Refinements of environmental assessment during an outbreak investigation of invasive aspergillosis in a leukemia and bone marrow transplant unit

OBJECTIVES: To investigate an outbreak of aspergillosis in a leukemia and b one marrow transplant (BMT) unit and to improve environmental assessment st rategies to detect Aspergillus. DESIGN: Epidemiological investigation and detailed environmental assessment . SETTING: A tertiary-care university hospital with a 37-bed leukemia and BMT unit. PARTICIPANTS: Leukemic or BMT patients with invasive aspergillosis identifi ed through prospective surveillance and confirmed by chart review. INTERVENTIONS: We verified the diagnosis of invasive fungal infection by re viewing medical charts of at-risk patients, performing a case-control study to determine risk factors for infection, instituting wet mopping to clean all floors, providing N95 masks to protect patients outside high-efficiency particulate air (HEPA)-filtered areas, altering traffic patterns into the unit. and performing molecular typing of selected Aspergillus flavus isolat es. To assess the environment, we verified pressure relationships between t he rooms and hallway and between buildings, and we compared the ability of large-volume (1,200 L) and small-volume (160 L) air samplers to detect Aspe rgillus spores. RESULTS: Of 29 potential invasive aspergillosis cases, 21 were confirmed by medical chart review. Risk factors for developing invasive aspergillosis i ncluded the length of time since malignancy was diagnosed (odds ratio [OR], 1.0; P=.05) and hospitalization in a patient room located near a stairwell door (OR 3.7; P=.05). Two of five A flavus patient isolates were identical to one of the environmental isolates. The pressure in most of the rooms wa s higher than in the corridors, but the pressure in the oncology unit was n egative with respect to the physically adjacent hospital; consequently, the unit acted essentially as a vacuum that siphoned non-HEPA-filtered air fro m the main hospital. Of the 78 samples obtained with a small-volume air sam pler, none grew an Aspergillus species, whereas 10 of 40 cultures obtained with a large-volume air sampler did. CONCLUSIONS: During active construction, Aspergillus spores may have entere d the oncology unit from the physically adjacent hospital because the air p ressure differed. Guidelines that establish the minimum acceptable pressure s and specify which pressure relationships to test in healthcare settings a re needed. Our data show that large-volume air samples are superior to smal l-volume samples to assess for Aspergillus in the healthcare environment (I nfect Control Hosp Epidemiol 2000;21:18-23).