A 66-year-old woman treated for hypersensitivity pneumonitis for more than 2 years achieved clinical and radiologic resolution after daily hot tub use was identified and treatment was redirected toward possible Mycobacterium avium complex (MAC) infection.
The woman was referred to a pulmonary clinic in August 2025 for progressive exertional dyspnea. She had developed flu-like symptoms, dyspnea, and oxygen desaturation in June 2023 and was diagnosed with nonfibrotic hypersensitivity pneumonitis attributed to exposure to pheasants and mouldy hay. She received prednisone and mycophenolate, but imaging abnormalities persisted, and she had recurrent hospitalizations for acute hypoxic respiratory failure. A subsequent detailed exposure history revealed daily indoor hot tub use since 2020, with respiratory symptoms developing after the exposure began.
Pulmonary function testing in September 2023 showed hyperinflation, air trapping, and mildly reduced diffusing capacity, with similar findings 1 year later. High-resolution computed tomography showed centrilobular ground-glass nodules and mosaic attenuation consistent with air trapping. Testing for avian antigens was negative, while immunoglobulin G levels against common moulds and thermophilic actinomycetes were within reference ranges.
A right upper lobe surgical lung biopsy showed predominantly nonnecrotizing airway-centered and airspace granulomas, along with organizing pneumonia and patchy chronic cellular interstitial infiltrates. Rare MAC organisms were detected by auramine-rhodamine staining, although bronchoalveolar lavage (BAL) culture was negative. Taken together, the findings were considered most consistent with hot tub lung due to hypersensitivity pneumonitis in the setting of MAC exposure.
With only transient improvement on corticosteroids and mycophenolate and subsequent imaging progression, the researchers considered a mixed mechanism involving hypersensitivity pneumonitis and MAC infection. The patient was counseled to strictly avoid hot tub use, immunosuppression was tapered, and azithromycin, ethambutol, and rifabutin were initiated in September 2025.
Following hot tub cessation and initiation of antimycobacterial therapy, the patient's respiratory symptoms markedly improved. She remained clinically stable, and serial imaging showed complete radiologic resolution approximately 24 months after presentation.
Hot tub lung may reflect a spectrum of immune-mediated and infectious mechanisms. In most reported cases, it behaves like nonfibrotic hypersensitivity pneumonitis, and eliminating hot tub exposure may be sufficient. Corticosteroids may be considered for more severe or persistent symptoms. Antimycobacterial therapy is generally reserved for selected patients with evidence suggesting MAC infection on biopsy or BAL or for those who fail to improve despite exposure avoidance and corticosteroid treatment.
The case had notable limitations. Hot tub water and filter components were not cultured, preventing definitive microbiologic confirmation that the patient's hot tub was the source of MAC. BAL culture was also negative. Although her exposure history, characteristic imaging, histopathologic findings, and detection of rare organisms supported the diagnosis, the researchers said definitive evidence of active infection remained limited.
The researchers said treatment-refractory interstitial lung disease may warrant reassessment of both the presumed antigen source and underlying disease mechanism, including targeted questioning about hot tub exposure. “This patient's complete resolution following triple-drug therapy and exposure cessation emphasises the importance of targeted questioning about hot tub exposure during diagnostic evaluation of persistent interstitial lung disease,” wrote lead author Fadi Abualhommos, of Pulmonary and Critical Care at Mayo Clinic in Phoenix, Arizona, and colleagues.
The authors reported no conflicts of interest or funding.
Source: Respirology Case Reports