogen reduction technologies may be needed. 72

Fundação Pro-Sangue and regional banks in Amazonas exemplify adaptive responses. For researchers, this signals demand for higher ed jobs in transfusion medicine and virology.

Public Health Challenges and Underestimated Burden

The study reveals massive underascertainment; symptom-based surveillance misses asymptomatics fueling chains. Manaus' 390,000 estimated cases imply herd immunity thresholds nearing, yet reassortment risks evolution. Climate change expands midge habitats, threatening urban sprawl. 62

Stakeholders: Ministry of Health, Fiocruz, UFAM urge integrated vector control, vaccines. PAHO warns of regional spread.PAHO update

Brazilian Universities and Fiocruz Driving OROV Research

Fiocruz Amazonas leads genomic surveillance, partnering with UFAM and UNICAMP. These institutions train postdocs, PhDs in arbovirology—vital amid outbreaks. Recent Fiocruz studies confirm vertical transmission, bolstering evidence. 83

• UFAM: Epidemiological modeling.
• Fiocruz: Sequencing hubs.
• USP/UNICAMP: Immune response insights.

Explore university jobs in Brazil's tropical medicine programs.

International Collaboration and Lessons Learned

US-Brazil ties, via Vitalant/UCSF and local partners, exemplify global health networks. Phylogenetic tools traced lineages, informing policy. Future: Pan-American surveillance, midge control trials.

Future Outlook: Vaccines, Surveillance, and Research Needs

No vaccines exist; candidates target conserved epitopes. AI-driven epidemiology, from Brazilian unis, promises early warnings. Careers in higher ed career advice for virologists abound. Proactive measures: Repellents, screens, blood NAT.

Photo by Leo_Visions on Unsplash

Opportunities in Arbovirus Research for Brazilian Academics

This publication spotlights demand for experts in molecular epidemiology. Institutions like Fiocruz offer postdoc positions; UFAM seeks lecturers. Aspiring researchers, check rate my professor for mentors. Brazil's research ecosystem thrives on such breakthroughs.