Environmental and Phylogenetic Investigations of Aspergillus flavus Outbreak Linked to Contaminated Building Materials, Denmark, 2025.

An Aspergillus flavus outbreak occurred in a tertiary hospital in Denmark. We compared environmental sampling methods, investigated the outbreak through short tandem-repeat genotyping, STRAfla, and analyzed isolate phylogeny using whole-genome sequencing. Paired sampling revealed that air sampling underestimated A. flavus burden (8 CFU/81 air samples vs. 585 CFU/81 surface samples), and culturing at 37°C was superior to 25°C (risk ratio 1.77; p<0.001). STRAfla (n = 145) confirmed clonality of the outbreak isolates. Active growth was identified in a kitchen inside the affected ward. Genetically related isolates were also found in the Department of Clinical Microbiology and in 4 unrelated wood-based building materials from retailers in Denmark. Phylogenetic analyses of 167 isolates supported introduction of A. flavus from building materials. We hypothesize that water damage enabled germination of dormant spores in precontaminated wood-based products. Our findings highlight a risk factor for outbreaks and should inform future hospital construction and infection prevention strategies.