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

## TL;DR

A hospital in Denmark experienced an Aspergillus flavus outbreak linked to contaminated building materials, with findings suggesting water damage enabled spore germination.

## Contribution

The study identifies contaminated wood-based building materials as a novel source of A. flavus outbreaks in hospitals.

## Key findings

- Air sampling underestimated A. flavus burden compared to surface sampling.
- Culturing at 37°C was more effective than at 25°C for detecting A. flavus.
- Phylogenetic analysis linked outbreak isolates to building materials from retailers.

## Abstract

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.