Background

An estimated 10.6 million people were diagnosed with tuberculosis (TB) worldwide in 2022, whereas in the same year, a total estimate of 1.3 million people died from TB [2]. According to the World Health Organization (WHO), for the distribution of the global TB burden for regions in 2022, Africa ranked second after Southeast Asia, with a total of 23% of the global TB cases [2].

Despite efforts by several stakeholders to stop TB, Uganda remains one of the 30 WHO-designated high-burden TB countries, with an incidence rate of 198 per 100,000 people [1,2]. High burden countries (HBCs) accounted for 87% of all of the estimated incidence of TB worldwide in 2022, as was the case in 2021 [2].

Notably, Uganda achieved 100% TB treatment coverage despite recording a total of 12,000 TB-related deaths in 2022 [2].

In 2018, the United Nations (UN) General Assembly designated casual labourers working at road construction sites as high-risk TB subpopulations due to their high occupational exposure to dust, especially from silica-containing surfaces [3]. Therefore, there is an urgent need to understand the burden of TB in this subpopulation and modifiable factors that can be leveraged to enhance prevention and control programs. Several studies have also reported that workers exposed to silica dust, such as miners, construction workers, and stone crushers (among others), are at increased risk of developing tuberculosis [4–6].

For example, miners have been reported to have a TB incidence ranging between 2.8% and 12.4% [7–11]. Casual labourers involved in the crushing and grading of aggregates from hard cores (rocks), excavation of gravel (murram) and sorting of gravel (murram), among others, are exposed to high amounts of silica dust [11,12].

The inhalation of respirable silica leads to silicosis, which is mainly characterized by granulomas with collagen nuclei surrounded by epithelioid cells and results in the formation of silicotic nodules that are dispersed throughout the lungs; as the condition worsens, these nodules can consolidate to form enormous masses to distort the parenchyma, thus leading to the development of many pulmonary diseases, including silica-associated PTB [13].

Individuals exposed to silica have a 2.8 to 3.9 times greater risk of developing pulmonary tuberculosis and a 3.7 times greater risk of developing extrapulmonary tuberculosis than does the general population [14]. Recent studies in Zimbabwe and India have shown that the prevalence of TB among workers exposed to silica dust (miners) ranges between 4% and 9.4%, which is relatively high [8,9].

Several studies in Uganda have focused on the burden of TB among other risk groups, such as health workers and slum dwellers [15–17]. However, no study has been conducted among road construction workers in Uganda, despite the high occupational exposure and widespread use of silica-containing surfaces.

Some Ugandan surfaces are made of sedimentary and asphaltic rocks and murram, which have been found to contain high amounts of respirable silica dust [3,6,18]; however, casual labourers often lack personal protective equipment and have long periods of exposure, which exposes them to high amounts of silica dust [19,20]. In addition, these casual labourers reside in poorly ventilated houses near construction sites and often stay and work as a group, which leads to overcrowding that enhances the transmission of PTB. Overcrowding was found to be associated with TB disease in a study among gold miners with the same occupational exposure in South Africa [21].

In addition, casual labourers have temporal migratory tendencies as they move from their homes to sites and later switch from one site to another. Recent studies have shown that geographical mobility and migration are associated with TB [10,22]. Based on these data, we aimed to determine the prevalence of PTB and associated factors among casual labourers working at road construction sites in central Uganda.

Methods

Results

Sociodemographic characteristics of the study participants

A total of 297 casual labourers participated in the study, representing a 100% response rate. The median (IQR) age of the participants was 29 years (25–33), and most participants were males (95.6%). Most of the participants were not married (66.7%), had a certificate-level education (81.5%) and had an exposure period of 0 to 1 year (51.7%). A few of the individuals were smokers (17.8%), and approximately one-quarter of the participants used dust masks (28.3%). More than half of the participants were nonalcohol users (51.2%). Most of the participants (64.0%) worked on sites in which the site setting was considered to be rural (Table 1). It should be noted that dust masks are contractually available from the employers.

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Table 1. Sociodemographic characteristics of the 297 study participants from selected road construction sites in central Uganda.

https://doi.org/10.1371/journal.pone.0304719.t001

Clinical factors

Only 10.1% of the study participants had ever been tested for TB; moreover, 3.4% were experiencing wasting, and 17.2% were hypertensive. Over two-thirds of the participants had ever been tested for HIV (68.7%), and 83.5% had ever received a BCG vaccine. Of those who had ever been tested for TB, only 13.3% had positive test results. Of those who had ever tested for HIV, only 2.9% tested positive (Table 2).

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Table 2. Clinical characteristics of the 297 study participants from selected road construction sites in central Uganda.

https://doi.org/10.1371/journal.pone.0304719.t002

The study used the standard definition of the WHO of wasting in participants as having a low weight for a particular height.

Bivariate and multivariate analyses of the factors associated with TB

Room occupancy, alcohol use, BMI, BCG scarring, exposure period, smoking status, marital status and age were the only factors that were considered for the multivariate analysis, because they had P values less than 0.3. After adjusting for covariates, only alcohol use (P = 0.001), staying in shared rooms (P<0.001), and absence of a BCG scar (P = 0.046) were found to be significantly associated with PTB (Table 3). No interactions or confounders were found in the final model.

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Table 3. Bivariate multivariate analyses of the factors associated with PTB.

https://doi.org/10.1371/journal.pone.0304719.t003

From Table 3, the only factors that were significantly associated with PTB among the casual labourers working in selected road construction sites in central Uganda (according to the multivariate analysis) were sharing rooms of residence (aPR = 8.133, 95% CI; 4.374, 15.123), alcohol use (aPR = 2.700, 95% CI; 1.521, 4.792) and absence of a BCG scar (aPR = 3.447, 95% CI; 1.024, 11.606).

Discussion

This study assessed the prevalence of PTB among casual labourers from selected road construction sites in central Uganda. In this cross-sectional study, the TB incidence was found to be 10 times greater than the national TB incidence (0.2%) in Uganda [24]. This observation can be attributed to the fact that this study concentrated on only a high-risk group of silica dust-exposed workers (casual labourers) involved in road work, whereas the coverage of the national TB prevalence survey 2015 [25] included other low-risk groups. Although there are no studies on the prevalence of TB among silica dust-exposed workers in Uganda, a study in Ghana found that the TB prevalence among miners (silica dust-exposed) was 2.65%, whereas it was 0.64% among nonminers who were screened in the community (Ohene et al., 2021). Another study among 451 small-scale artisanal miners who were also silica-dust-exposed workers in Zimbabwe reported a TB incidence of 4% (Moyo et al., 2021). The difference in incidence could be due to a number of reasons, such as greater precision given the large sample size that was used in the later study. In 2017, the Centers for Disease Control reported that the prevalence of TB among South African miners is 15 times greater than that among the rest of the South African population [26].

This high prevalence of TB among casual labourers involved in road work implies that road construction workers could have a considerable burden of infectious TB disease, which calls for urgent attention and implementation of TB prevention and control measures among road construction workers.

The study participants who reported of sleeping in shared rooms of residence were 8.1 times more likely to have PTB than those who did not share their rooms. A more recent study in Uganda by Kirenga et al. showed that overcrowding was a strong risk factor associated with TB [17]. This finding concurs with the findings of several other studies evaluating environmental risk factors associated with TB [27,28].

It was also found that study participants who drank alcohol were 2.7 times more likely to have PTB than those who did not drink alcohol. Several other studies by Lienhardt [29], among others, have also demonstrated alcohol use to be among the top modifiable risk factors associated with TB [30–33]. This can be attributed to the fact that most bars are congested, and those drinking alcohol are consistently in close contact, which puts them at risk of contracting TB if there is a positive TB case. The WHO global TB report of 2022 also highlighted alcohol use as being among the five risk factors that are strongly associated with many TB cases [1].

In this study, participants without a BCG scar were 3.4 times more likely to have PTB than those with a BCG scar. This finding could be best explained by the absence of immunity against TB disease on the assumption that the absence of a BCG scar implies that a particular participant was either not vaccinated with BCG at birth or may not have elicited an immune response when vaccinated. This finding is also consistent with a study in the United States that demonstrated BCG vaccination to be protective against TB disease [34]. Some countries experiencing high TB incidence have resorted to targeted BCG vaccination in adults younger than 35 years of age among high-risk groups [6,35–37].

Two strengths of this study were the fact that an updated version (7.0) of the CAD4TB software with greater sensitivity (91.5%) was used in the study and that the study was the first to be performed among silica dust-exposed workers in Uganda, particularly casual labourers working at road construction sites who represent a high-risk group. Notably, all casual labourers who were approached consented voluntarily, thus implying that the study achieved a 100% response rate.

This study had limitations that should be noted. Information bias could be detected in the way that HIV status (which is a known predictor of TB disease) may have been underreported by some of the participants, due to the fact that the study did not perform HIV tests but rather used self-reports. Due to the fact that some study participants failed to produce sputum, there may have been under declaration of TB in this study. The study was prone to random error in such a way that the sample size was not adequate to examine all of the associated factors. As a result, the study lacked enough power to detect small effects. This study may not be generalizable to the entire country, such as Uganda, due to ecological differences in earth surfaces that contain different amounts of silicon, thus implying varying exposure levels for different regions in the country.

Conclusions

There was a high prevalence of TB among casual labourers working at road construction sites in central Uganda. Study participants living in shared rooms had greater odds of contracting TB than did those sleeping one person per room. TB incidence was also greater among alcohol users than among nonusers. Participants without a BCG scar had relatively greater odds of developing PTB than those with a BCG scar. Routine TB screening of road construction workers at all road construction sites is recommended, and targeted BCG vaccination in road construction workers younger than 35 years could be adopted to increase immunity against TB disease. A more comprehensive study covering all regions in the country should be conducted to ascertain the overall TB incidence among road construction workers.

Acknowledgments

We wish to extend our gratitude to the research assistants, Uganda National Roads Authority, Ministry of Works and Transport, and National Tuberculosis and Leprosy Program under the Ministry of Health, USAID, The Global Fund, all who made this research possible. We also wish to thank the project managers of all of the selected road construction projects that participated in this research.

We thank American Journal Experts (AJE) for English language editing.