Aspergillus Flavus Otbreak Linked to Building Materials: Sampling and Genomic Linkage

Investigators reporting in the CDC Emerging Infectious Diseases Aspergillus flavus outbreak investigation describe using broad environmental sampling to localize contamination and identify a microscopy-confirmed plywood cupboard source, while STRAfla microsatellite typing and whole-genome sequencing were used to assess isolate relatedness and inform source hypotheses.

The investigation took place in a Danish tertiary-hospital setting involving hematology wards and other hospital areas, with sampling undertaken to map where A. flavus was present and to support subsequent typing work. Across March–June 2024, the authors report collecting 449 environmental samples from 299 locations, including paired air-versus-surface sampling across 81 paired sites and additional targeted sampling guided by initial findings. Methods included culturing paired sets of plates at 25°C (read at 7 days) and 37°C (read at 3 days) as part of a temperature comparison described by the investigators. Overall, the approach combined environmental mapping with laboratory identification and downstream typing to connect environmental and clinical isolates within the outbreak narrative.

In the paired sampling comparison, the authors report that surface sampling yielded 585 A. flavus CFU versus 8 A. flavus CFU from matched air samples, and they note that air sampling was “nonaggressive” because disturbing dust was considered unacceptable for hospital staff. For culture conditions, they report higher recovery at 37°C than at 25°C for surface samples (risk ratio 1.77; p<0.001) in their analysis. As sampling became more targeted, the investigators describe peak contamination in staff-associated areas, including a combined staff meeting/lunchroom and ward kitchen, and they report that dismantling work in a kitchenette led to identification of active A. flavus growth on a plywood cupboard, confirmed by microscopy. These observations describe how the team moved from broad paired sampling toward localization of specific reservoirs within staff and kitchen-adjacent spaces.

Genotyping and sequencing were then used to characterize relatedness and distribution of isolates across patients, hospital interiors, and materials. Using STRAfla microsatellite typing, the authors report analyzing 145 isolates and identify a dominant outbreak genotype (A) concentrated on the “outbreak floor.” Whole-genome sequencing of 167 isolates grouped the outbreak isolates into a defined phylogenetic cluster (cluster 10) that, as reported, comprised hospital isolates except for 4 isolates from wood-based building materials. The authors’ time-scaled phylogeny is presented as consistent with an earlier introduction followed by later clonal expansion, and the authors hypothesize that dormant spores in precontaminated wood products could germinate after water exposure. The article does not describe a standardized patient screening, isolation, and follow-up protocol in procedural detail, but it frames the investigation as integrating clinical and environmental isolate collection with multidisciplinary outbreak control efforts.

Key Takeaways:

• The authors report higher detection from surface sampling than from nonaggressive air sampling, and they report greater recovery at higher incubation temperature in their culture comparison.
• Peak A. flavus contamination was reported in staff- and kitchen-associated areas, and microscopy confirmed A. flavus growth on a plywood cupboard.
• Genomic results are reported as showing a dominant outbreak genotype on the outbreak floor and a cluster 10 grouping linking hospital isolates with isolates from unrelated wood-based materials, alongside the authors’ proposed introduction-and-expansion scenario.