This will briefly delineate why South African gold miners face particularly high rates of tuberculosis. In the South African gold mining industry, physical, biological and social factors combine to a situation colloquially referred to as a ‘perfect storm of disease,’ where HIV, silica dust, and tuberculosis (TB) interact in concert with one another to result in some of the highest known rates of these diseases in the world. While the interaction of the host immune system with these agents increases individual morbidity and mortality, social and environmental factors allow these health concerns to run rampant in both the localized and general population. In a setting where over 90 percent of the gold miners migrate from rural areas of southern Africa, and with the growing rate of drug resistance, this situation presents itself as one of the largest public health threats that the health community faces today.
Silicosis, a fibrotic disease caused by silica dust exposure, predisposes patients to TB and increases the risk of active TB infection to a risk factor comparable to HIV infection1-5. Importantly, exposure to silica dust even without the development of silicosis remains associated with an increased TB risk2,6. Since the body is unable to eradicate silica dust once it enters the lungs, the risk of TB disease associated with silica lasts for the duration of the patient’s lifetime; thus silica dust continues to act as a predisposition to TB6. While the miner is at risk of silica exposure in the mine, social factors, such as single-sex hostel living quarters and migrancy, simultaneously increase the risk of HIV infection:7-9 HIV infection in the mining compound reaches close to thirty percent of the population10. When a patient is infected with HIV, the risk for TB is also increased; if that patient additionally has silicosis, the end result is a synergistic risk probability on a multiplicative, as opposed to additive, scale5.
Consequentially, the highest recorded rates of TB in the world are found on the South African gold mines11, and throughout history, this number has only risen12. Death from TB (mortality) exceeds death from accidents on the mines13,14. The prevalence of TB in the gold mining industry increased from 806 per 100,000 in 1991 to 3821 in 200415; concomitantly, HIV has increased from <1 percent in 198716 to >27 percent in 200010. Recent data, particularly after the release of ARV’s, are not readily available.
TB, Silicosis-related TB, and HIV
The association between tuberculosis and silicosis has long been recognized and it is universally accepted that the presence of silica dust increases the risk of pulmonary active TB2,6,17-25. Quantified analysis investigating the relationship between the two indicates that in the South African gold mining industry, TB incidence of men with and without silicosis varied greatly, at infection rates of 981 vs 2,707, respectively (rates per 100,000 population). Relative risk of clinical TB was 2.8 for those with silicosis as opposed to men without silicosis1. Other studies have also shown silica dust increases the risk of pulmonary TB in miners by about threefold compared to miners without silicosis26. Additionally, as silicosis continues to act as a predisposition for TB for the duration of one’s life, incidence of TB continued to climb for men with silicosis, reaching an increase of up to 6.3% per year for those with category 3 silicosis. Consequently, one-quarter of the men suffering from silicosis will develop TB by 60 years of age1.
Deep level gold mine shafts are often only 2-3 meters wide and can reach up to several kilometers underground27. This makes crucial TB preventative strategies, such as air circulation and ventilation, extremely difficult. Personal protection equipment (PPE) is required to enter the mineshaft, however is rendered useless in practice because the worker, due to poor enforcement and increased difficulty breathing in a hot, sweaty environment, often removes his or her PPE28. With these conditions, it is to little surprise that the rate of recurrent TB infection (a secondary TB infection in the same host) in the gold mining industry, whether or not complicated by silicosis, more than triples the rate in the general population. Moreover, 69% of these recurrent infections are attributed to exogenous reinfection, as opposed to relapse of an existing endogenous infection, which far exceeds the general population11,30.
Silica dust is physiologically responsible for this increase in TB largely because it affects the alveolar macrophage, which is also the primary immune cell against tuberculosis. Alveolar macrophages (as well as other macrophages and scavenger cells in the lung parenchyma to a lesser extent) engulf the crystalline dust particle as it would a biological pathogen. This contorts the macrophage, resulting in the silica dust impairing its function and lowering the host’s physiological immunity to TB. Simultaneously, it elicits pro-inflammatory responses to develop fibrotic nodes, further exposing the lung to risk of TB infection. The recruitment of other macrophages and monocytes from the normal cytokine response subsequently recruits more cells for Mtb to invade (see Paper III: ‘Physiopathology’).
Because of this exposure to silica dust, gold miners have historically been more susceptible to TB than the general population1-6,29. However, in the past three decades, the emergence of HIV has made the gold mining community exquisitely susceptible to TB infection, and infection rates have skyrocketed12. The added factor of HIV distinguishes gold mining in South Africa from all other gold mining regions of the world. Socioeconomic conditions, contextual factors related to migrancy, living conditions, and social dynamics contribute to a dramatic rise in HIV infection in the mining communities, with some having a prevalence of up to 70% in resident sex workers31,32. Because of these conditions, a recent Deloitte audit on the mining industry indicates that 1 in 3 new mineworkers will become infected with HIV within the first 18 months of employment33. The increase of TB in the gold mines has been attributed largely in part to the concomitant epidemic of HIV34. Unlike silicosis, HIV infection does not only increase the risk of TB infection, but also the risk of death, shortens survival time, and increases the chance of latent infection developing into active TB infection. Furthermore, it complicates diagnostic test for TB and MDR-TB, and results in higher atypical and extrapulmonary TB manifestations35.
HIV uses the CD4+ glycoprotein (a receptor protein commonly expressed on the host’s immune cells, specifically T-cells, as well as other monocytes, macrophages, and dendritic cells) to gain entry and infect the host’s immune cells. The paramount concern in the gold mining industry is the multiplicative relationship of silicosis and HIV in regards to TB infection: because they both act on the host’s immune system, these risks do not add together but rather multiply the chance of developing active TB. HIV and silicosis result in approximately 15 times the risk of TB infection compared to HIV negative, silicosis free miners36.
Overview: Impact of TB in the Mining Industry on the General Population
Today in South Africa, thousands of men migrate to and from the mines from rural areas of South Africa and surrounding countries. The abolition of Apartheid has relaxed the restrictions on home visit regulations, allowing for this oscillation to become more frequent and practiced37. In Lesotho alone, over 50,000 men migrate to the South African mines each year. Under Apartheid, these men would typically travel home one to two times a year, however today under democratic rule, 60% of these men travel home at least once a month38. As a result, an estimated 760,000 cases of incident tuberculosis in the general population of sub-Saharan Africa is directly attributable to the mining industry4.
This exposes rural populations with a typically low prevalence of both TB and HIV to these pathogens. In these rural settings, individuals have a significant lack of healthcare and are unable to obtain critical medicines needed to treat and cure these diseases. This migration pattern also creates serious complications with diagnostics, continuation of care and referrals for those mineworkers infected with TB, as well as adherence issues for those who are able to obtain medication.
These issues further complicate the impact of mining on the health of the region because they directly lead to the to the development of drug resistant TB (DR-TB). In a recent cross sectional study, 18% of South African gold mine workers infected with TB had acquired drug resistance, and another 9% had contracted DR-TB as a primary infection40. Moreover, 25% of DR-TB cases in Lesotho were among former mine workers in South African mines39. When these miners travel home they pose a high risk of transmission in communities that severely lack the diagnostic capability to detect and treat not only TB, but particularly DR-TB.
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