Humidifier lung diagnosed by integrating environmental assessment and inhalation provocation testing: a case with Purpureocillium lilacinum as the most likely causative antigen

Humidifier lung is a rare phenotype of hypersensitivity pneumonitis caused by inhalation of aerosolized antigens from contaminated humidifiers, and identification of the causative antigen is often challenging. We report a case of humidifier lung in a 63-year-old woman who presented with recurrent episodes of fever, cough, and dyspnea. Chest computed tomography showed diffuse ground-glass opacities with mosaic attenuation, and bronchoalveolar lavage fluid revealed lymphocytosis. Her symptoms improved without treatment during hospitalization but recurred 5 hours after returning home, suggesting re-exposure to an environmental antigen. A humidifier challenge test reproduced symptoms, including fever, cough, and hypoxemia, necessitating systemic corticosteroid therapy. Fungal contamination was identified within the internal components of the humidifier, from which Purpureocillium lilacinum was isolated. A serum precipitin test using an antigen prepared from the isolate showed a positive reaction. Although precipitin testing with a routine antigen panel showed positivity for multiple antigens, possible cross-reactivity was considered. Taken together, these findings led to the diagnosis of humidifier lung, with Purpureocillium lilacinum considered the most likely causative antigen. This case highlights the importance of integrating the clinical course, environmental assessment, and provocation testing in identifying causative antigens in humidifier lung.