Background
Tuberculosis (TB) is a chronic respiratory infectious disease caused by
Mycobacterium tuberculosis (MTB) [
1]. According to the World Health Organization, approximately 1.7 billion (23%) people have latent TB infections and an estimated 10.0 million (range, 9.0–11.1 million) new cases of TB are reported annually. Two-thirds of these cases occur in eight countries: India (27%), China (9%), Indonesia (8%), the Philippines (6%), and Pakistan (5%), Nigeria (4%), Bangladesh (4%) and South Africa (3%). In addition, TB caused an estimated 1.3 million (range, 1.2–1.4 million) deaths in 2017. Notably, China has the fourth highest number of incident TB cases, and second number of multidrug resistant TB cases in 30 high‑burden countries worldwide [
2]. Thus, TB is a critical public health challenge worldwide, particularly in low- and middle-income countries.
The Fifth Chinese Epidemiological Survey in 2010 [
3] showed that the prevalences of active TB, smear-positive TB, and bacteriologically positive TB in western China were 695/100,000, 105/100,000, and 198/100,000, respectively; these numbers are higher than those for the central and eastern regions, but lower than that for China as a whole. Inner Mongolia is located in northern China, and has up to 200,000 TB cases; about 3,500 people in Inner Mongolia die from TB annually. In the 2011 National Surveillance of TB Report [
4], the prevalences of active TB, smear-positive TB, and bacteriologically positive TB in Inner Mongolia were 757/100,000, 78/100,000, and 183/100,000, respectively; these numbers are higher than the national average. The factors associated with TB are of interest to healthcare personnel responsible for the care of high-risk patients, and include sociodemographic [
5,
6] and genetic [
7,
8] factors. However, the effects of environmental factors [
9‐
11], particularly those related to meteorology, on the TB prevalence are typically overlooked.
Geographic information systems have been used in studies of infectious diseases,
e.
g., to detect hot spots and epidemics [
12,
13]. However, assessment of the effects of various factors on the TB prevalence is hampered by the dearth of data on incident cases in Inner Mongolia. Therefore, we investigated the spatiotemporal trends and geographic variations of the TB prevalence in Inner Mongolia from 2010 to 2014 and identified related meteorological factors.
Discussion
Inner Mongolia is a critical area in China for TB prevention and control due to its poor conditions, backward economy, multi-ethnic settlements, and ongoing TB epidemic [
16]. MTB is generally transmitted via the airborne route, with association between cases and epidemic interaction between neighboring areas [
17,
18], meaning that areas close to high-epidemic areas are at elevated risk of TB epidemics.
In this study, the TB prevalence was higher among males than among females, possibly due to the former being at greater risk of exposure to MTB as a result of differences in socioeconomic and cultural factors. In addition, males frequently smoke tobacco and drink alcohol; the latter may lead to liver damage [
19], increasing the risk of MTB infection. The TB prevalence was highest in those aged 46–60 years, likely due to age-related degeneration of physical function and T‑cell–mediated immune dysfunction [
20], consistent with prior reports [
21,
22]. Additionally, farming was the occupation at greatest risk of TB. Rural areas typically have poor economic conditions and scarce public health services, particularly in the absence of community resources such as roads, water, and electricity [
23]. MTB is inactivated under dry, sunny conditions; in contrast, humidity and poor ventilation increase the risk of TB [
24].
The birth prevalence was associated positively with the TB prevalence. Children, and particularly infants, have weak cellular immunity and immature lungs, which increases their risk of MTB infection [
25]. In addition, MTB can be transmitted to infants by family members and neighbors [
26]. The TB prevalence among children aged 0–14 years was 0.005%, lower than both that reported by the Fourth TB Epidemiological Survey in China and the prevalences in other high-TB‑burden areas,
e.
g., Mexico (7.8–32.0%) [
27], Turkey (3.4%), and other middle-income countries [
28]. In Inner Mongolia, the TB prevalence is lower in children than in adults, but TB in children tends to be atypical, progress rapidly, and disseminate readily [
29].
Population density and the number of beds were associated negatively with the TB prevalence. The TB prevalence was higher in several undeveloped areas, which have poor living environment, a dearth of human resources, greater dispersal of residents, road transportation problems, and insufficient medical resources [
30]. A low-rise, high-density residential environment is associated negatively with the TB prevalence [
31]. Residents of high-rise buildings benefit from better ventilation and more direct sunlight, which reduce the risk of MTB infection. In contrast, the low air quality and little sunshine experienced by residents of impoverished areas increase the risk of TB, which may explain the positive association between rural residence and the TB prevalence. Most rural areas are underdeveloped with low population density and limited number of beds, and the associated low income, educational level, and awareness of health-related issues facilitate the spread of TB [
31‐
33].
The impact of meteorological factors on human health, and particularly infectious disease, is of interest worldwide [
34,
35]. Sunshine duration and air pressure were associated positively with the TB prevalence, consistent with previous reports [
36,
37]. Humidity tends to decrease and temperature to increase with increasing sunshine duration. Furthermore, air conditioning in rooms with limited ventilation facilitates the spread of MTB [
38]. The continental climate of Inner Mongolia is dry and arid, and the air pressure during winter and spring is higher than that in summer. In this study, the TB prevalence was higher in winter and spring, suggesting a positive correlation with air pressure. This may be because high air pressure enhances atmospheric flow, promoting the transmission of MTB [
11]. Precipitation was associated negatively with the TB prevalence, possibly because rain tends to increase the air quality and reduce the MTB concentration [
39]. Therefore, prevention and management of TB should be strengthened in areas that receive little precipitation.
Our study has several strengths. To our knowledge, it is the largest study of the trend of, and factors associated with, the TB prevalence in Inner Mongolia from 2010 to 2014. Moreover, we estimated the TB prevalence according to demographic characteristics, e.g., age and sex. However, this study also has several limitations. First, it was a retrospective analysis; a prospective study would have enabled prediction of the trend of the TB prevalence. Second, aggregation of individual cases to the district level may have resulted in individual differences being missed; however, we assessed the effects of multiple factors on the TB prevalence in each district. Finally, the survey covered only certain regions in China, which hampers extrapolation of our findings.