Epidemiology and phylogenomic characterisation of the 2025 Ebola virus disease outbreak in Bulape, Kasaï province, Democratic Republic of the Congo: a retrospective observational study.

BACKGROUND: An Ebola virus disease (EVD) outbreak in Bulape health zone, Kasaï province, DR Congo, was declared on Sept 4, 2025, and ended on Dec 1, 2025. We describe the dynamics of this outbreak and the molecular characteristics of the causative Ebola virus variant (species Orthoebolavirus zairense).

METHODS: This retrospective observational study included probable and PCR-confirmed EVD cases reported between Aug 4 and Dec 1, 2025. Individuals who tested PCR-negative for Ebola virus were classified as discarded cases and excluded. Trained field investigators and health professionals recorded epidemiological and clinical data during initial case investigation. The primary outcome was the generation of complete Ebola virus genomes from clinical samples tested PCR-positive. Ebola virus RNA was detected by real-time PCR and subjected to whole-genome sequencing to generate complete Ebola virus genomes. Ebola virus sequences were integrated with detailed epidemiological data to infer case linkages and reconstruct transmission chains. Bayesian analysis was performed to infer time-scaled phylogenetic trees.

FINDINGS: In total, 64 EVD cases (53 confirmed and 11 probable) were reported, including 45 deaths, corresponding to a case-fatality rate of 70·3% (45 of 64, 95% CI 57·6-81·1%). The median age of affected individuals was 25 years (IQR 6-51) and 58% (37 of 64) were women. Our analysis identified five generations of transmission originating from an undetected index case, with transmission occurring predominantly in health-care settings. One inferred superspreading patient generated 11 secondary infections, indicating marked heterogeneity in early transmission. The mean serial interval was 15·7 days (95% CI 13·9-17·6 days). Two hypothetical unsampled cases were inferred within the transmission chain: one representing the superspreading patient within the chain, and another representing a case that did not lead to any further secondary human-to-human transmission. Phylogenetic analysis showed two distinct clusters, each likely arising from a single source and both sharing a recent common ancestor. The 2025 Ebola virus lineages are most closely related to genome sequences from the 1976 Yambuku-Mayinga outbreak.

INTERPRETATION: The 2025 EVD outbreak likely resulted from a novel zoonotic spillover; health care-associated transmission substantially amplified the outbreak. Delayed recognition and inadequate infection prevention and control capacity likely facilitated superspreading. Integrated genomic and epidemiological investigation strengthens transmission chain inference and supports a rapid public health response. Strengthening infection prevention and control measures in health-care facilities, alongside integrated epidemiological-genomic and ecological surveillance, rapid response, and vaccination among health-care workers, is essential to minimising the effect of future EVD outbreaks.

FUNDING: WHO; Africa Centres for Disease Control and Prevention; Culmen International LLC; and the Belgian Directorate-General for Development Cooperation and Humanitarian Aid under the extended Framework Agreement 5.