Malaria in China: a longitudinal population-based surveillance study

Ding, Cheng; Huang, Chenyang; Zhou, Yuqing; Fu, Xiaofang; Liu, Xiaoxiao; Wu, Jie; Deng, Min; Li, Lanjuan; Yang, Shigui

Most malaria cases (92%) were in the WHO African region [4] . The geographical distribution of malaria was also unbalanced within China. The high-incidence areas were mainly concentrated in central and southern areas of China (Henan, Anhui, Jiangsu, Hubei, Sichuan, Yunnan, Guangdong and Guangxi), while relatively low in northern areas. A previous study reported that the number of malaria cases in Anhui, Yunnan and Hainan provinces accounted for 77.48% of the total cases from 2006 to 2010 and concluded that provinces with high incidence rates were concentrated in central and southwestern China and Hainan [15] . Furthermore, the geographical distribution of malaria tended to concentrate as time progressed from this study. It was also revealed that the spatiotemporal distribution of malaria in China changed from relatively scattered (2004-2009) to relatively clustered (2010-2014) [16] . This mainly resulted from imported cases, since these imported ones from trade activities and international workers would directly increase the number of cases which were not attributed to local climate conditions or mosquitoes density. For mortality, the rates were relatively high in southern China in the 1950s, and the disparity between areas was not obvious and remained at low levels since the 1970s. Seasonal patterns of malaria in China were obvious with the incidence rate peaked in July through September and mortality peaked in August through October. Another study also showed that malaria cases were mostly concentrated in the second half of the year, particularly in July through October [15] . The seasonal and geographical distribution of malaria observed might be associated with the seasonal dynamic of Anopheles mosquito vectors and human population distributions. National surveillance of mosquitoes showed that the mosquito density in the areas south of the Yangtze River was higher than that in other areas, and this corresponded to areas of higher malaria incidences. The density of Anopheles mosquitoes was peaked during June through August [17] . Natural and climatic conditions in summer are more suitable for mosquitoes compared to those in winter. Additionally, the distribution of the population was regional (mainly concentrated in the southeast and central regions), which was also associated with factors such as river network density, temperature and the warming index [18] . The susceptible population was consistent with that of mosquitoes, and these conditions were conducive to the occurrence of the malaria epidemic.