Hot Tub Lung

Hot tub lung is a hypersensitivity like pneumonitis caused by inhaling aerosolised Mycobacterium avium complex (MAC, most often Mycobacterium avium) from poorly maintained hot tubs, spas, indoor swimming pools, humidifiers and showers. The organism resists chlorine and thrives in warm water biofilm. Inhaled bacterial antigen provokes granulomatous inflammation, and viable organisms are present in lung tissue, so the disease has both infective and immunological components.

It occurs in otherwise healthy adults, usually non-smokers, after weeks to months of repeated exposure. Prognosis is good if exposure ceases, and most patients recover fully; relapse follows re-exposure. Rarely, residual fibrosis or respiratory failure results. Heated indoor pools at hotels and clubs, and domestic water heaters, are relevant sources in Kenya.

• Subacute dry cough, exertional dyspnoea, low grade fever, chills and fatigue.
• Weight loss, chest tightness, wheeze.
• Crackles in about half of patients; hypoxaemia in severe cases.
• Symptoms improve away from the exposure source, for instance after leaving a holiday resort.
• Severe cases cause acute respiratory failure needing ventilation.
• Red flags: SpO2 below 90 percent, rapid progression, ground glass opacities involving most of the lung, immunosuppression.

• Exposure history: hot tub, spa, indoor pool or humidifier use, frequency and recent maintenance.
• Chest radiograph: diffuse nodular or ground glass opacities, often normal early.
• High resolution CT: diffuse centrilobular nodules and ground glass attenuation with mosaic perfusion and air trapping; no cavitation, no tree-in-bud pattern predominance (which suggests airway disease from nodular bronchiectatic MAC).
• Spirometry: restrictive or mixed; reduced DLCO.
• Sputum or lavage acid fast bacilli smear and culture (MAC grown on liquid and solid media over 6 weeks); lavage lymphocytosis above 30 percent.
• Lung biopsy: poorly formed non-necrotising granulomas and bronchiolitis; acid fast bacilli may be scarce.
• Water culture from the implicated tub if feasible.
• Exclude tuberculosis (GeneXpert), HIV, pneumocystis, sarcoidosis, other hypersensitivity pneumonitis, and nodular bronchiectatic MAC disease in older patients.

• Stop exposure at once, and keep the tub drained and cleaned. This is the key treatment.
• Mild disease: avoidance alone; most improve within 2 to 6 weeks.
• Hypoxaemic or moderate disease: oxygen to SpO2 92 percent or more; prednisolone 0.5 to 1 mg/kg orally once daily (usual 40 mg) for 2 to 4 weeks, then taper over 4 to 8 weeks.
• Severe or persistent disease, or positive cultures with progression: add antimycobacterial therapy with clarithromycin 500 mg orally twice daily, rifampicin 600 mg orally once daily (450 mg if under 50 kg) and ethambutol 15 mg/kg orally once daily, for 6 months; check baseline visual acuity and colour vision, then monthly review for ethambutol toxicity; check liver enzymes at 2 weeks and monthly; ECG QTc before and 2 weeks after clarithromycin.
• Immunosuppressed patients (HIV with CD4 under 100 per microlitre): treat as disseminated MAC: clarithromycin 500 mg twice daily and ethambutol 15 mg/kg daily for at least 12 months; start antiretroviral therapy within 2 weeks.
• Respiratory failure: ventilatory support with lung protective settings, methylprednisolone 500 mg intravenously once daily for 3 days.
• Environment: water heated to 60 degrees C or more kills the organism; hot tubs need weekly shock disinfection, filter replacement every 3 months, and drain and clean every 3 months.
• Follow up: clinic review at 2 weeks, 3 months and 6 months with spirometry and CT; sputum culture monthly while on therapy.
• Counselling: do not use implicated tub until cleaned and disinfected; relapse follows re-exposure.