Silicosis and Silicotuberculosis Among Small Scale Gemstone Miners in Northern Tanzania: SilicoTB

Background Artisanal and small-scale mining (ASM) produces a quarter of the global supply of key metals for the electronics industry and provides direct employment for an estimated 45 million people. Approximately 10 million are in sub-Saharan Africa (SSA), of whom over 1 million work in Tanzania. ASM is characterized by occupational insecurity and low levels of safety and environmental protection, but there is little research into its risks, particularly respiratory and communicable diseases.

Silicosis is an occupational respiratory disease caused by inhalation of respirable crystalline silica (RCS). It usually presents irreversibly after decades of exposure, although accelerated disease associated with high initial exposures can lead to respiratory failure within 1-2 years. To date, silicosis has been examined in only two ASM populations: in Brazil, where radiographic evidence was reported in 37% of 348 current and ex-miners, and in Zimbabwe, where a prevalence of 11% was reported among 514 current miners.

Silicosis and RCS exposure increase the risk of pulmonary tuberculosis (PTB). In miners with silicosis, human immunodeficiency virus (HIV) is also known to multiplicatively increase the risk of TB. Correspondingly, a high PTB prevalence of between 3% to 14% has been reported in several studies of sub-Saharan African ASM workers. The role of miners in community PTB transmission is of considerable public health importance but remains unclear; whole genome sequencing (WGS) is the most accurate available method for assessing this. HIV prevalence across small-scale mining populations ranges from 1.8-18%. The role of HIV in the development and progression of silicosis is unclear and requires further study, with conflicting evidence on whether it reduces or increases the risk of silicosis.

The World Health Organization (WHO) provides a strong recommendation for screening among silica-exposed populations. This recommendation is based on an estimated number needed to screen of 36 in high incidence settings and an increased mortality from TB amongst miners with silicosis, with a corresponding 82% reduction in mortality in screened compared to unscreened miners with TB. Despite this recommendation, there is limited evidence, particularly from Sub-Saharan Africa and in the last two decades, of optimized methods for doing this. Beyond screening, differentiating silicosis, tuberculosis, and silicotuberculosis is clinically challenging, leading to misclassification across all diagnoses. Symptoms are common among miners; for example, in ASM TB screening studies, between 23-39% meet symptomatic criteria for presumptive TB. There is also a growing appreciation that subclinical TB is common and plays an important role in transmission. A screening tool that, either alone or in tandem with current approaches, improves the sensitivity and specificity for TB would be useful diagnostically and programmatically, potentially allowing allocation of resources towards higher risk individuals. One recently investigated method which has shown promise in meeting the WHO target product profile of 90% sensitivity and 70% specificity for a triage tool is C-reactive protein (CRP). CRP testing can be performed at the point of care for less than $2-3. Most recently, WHO guidance has recommended CRP (with a threshold of >5mg/L) may be used in screening for TB disease among adults and adolescents living with HIV.

The tanzanite gemstone is mined exclusively in a 5x2 km strip of land in Mererani, Northern Tanzania. Miners use deep shafts and basic techniques. Three sources of evidence raise concern that the burden of silicosis and PTB among the estimated 9,000-12,000 ASM tanzanite miners is high. First, a recent cross-sectional survey of 330 miners estimated a silicosis prevalence of 30% on chest radiograph and a microbiologically confirmed PTB prevalence of 6% among miners and 8% in the surrounding community. Miners had a median age of 35 years and just 5 years of employment. Second, a case series from the local tertiary referral hospital found a primary diagnosis of silicosis or silicotuberculosis made up 22% of respiratory admissions; over half were under 45 years old and 63% required oxygen therapy. Third, in an occupational clinic for miners and their families, 22% of 315 miners had silicosis on chest X-ray (CXR) and 18% of 243 tested had confirmed PTB.