Silent WEEV Viremia Reached the Blood Supply During Argentina’s Outbreak
Western equine encephalitis virus returned to Argentina in 2023–2024 with a pattern that should concern both infectious disease clinicians and blood bankers: most infections were invisible. During the outbreak, regional hemovigilance in Córdoba identified one asymptomatic viremic donor among 637 donors residing in affected areas, proving that WEEV can enter the donation stream without warning signs and without any clinical trigger to halt collection.
The finding carries immediate transfusion-safety implications. WEEV behaves like other arboviruses that can circulate silently in humans for a short period before symptoms appear, and that window is enough for donation to occur. In a setting where the virus was actively circulating across multiple provinces, the blood supply could not rely on symptom-based deferral alone.
Outbreak Conditions Made Donor Surveillance Necessary
Argentina’s 2023–2024 outbreak was not a minor spillover event. Between epidemiologic week 48 of 2023 and week 26 of 2024, SENASA confirmed 1,529 equine outbreaks across 17 provinces, with no new equine outbreaks detected after week 16 of 2024. Human disease followed the same regional pattern: 572 suspected cases were reported across 21 provinces, 107 were confirmed, 21 were probable, and 115 were discarded. The last confirmed human cases appeared in week 15 of 2024.
Confirmed illness clustered heavily in adults of working age, the same demographic eligible for donation. Median age among confirmed cases was 58 years, with cases spanning 4 months to 81 years. Sixty percent occurred in people 50 to 69 years old, and 86% were in men. Twelve deaths were reported among confirmed cases, including fatal infections in people with rural or semirural exposure and in those with underlying conditions.
Silent transmission is the problem here. The article’s epidemiologic framing makes clear why: WEEV infections are often asymptomatic, and the estimated symptomatic-to-asymptomatic ratio in adults is about 1:1,150. In children younger than 4 years, the ratio is still striking at roughly 1:58. Those numbers mean reported encephalitis cases likely represent only a fraction of the total infections moving through communities, with transient viremia creating a narrow but consequential transfusion risk.
Córdoba’s Hemovigilance Network Used Geography as a Screening Tool
The Fundación Banco Central de Sangre in Córdoba built a response that matched the outbreak’s geography. As a provincial reference blood bank coordinating transfusion services across a territory of 165,321 km2, it already handled about half of all blood units collected in Córdoba for serologic and molecular pretransfusion screening. That scale made it an ideal sentinel site.
The intervention layered three strategies onto routine blood collection: epidemiologic mapping, enhanced donor selection criteria, and alphavirus-specific nucleic acid testing. Real-time maps were created to identify suspected WEEV hotspots, since official epidemiologic reports did not always specify the localities where cases were being detected. Blood collection teams, community promoters, and local organizers contributed field information, and donors were asked about probable cases in their communities during predonation screening. Repeated reports from the same areas were treated as hotspots, allowing the blood bank to track the outbreak’s spread in near real time.
A single asymptomatic infection in a donor can matter more than many symptomatic cases in the community because donation occurs at the exact moment the pathogen is least visible. In arboviral outbreaks, blood safety depends on finding that moving target before the blood leaves the collection center.
One Positive NAT Result Confirmed Ongoing Regional Transmission
Among 637 donors residing in outbreak-affected areas, one donor tested positive for WEEV RNA and had no symptoms. Phylogenetic analysis placed the sequence within the contemporary South American outbreak lineage, linking the donation directly to the broader regional epidemic rather than to an isolated spillover event.
The biologic meaning is straightforward. WEEV can produce a viremic phase before or without symptomatic disease, and molecular screening is the only practical way to identify that phase in donors who feel well. The positive NAT result therefore did more than identify one unsafe donation; it validated the hemovigilance model as a surveillance tool for a pathogen that moves under the clinical radar.
The study also fits a wider pattern seen with arboviruses such as West Nile virus, dengue, Zika virus, and chikungunya virus. All have shown how mosquito-borne outbreaks can threaten both the safety and availability of blood supplies. Deferral policies help, but when symptom recognition is unreliable and many infections are subclinical, molecular testing becomes the decisive barrier.
Blood Banks Can Function as Sentinel Sites for Emerging Arboviruses
This outbreak exposed a practical gap in transfusion medicine. Before the response in Córdoba, there were no laboratory screening tools or specific donor deferral criteria for WEEV. Once human cases were recognized, the blood bank had to build a response from first principles: map exposure, ask better questions, and test selected donations molecularly.
The result supports a broader public health role for hemovigilance. Blood donor surveillance did not merely protect recipients; it also offered a window into viral circulation in the community. Because donors are generally healthy adults and because donations are collected continuously across a large geographic area, blood services can detect silent arboviral activity before routine clinical reporting fully captures it.
For regions where equine encephalomyelitis viruses circulate, this approach is especially relevant. WEEV is maintained in enzootic cycles involving birds, small mammals, and mosquitoes, and spillover can appear suddenly when ecological conditions align. The abrupt reappearance in Argentina after the last human case in 1996 shows how quickly surveillance gaps can open when animal vaccination coverage falls and vector-driven transmission resumes.
The study leaves one practical question open: how widely such molecular screening should be extended during future outbreaks. Even so, the clinical message is already firm. When WEEV circulates, apparently healthy donors can still carry virus, and transfusion safety depends on finding those infections before blood is issued.