Descriptive analysis
244,348 human brucellosis cases were reported from 31 provinces during the study period. Among all the represented years, 2014 had the largest number of cases (57,222) with the reported cases of all provinces, except for the Tibet province. There was a small increase in the incidence during the study period (2.9328/100,000 in 2012 to 3.4388/100,000 in 2016).
Clustering analysis
Human clustering analysis
At present, the incidence of brucellosis is gradually expanding to include all age groups in addition to the previously affected young and middle-aged population. During the period of 2012-2016, the average incidence of human brucellosis was higher in the age group of 40-65 years. On the other hand, the incidence of brucellosis in other age groups was relatively low, and the age groups under 20 and over 80 had a much lower incidence. The concrete incidence distribution of each age group is shown in Table 1.
As shown in Fig. 1, the results of the hierarchical clustering show that the average incidence of human brucellosis among different age groups during the period of 2012 to 2016 was divided into three clusters. The highest incidence was in the cluster of the 40~65 groups. The second cluster included the groups of 10~, 15~, 80 and above~, 20~ and 70~. Finally, the incidence rates of the 30~, 35~, 5~, 25~, 70~ and 0~ groups were clustered into another cluster.
Temporal clustering analysis
The results of the seasonal decomposition of the brucellosis incidence showed strong seasonal characteristics, as shown in Fig. 2. Similarly, the temporal cluster analysis showed a consistent result, with a high incidence of brucellosis occurring between March and July annually (Table 2), which was in accordance with the previous studies. The time frame with a high clustering for human brucellosis in the whole study period was observed from January 2014 to December 2015. During this period, a total of 113,111 human brucellosis cases were reported, and the risk of human brucellosis-related events was 31% (relative risk (RR) = 1.31, P = 0.001) higher than the risk during other periods. In addition, the overall brucellosis incidence increased during the study period, but slightly declined in 2016.
Spatial clustering analysis
We obtained the Moran’s I value, variance, Z score and P value from 2012 to 2016 using provincial units to carry out the global autocorrelation analysis (see Table 3). The values of Moran’s I were 0.1179 and 0.1181 for 2013 and 2014, respectively, while the Z values were greater than 1.96 (all P < 0.05), which indicates that the incidence of brucellosis in China between 2013 to 2014 had a non-random distribution; thus, further spatial clustering analysis of human brucellosis was needed.
As shown in Fig. 3, the spatial clustering analysis of the incidence of human brucellosis from 2012 to 2016 showed that the location of the brucellosis clustering in mainland China remained relatively stationary, mainly concentrated in most parts of northern China. From 2012 to 2015, one region was located in the northeast of mainland China, and included Heilongjiang, Jilin, Liaoning and Inner Mongolia, while the other region contained Tibet, Xinjiang, Qinghai, Ningxia, Gansu and Shanxi. In 2016, there were relatively few clustering regions, such that the primary clustering area included Heilongjiang, Jilin, Liaoning and Inner Mongolia, and the secondary clustering area included Gansu, Ningxia and Shanxi provinces.
Spatial-temporal clustering analysis
A heat map was drawn for the regions and time periods of the human brucellosis incidence. It was found that the areas of Xinjiang, Ningxia, Heilongjiang, Inner Mongolia and Shanxi had a high incidence during the study period (Fig. 4A), which was much higher than that in other regions for the same period. At the same time, brucellosis tends to occur from March to August, with the highest incidence in May, particularly in 2014 and 2015 (Fig. 4B).
Finally, we identified both spatial and temporal clusters of high incidence districts per zone. As shown in Table 4 and Fig. 5, the results of the spatial-temporal cluster analysis for the reported human brucellosis cases in 31 provinces of mainland China from 2012 to 2016 included one primary clustering area and three secondary clustering areas. The primary clustering area was located in the northeast of China, including Inner Mongolia, Heilongjiang, Jilin and Liaoning, and the high-risk time frame was from January 2012 to December 2013 (RR = 5.17, LLR = 33228.98, P < 0.001). The three secondary clusters were mainly distributed in the northeast of China with several relatively small areas in central China, and the cluster time frames mainly ranged from January 2014 to December 2016.