In the current study, we try to investigate the epidemiological and spatiotemporal characteristics of BD for the purpose of a good understanding of the disease's distribution in Sichuan. Public health researchers are usually interested in using data visualization methods to describe the distribution of diseases, while considering time factors and population effects. The reason is that the visualization of high-risk disease areas can guide managers to prioritize the optimal allocation of investment, personnel, and services, so as to realize the optimal allocation of resources among regions[20]. In this study, the incidence data of BD at the county level in Sichuan from 2011 to 2019 were used to discuss the epidemiological characteristics of the disease and investigate its spatial and temporal distribution rules and possible hotspots.
The incidence of BD in Sichuan showed a downward trend from 2011 to 2019. It is possible that rapid economic growth has resulted in significant improvements in water supply and sanitation facilities, as well as significant changes in population hygiene practices[21]. In terms of age, BD mainly affected children under the age of five, followed by children between the ages of five and nine, which was consistent with the prior studies. [22–24].We speculated that the poor awareness of disease prevention and poor hygiene could lead to the infections, moreover, hypo immunity may further cause the disease progression. Under this situation, targeted prevention and control measures for children could have great public health significance to control the spread of BD, which has already been confirmed by prior study[25], Our study found that the occupations of the cases were mainly scattered children and farmers, we hypothesis that this may be related to the low prevalence of running water and sanitary toilets in rural areas.
For the temporal characteristics, there were new cases every month, and the incidence rates showed an obvious seasonal distribution. The same as the prior studies[26–28], the peak of BD appeared early and lasted from May to October. As we all know, the occurrence of intestinal infectious diseases is related to climatic factors such as sunshine, temperature, humidity, and the quality of food or drinking water[29]. The high temperature and humidity of Sichuan in summer and autumn accelerate bacterial reproduction. Once the food and water were contaminated, it was easy to get BD.
In the current study, the distribution of BD was heterogeneous. The areas of high incidence mainly concentrated in the northwest and southwest regions where the economy was relatively backward. On the contrary, the incidence was low in the eastern and central regions with a relatively developed economy, which suggested that the more developed the regional economy was, the lower the incidence rates were. And the prior studies have also indicated that economic development always means superior water supply, more complete sanitation facilities and better medical care, which could to some extent prevent the further spread of BD and reduce its incidence rates[30, 31] [32].It also explained why farmers were more easily infected by BD. Besides, the animal husbandry, a factor that promote the spread of bacteria, was more common in western Sichuan, which may also play a role in the epidemic of BD[33].
According to the spatial autocorrelation analysis, we found that BD was not randomly distributed at county level in Sichuan from 2011 to 2019. According to the LISA map, high-high clusters were mainly distributed in in the northwest and southwest of Sichuan. The hot spots of geographical environment presented similar characteristics, including relief, sparsely populated and relatively backward economy. But a low-high cluster was detected in Ganzi Prefecture, which also belongs to the western areas. We speculated that the effective infectious disease control measures taken in Ganzi Prefecture in recent years have improved the epidemic. However, inaccurate diagnosis or delayed reports of BD could also cause the difference detected in our study. Besides, Ganzi Prefecture had a unique plateau climate, which has been reported to be not suitable for the reproduction of the bacteria for its coldness and dryness [34].
For the results of the spatiotemporal cluster analysis of BD in Sichuan, we noted that the most likely clusters located in the southwest and northwest of Sichuan, and gradually concentrated in the southwest, which partially indicated that the disease control measures taken these years have made sense, especially in the northwest. Simultaneously, we found that the 2nd secondary clusters mainly concentrated in Chengdu and its surrounding areas, which may be due to the high population density and high population mobility in such areas. In the current analysis, we noted that no clusters existed in eastern Sichuan, where the terrain is relatively flat and the altitude is relatively low. Prior study has confirmed that developed economy, high altitude, relief and minority areas were the risk factors for BD[35], which may explain the phenomenon found in eastern areas to some extent.
Limitations should be considered when interpreting the findings of this study. First, the data of this study came from the China's current surveillance system, which may have missed some BD cases for reasons as unreported, undiagnosed or misdiagnosed. Second, this study was confined to Sichuan, a boarder range of study was needed for the purpose of a better allocation of the public health resources and a further control of the epidemic of BD.