Trends in silicosis prevalence and the healthy worker effect among gold miners in South Africa: a prevalence study with follow up of employment status

BACKGROUND: Given the intimate association between silicosis and tuberculosis, understanding the epidemiology of the South African gold mining industry silicosis epidemic is essential to current initiatives to control both silicosis and tuberculosis in this population, one of the most heavily affected globally. The study's objectives were to compare the prevalence of silicosis among working black gold miners in South Africa during 2004-2009 to that of previous studies, including autopsy series, and to analyse the influence of silicosis and/or tuberculosis on exiting employment. METHODS: Routine chest radiographs from a cohort of gold miners were read for silicosis by an experienced reader (I), and a subset re-read by a B-trained reader (II). Two methods of presenting the readings were used. Additionally, with baseline status of silicosis and previous or active tuberculosis as predictors, survival analysis examined the probability of exiting the workforce for any reason during 2006-2011. RESULTS: Reader I read 11 557 chest radiographs and reader II re-read 841. Overall, silicosis prevalence (ILO ≥ 1/0: 5.7 and 6.2% depending on reader method) was similar to the age adjusted prevalence found in a large study in 1984 (5.0%). When comparison was restricted to a single mine shaft previously studied in 2000, a decline in prevalence (ILO ≥ 1/1) was suggested for one of the reading methods (duration adjusted 20.5% vs. 13.0% in the current study). These findings are discordant with a long-term rising autopsy prevalence of silicosis over this period. Overall, relative to miners with neither disease, the adjusted hazard ratio for exiting employment during the follow-up period was 1.54 for baseline silicosis [95% confidence interval (CI) 1.17, 2.04], 1.71 for tuberculosis (95% CI 1.51, 1.94) and 1.53 for combined disease (95% CI 1.20, 1.96). CONCLUSIONS: This study found, a) there was no significant decline in overall silicosis prevalence among working black miners in the South African gold mining industry between 1984 and 2004-2009, and b) a possible decline at one mine shaft more recently. In the absence of evidence of declining respirable silica concentrations between the 1980s and 2000s, the trends found are plausibly due to a healthy worker survivor effect, which may be accelerating.