Descriptive analysis
There were 244,348 cases of human brucellosis reported over 31 provinces during the study period . During all years represented, 2014 had the largest number of cases (57,222) with all provinces reporting, except the Tibet. There was a small increase in incidence during the study period(2.9328/100,000 in 2012 to 3.4388/100,000 in 2016).
Clustering analysis
1 Human clustering analysis
At present, the incidence of brucellosis is gradually expanding, from the previous young and middle-aged population to all age groups. During 2012-2016, the average incidence of human brucellosis was higher in the 40-65 age group. While the incidence of brucellosis in other age groups was relatively low, and the age groups under 20 and over 80 were much lower, and the concrete incidence distribution of each age group was as follows(Table 1).
According to the hierarchical clustering result(Fig 1), the average incidence of human brucellosis among different age groups during 2012 to 2016 was divided into three clusters. The highest incidence was in the 40~65 groups. Secondly, 10~, 15~, 80 and above~, 20~, 70~ were grouped together. Once again, the incidence rates of 30~, 35~, 5~, 25~, 70~and 0~were clustered into another cluster.
2 Temporal clustering analysis
The seasonal decomposition of brucellosis incidence showed strong seasonal characteristics(Fig.2). Similarly, the temporal cluster analysis showed the consistent result, with the high incidence of brucellosis occurring between march and July annually(Table 2), which was similar to the previous studies. The high clustering time 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% (RR = 1.31, P = 0.001) higher than during other periods. In addition, brucellosis incidence increased overall during the study period, but declined slightly in 2016.
3 Spatial clustering analysis
Using provincial units to carry out the global auto-correlation analysis, we obtained Moran’s I value, variance, Z score and P value from 20012 to 2016, respectively (see Table 3). The values of Moran’s I were 0.1179 and 0.1181 respectively for 2013 and 2014, while Z values were greater than 1.96 (all P < 0.05), indicating that incidences of brucellosis in China between 2013 to 2014 had a non-random distribution. So further spatial clustering analysis of human brucellosis were needed.
The spatial clustering analysis of the incidence of human brucellosis from 2012 to 2016 showed that the location of brucellosis clustering in mainland China remained relatively stationary, mainly concentrated in most parts of northern China(Fig 3). One region located in the northeast of mainland China, including Heilongjiang, Jilin, Liaoning, Inner Mongolia. The other region contained Tibet, Xinjiang, Qinghai, Ningxia, Gansu and Shanxi in the year of 2012-2015. In 2016, there were relatively few clustering regions, the primary clustering area included Heilongjiang, Jilin, Liaoning and Inner Mongolia, and the secondary clustering area included Gansu, Ningxia and Shannxi provinces.
4 Spatial-temporal clustering analysis
A heat-map was drawn for the regions and time of human brucellosis. It was found that the high brucellosis occurred in Xinjiang, Ningxia, Heilongjiang, Inner Mongolia and Shanxi during the study period (figure 4A), and the incidence was much higher than that in other regions during 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(figure 4B).
Finally, both spatial and temporal clusters of high incidence districts per zone were identified. The results of spatial-temporal cluster analysis for reported human brucellosis in 31 provinces of mainland China from 2012 to 2016 were shown in Table 4 and Fig. 5. There were one primary cluster area and three secondary cluster areas. The primary cluster located in northeast of China, including Inner Mongolia, Heilongjiang, Jilin and Liaoning, and the high-risk time was from January 2012 to December 2013(RR=5.17, LLR=33228.98, P<0.001). Other three secondary clusters were mainly distributed in the northeastern China and several relatively small areas in the center China. And the cluster time frames mainly ranged from January 2014 to December 2016.