Regional signals preceding the 2026 Bundibugyo virus disease outbreak

Abstract

The Bundibugyo virus circulated undetected for months before outbreak declaration.•Four earlier regional hemorrhagic fever clusters flagged by open surveillance are unresolved.•These clusters warrant urgent reanalysis due to concern for regional spread. The May 2026 Bundibugyo virus disease outbreak in the Democratic Republic of the Congo (DRC) was declared a Public Health Emergency of International Concern after substantial undetected community transmission. We describe regional surveillance signals reported by the Biothreats Emergence, Analysis, and Communications Network (BEACON), our open-access event-based surveillance program, in the weeks preceding outbreak declaration. We reviewed BEACON reports of viral hemorrhagic fever (VHF)–compatible illness clusters detected in the transboundary DRC-Uganda-Burundi-South Sudan region during March-April 2026, before the May 15 laboratory confirmation of Bundibugyo virus (BDBV). BEACON detected four temporally proximal viral hemorrhagic fever (VHF) compatible illness signals: (i) March 9, North Kivu Province—suspected Ebola case under investigation with unresolved laboratory results; (iii) March 10, Kasaï Province—fatal hemorrhagic illness with secondary cases and negative Ebola polymerase chain reaction; (iii) March 30, Burundi—35 cases of undiagnosed cluster near the DRC border with five deaths, negative testing for major filoviruses and >200 pathogens, pending metagenomic sequencing; and (iv) April 22, South Sudan—three suspected VHF cases with negative initial testing. All four signals shared a similar diagnostic phenotype: VHF-compatible presentation, mobilization of investigation teams, negative initial testing, and no publicly reported confirmed etiology. None were formally reported to have been resolved. Our detection of four unresolved VHF like signals preceding the confirmed BDBV outbreak highlights possible gaps in follow-up mechanisms for cases negative for common pathogens. In light of confirmed BDBV circulation and Africa Centers for Disease Control’s identification of 10 countries at high risk for spread, these preceding signals warrant urgent retrospective investigation.