Identification of fungi causing humidifier lung: 2 rare cases and a review of the literature

BACKGROUND: Humidifier lung (HL) is a hypersensitivity pneumonitis resulting from exposure to humidifiers, with fungi from the humidifier as one of the etiologic agents. However, identification of the fungal species responsible for each case can be challenging because of difficulties in culturing fungi, their accurate identification, and interpreting the results of specific serum IgG testing. OBJECTIVE: To clarify the best way to determine the causative fungal species of each HL case. METHODS: We report 2 cases with HL in which rare fungi were identified as causative agents. In addition, we searched MEDLINE for previous publications on HL caused by fungi and performed a literature review focusing on clinical testing for the determination of causal fungal species. RESULTS: In our 2 cases, we identified Fusarium oxysporum species complex, Purpureocillium lilacinum, Acremonium sclerotigenum/egyptiacum as the causative fungal species, based on findings that these could be cultured from humidifier water (HW) and precipitins against these fungi were also positive. The literature review identified 31 HL cases in which the causative fungal species had been documented. In more than half of the cases (17 of 31) there was a concordance between the fungal species cultured from HW and the presence of specific IgG in the blood. CONCLUSION: We recommend performing culture of fungi from HW and specific serum IgG testing for the accurate determination of the causative fungal species in HL, and concordance between them serves as a rationale for the determination of causative fungal species.