We collected the number of TB cases in Chongqing from 2014 to 2020 which to explore the spatial cluster of different populations' risk of TB and to identify the factors associated with TB by building a Bayesian Spatio-temporal model. In this model, we found that the overall risk of TB continued to decline in this period, and it showed that the heavy TB burden in Chongqing was related to rising SO2, less UR, and more LINA after adjusting the spatial effect, temporal effect, and Spatio-temporal interactions. The southeast and northeast areas of Chongqing were regarded as the areas with the highest TB risk, while the risk of the main urban area showed an upward trend during the study period.
We found that the number of TB cases showed a downward trend and had an obvious seasonal trend, mainly concentrated in the first half of each year, especially in January and March during the study period. Firstly, Mycobacterium tuberculosis is resistant to dampness and microtherm[24], and Chongqing is rainy and cold in winter, which provides a good climate environment for its survival of it. Secondly, at the end of winter and the beginning of spring, the descending vitamin D in the body due to the less exposure to sunlight (skin synthesis of vitamin D depends on sunlight)[25, 26] may affect the function of cellular immunity and macrophages which increases the risk of infection with Mycobacterium tuberculosis. Finally, The high indoor population density and poor ventilation environment lead to a relatively high concentration of pathogenic bacteria in the air during the winter. Especially, the Chinese traditional Spring Festival is always accompanied by large-scale population flow, which is more conducive to the spread of Mycobacterium tuberculosis in January. In addition, there is an incubation period between the infection of Mycobacterium tuberculosis and the occurrence of TB, and some people may avoid seeking medical treatment because of local cultural traditions during the spring festival. Therefore, transmission in winter may lead to the peak of TB-reported cases in spring[27].
The study on the whole population in Chongqing found that the areas with a high risk of TB in Chongqing were concentrated in the Southeast and Northeast, and the RR of TB in the Southeast and main urban areas showed an upward trend during the study period. In gender groups, there was a higher RR of TB in the main urban area in women, meanwhile, it was also found that UR had a positive effect on the incidence of TB in the group. Previous studies had found that the women with higher employment rates in urban areas could contact more floating people and the social circle of women in rural areas was small because most of them must stay at home to take care of the elderly and children[28]. Therefore, women in urban areas were more likely to be exposed to TB than women in rural areas. In age groups, People under the age of 30 had a higher RR of TB in the central main area, and UR had a positive impact on the incidence of TB in this population. At the same time, the risk of people aged 30 to 44 in the main urban area had a significant upward trend. On the one hand, there were some students in this group, who lived in the schools where people are concentrated, thus, it was easy to spread in that place if teachers or students were infected with TB. On the other hand, Young people tended to flow to urban centers due to economic factors[29]. However, it was difficult for them to have a healthy living condition under low income and heavy work pressure so they were more susceptible to TB[30]. Concurrently, there was a most obvious upward trend of disease RR in the Southeast Chongqing in the group over 60 years old. On the one side, Southeast Chongqing, which belonged to the minority gathering area of Chongqing and was close to the multi-ethnic province Guizhou Province, had a low economic level and educational level which caused the low literacy rate of Chinese characters, especially in the people aged over 60[31, 32]. Hence, this population was difficult to obtain and understand the basic knowledge of TB prevention and treatment. On the other side, the problem of the rural empty nest was serious and people over the age of 60 were less likely to seek medical treatment or only chose primary care facilities near their homes, which led to their terrible physical condition[33]. This might lead to the old infected with TB could not be detected and treated in time.
Regarding the covariates, The analysis in Chengdu and other places in China found that higher SO2 levels had an acute effect on TB[34–36] which was consistent with our study. Firstly, An in vitro study found that short-term exposure to SO2 caused 62% of alveolar macrophages to die within 30 minutes at a concentration of 12.5 ppm[11], which could affect the body's ability to eliminate inhaled Mycobacterium tuberculosis through macrophage phagocytosis. Secondly, SO2 was produced from fuel combustion, in which sulfur-containing coal was the main source of SO2. In some rural areas, coal was still used for household cooking and heating in winter which could cause poor indoor ventilation, serious air pollution, and increased the probability of contracting TB[14]. Urbanization had a negative effect on the RR of TB, with the most significant impact on people aged 30 to 44 years and the conclusion was the same in a study in Beijing, China[37]. Urbanization would bring better health facilities, medical facilities, and educational facilities, which would play a significant role in reducing the cases of TB. Moreover, the result that the PEIA was associated with an increase in the risk of TB which might be caused by the high smoking rate of farmers engaged in agriculture work and the harsh environment with fungal spores, pollen, animal dander, and grain dust[38, 39]. There was a positive relationship between LINA per 1000 persons and TB risk, especially in women and people over 60 years old. The Global TB Report 2021 stated that poverty was a determinant of people infected with TB[1, 40]. Most urban poor women were widowed and divorced with no land and no housing, and only relied on their sources of income. However, gender discrimination was still common in the workplace, so women were easy to be laid off, and difficult to find employment. Thus, there was a difficult challenge for urban women to change their poverty status. meanwhile, their health was more vulnerable to TB because of poor living conditions[41]. In the research on the poor people over 60 years old, it was found that most of this group, composed of women, with low education levels and living alone, were marginal people in the city and always were neglected by the security policy. Hence, Due to basic living standards could not being fully guaranteed by policies and the loss of working ability, this population with absolute poverty was easy to be affected by mycobacterium tuberculosis[42].
This study not only considered the risk of TB in different populations but also added the adjacent relationship between regions to the model so that the results were more accurate and were not easily affected by sample size and extreme values. However, There were also some limitations in our analysis. Firstly, some important variables that were significantly associated with TB, such as education level, smoking history, and drinking history, were not collected. Secondly, reported TB cases in the poverty-stricken region, such as southeast of Chongqing, might be affected by underreporting or data loss. Finally, The analysis level of this study was divided according to districts (counties). We can consider using smaller geographical units, such as streets and towns to explore the districts with a high risk of TB like Southeast Chongqing and Northeast Chongqing in a future study.