Silicotuberculosis

Silicotuberculosis is active tuberculosis in a person with silicosis. Silica exposure impairs the macrophage defence against Mycobacterium tuberculosis, so the risk of tuberculosis is about 3 times that of the general population in simple silicosis and much higher in advanced disease, and it is one of the largest occupational health problems in southern African gold and other mines.

Exposure also occurs in quarrying, stone cutting, sandblasting, foundry work and tunnelling. Silica exposure with HIV infection multiplies the risk further, and tuberculosis may continue to appear years after leaving the work.

Pathophysiology. Silica particles are ingested by alveolar macrophages and damage the phagolysosome, causing macrophage death, fibrosis and poor ability to kill the bacillus. Silicotic nodules and lymph nodes form a poor perfusion focus in which tubercle bacilli grow and cavitate. Silica also reduces drug penetration into fibrotic tissue.

Cough, sputum, fever, night sweats, weight loss, haemoptysis, worsening dyspnoea and chest pain, in a worker with a record of silica exposure. Features that suggest tuberculosis on a background of silicosis are new constitutional symptoms, worsening dyspnoea and a change in the chest radiograph.

Haemoptysis over 100 mL, respiratory failure or pneumothorax need urgent assessment.

• Occupational history with duration and intensity of exposure, and a record of previous radiographs.
• Chest radiograph and CT: small nodules and conglomerate masses (progressive massive fibrosis) in the upper zones, with new cavities, rapid change, pleural effusion or lymph node enlargement suggesting tuberculosis. Eggshell calcification of nodes points to silicosis itself.
• Sputum smear and nucleic acid amplification (Xpert MTB/RIF) with culture and susceptibility, repeated several times because smear is often negative. Bronchoscopy with lavage when sputum is negative.
• HIV test, diabetes screening, and baseline liver and renal function.
• Interferon gamma release tests and tuberculin skin tests are less reliable in advanced silicosis and HIV, and do not exclude active disease.

Differential diagnosis.

• Progressive massive fibrosis without infection
• Lung cancer, which is also raised in silica exposure
• Nontuberculous mycobacterial disease
• Chronic pulmonary aspergillosis
• Fungal infection (histoplasmosis)
• Sarcoidosis

• Antituberculosis therapy for a longer time than usual because of poor drug penetration and high relapse rates: rifampicin 10 mg/kg (maximum 600 mg), isoniazid 5 mg/kg (maximum 300 mg), pyrazinamide 25 mg/kg and ethambutol 15 mg/kg daily for 2 months, then rifampicin and isoniazid daily for at least 6 months, extended to a total of 8 to 9 months, with pyridoxine 10 to 25 mg daily. Manage drug resistance with a specialist regimen. Directly observed therapy supports adherence.
• Latent tuberculosis in a person with silicosis, once active disease is excluded: rifampicin 10 mg/kg daily for 4 months, or isoniazid 300 mg daily with pyridoxine for 6 to 9 months, or weekly isoniazid 15 mg/kg with rifapentine (750 mg at 32 to 49 kg, 900 mg at 50 kg or more) for 12 weeks. Annual screening for symptoms and chest radiograph.
• Stop further silica exposure and arrange occupational rehabilitation, with notification and compensation processes. Dust control at the workplace.
• HIV: antiretroviral therapy started within 2 weeks in those with CD4 under 50, otherwise by 8 weeks, with drug interactions with rifampicin checked.
• Smoking cessation (varenicline 0.5 mg daily for days 1 to 3, 0.5 mg twice daily for days 4 to 7, then 1 mg twice daily), and vaccination against influenza, pneumococcus and COVID-19.
• Pulmonary rehabilitation, bronchodilators for airflow obstruction, and oxygen for resting hypoxaemia.
• Lung transplantation for suitable patients with advanced silicosis after cure of tuberculosis.

Complications and prognosis. Progressive massive fibrosis, respiratory failure, cor pulmonale, pneumothorax and lung cancer. Mortality is higher than for either condition alone, and relapse is common.

Monitoring. Monthly clinic review in treatment with sputum at 2 and 5 months, liver function, and chest imaging at the end of treatment, then yearly for life with screening for lung cancer.

Referral and escalation. Refer to a tuberculosis service and an occupational lung disease clinic, and refer for drug resistant disease.