Introduction and objective:
Mushroom compost worker’s lung (MCWL) is a hypersensitivity pneumonitis (HP) entity occurring in workers employed at preparing compost for the cultivation of white button mushrooms. The aim of the study is to identify the strains of thermophilic actinobacteria as the potential causative agents of MCWL symptoms in workers.

Material and methods:
Clinical examinations included chest X–ray, high-resolution computed tomography (HRCT), bronchoscopy with bronchoalveolar lavage (BAL), pulmonary function tests comprising spirometric measurements, DLCO measurements, body pletysmography, and arterial – blood gas tests. Identification of thermophilic actinobacteria was performed by microscopy, culture and genetic methods. Allergological tests included the agar gel precipitation test and inhalation challenge with actinomycetal antigens.

Results:
Among a group of 20 patients, 15 showed in HRCT ground-glass attenuations solely and were diagnosed with acute HP, 3 showed ground-glass attenuations with pulmonary fibrosis and were diagnosed with subacute HP, while 2 patients were diagnosed with Organic Dust Toxic Syndrome (ODTS). Thermobifida fusca and Laceyella sacchari strains were isolated from the symptom-causing mushroom compost. In 3 workers, strong precipitin reactions to Th. fusca were found, whereas there were no reactions to L. sacchari. Four workers showed a positive inhalation response to small doses of Th. fusca antigen.

Conclusions:
The results of this study indicate that Th. fusca is most probably a major causative agent of MCWL and should be considered in the diagnosis of HP in workers of the mushroom compost facility.