Maple Bark Stripper Disease

Maple bark stripper disease is hypersensitivity pneumonitis caused by inhaling spores of the black fungus Cryptostroma corticale, which grows beneath the bark of stored maple logs (and occasionally sycamore, plane and other hardwoods). Exposure occurs when workers strip bark, handle, chip or saw infected timber in enclosed sheds or sawmills, and when people clear affected trees. Outbreaks are linked to warm, dry summers, and incidence is rising with climate change.

The disease appears as acute or subacute alveolitis, and chronic disease can lead to fibrosis. The spore load is large and the antigen is potent, so a short heavy exposure can cause severe acute respiratory failure. The disease is rare in tropical countries but imported timber, wood processing, and arborist work with hardwoods make it relevant to clinicians dealing with occupational lung disease.

• Acute onset 4 to 12 hours after exposure: fever, chills, headache, myalgia, dry cough, dyspnoea.
• Subacute: cough, exertional breathlessness, fatigue, weight loss, night sweats.
• Chronic: progressive dyspnoea, clubbing, basal or diffuse crackles, inspiratory squeaks.
• Wheeze may accompany exposure when allergy is present.
• Symptom pattern linked to work; improvement on days away.
• Red flags: hypoxaemia at rest, respiratory failure, pneumothorax, weight loss with night sweats (tuberculosis must be excluded).

• Occupational history: handling of bark or logs, visible black sooty spore layer under bark.
• Chest radiograph: diffuse reticulonodular or ground glass opacities.
• High resolution CT: diffuse ground glass attenuation, centrilobular nodules, air trapping on expiration, fibrosis with traction bronchiectasis in chronic disease.
• Spirometry: restrictive pattern, reduced DLCO, exertional desaturation.
• Serum IgG precipitins to Cryptostroma corticale (positive in sensitised workers).
• Bronchoalveolar lavage: lymphocytosis exceeding 30 percent, low CD4/CD8 ratio.
• Biopsy if doubt remains: poorly formed non-necrotising granulomas with interstitial lymphocytic inflammation; spores may be identified in lavage.
• Exclude infection, pneumocystis, tuberculosis, sarcoidosis, organic dust toxic syndrome, and other causes of hypersensitivity pneumonitis.

• Remove from exposure immediately. Re-exposure causes relapse, which can be severe.
• Mild disease: avoidance alone with review at 2 weeks.
• Moderate to severe disease: oxygen to SpO2 92 percent or more; prednisolone 0.5 to 1 mg/kg orally once daily (maximum 60 mg) for 2 to 4 weeks, then taper by 5 mg weekly over 8 to 12 weeks.
• Life threatening respiratory failure: methylprednisolone 500 mg to 1 g intravenously once daily for 3 days followed by oral prednisolone 1 mg/kg daily; non-invasive ventilation or intubation as indicated using lung protective settings (tidal volume 6 mL/kg predicted body weight).
• Chronic fibrotic disease: prednisolone for 3 to 6 months; add azathioprine 1 to 2 mg/kg once daily or mycophenolate mofetil 500 mg twice daily increased to 1 g twice daily for steroid sparing; transplant assessment when progressive.
• Add co-trimoxazole 480 mg orally once daily, omeprazole 20 mg once daily, calcium and vitamin D when prednisolone exceeds 20 mg daily for over 4 weeks.
• Workplace control: store logs dry, remove bark with wet methods, use powered air purifying respirators, inspect logs for sooty bark, and report cases.
• Follow up: spirometry and CT at 4 to 6 weeks, 3 months and 12 months; annual review in chronic disease.
• Counselling: avoid all further exposure to infected wood.