WEEV blood donor finding adds to Argentina outbreak picture

A newly published Emerging Infectious Diseases report adds an important new layer to the story of western equine encephalitis virus in Argentina: during the 2023–2024 outbreak, investigators detected WEEV in an asymptomatic blood donor living in an outbreak hotspot in Córdoba. The team screened donors in affected areas using epidemiologic mapping, enhanced donor selection criteria, and alphavirus-specific nucleic acid testing, then confirmed that the donor-derived sequence matched the lineage circulating in the broader South American outbreak. (wwwnc.cdc.gov)

That finding comes after WEEV’s striking return in late 2023, when Argentina reported equine cases after a long quiet period and then began documenting human infections. WHO said the first human case notified in December 2023 was the country’s first reported human WEE case in more than two decades, while CDC notes the regional outbreak in Argentina and Uruguay ended in April 2024. In horses, the event was substantial: Argentine animal health authorities logged more than 1,200 equine outbreaks by early January 2024, and later research described the outbreak as occurring after a nearly 40-year intermission period in Argentina’s horse population. (who.int)

The blood donor study was carried out through the Fundación Banco Central de Sangre in Córdoba, which coordinates pretransfusion screening for a large share of the province’s blood supply. After a human WEEV case was reported in Córdoba on January 26, 2024, the team identified suspected circulation hotspots and ran interventions from February 5 through the first week of April. During that period, 4,048 people donated blood, and 637 of them lived in mapped hotspot areas. Those hotspot-area samples underwent alphavirus NAT screening, which identified one viremic donor. Researchers said the viral sequence was most similar to strains reported from Argentina and Uruguay during the same outbreak and found no evidence of introduction from North America or other continents. (wwwnc.cdc.gov)

The broader outbreak context helps explain why this matters to veterinary readers. PAHO reported 1,258 animal outbreaks in Argentina and 56 in Uruguay as of January 9, 2024, alongside 21 confirmed human cases, including one death, all in Argentina. A separate molecular epidemiology study later summarized the South American outbreak as causing 112 human and 127 equine laboratory-confirmed cases in Argentina and Uruguay between November 18, 2023, and April 6, 2024. Meanwhile, a horse-focused study from Argentina found confirmed equine infections in working horses with no history of prior vaccination, underscoring the consequences of immunity gaps during reemergence. (paho.org)

The study authors frame donor surveillance as more than a blood-bank issue. They argue that community-informed hemovigilance helped surface local transmission signals that were not fully captured in official reports, and that donor interviews added useful on-the-ground intelligence about probable cases in affected communities. That perspective lines up with broader international discussion of equine encephalitides as a One Health problem: WOAH has been updating standards for equine encephalitides with an emphasis on surveillance and public health protection, and another 2026 EID report noted that Argentina reinstated mandatory vaccination against eastern and western equine encephalitis in January 2024 after declaring an animal health emergency. (wwwnc.cdc.gov)

Why it matters: For veterinary professionals, the key takeaway is that horses may again serve as an early warning system for risks that extend into human healthcare infrastructure. The Argentina outbreak showed how quickly WEEV activity could move across equine populations and cross into human disease, while this donor study suggests asymptomatic human infections can occur in the same transmission landscape. That strengthens the case for rapid neurologic disease workups in horses, prompt reporting, vaccination where recommended, vector control, and tighter communication among equine veterinarians, public health officials, and blood services during arboviral events. (ri.conicet.gov.ar)

It also raises practical surveillance questions. If equine outbreaks can coincide with silent human viremia, veterinary field reports may help trigger earlier risk assessments for transfusion safety, especially in rural areas where people and horses share mosquito exposure. That’s an inference from the available evidence rather than a direct recommendation from regulators, but it’s consistent with the study’s emphasis on adaptive, local, context-informed interventions. (wwwnc.cdc.gov)

What to watch: The next step is whether public health and animal health agencies build these lessons into standing outbreak playbooks, including clearer triggers for equine vaccination campaigns, vector surveillance, and temporary blood donor screening or deferral measures when WEEV reappears. (wwwnc.cdc.gov)