Discussion

The trend of malaria in China between 1950 and 2016 was summarised. The incidence and mortality of malaria fluctuated during some periods. In general, the burden of malaria among the Chinese population was dramatically reduced. Consistent with this trend, the global incidence rate of malaria was estimated to have decreased by 18% between 2010 and 2017, and the WHO South-east Asia region recorded the largest decline (59%) [4]. These achievements may largely be attributed to the economic development and the improvement of disease management.

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 [Reference Ya-Nan15]. 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) [Reference Huang16]. 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 [Reference Ya-Nan15].

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 [Reference Wu17]. 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 [Reference Fang18]. The susceptible population was consistent with that of mosquitoes, and these conditions were conducive to the occurrence of the malaria epidemic.

Males suffered more disease burden of malaria than females from this study, males accounted for 64.8%–87.0% of malaria cases in previous studies [Reference Ya-Nan15, Reference Kong19]. This may be associated with the work activities, for example, farmers (mostly males) were more likely to be exposed to the environment of Anopheles mosquitoes. It has been observed that all age groups were at risk of malaria [Reference Kong19]. Another study between 2006 and 2010 reported the lowest incidence rates in those under 5 years of age (1.32 per 100 000) and highest rates in those over 60 years (3.41 per 100 000) [Reference Ya-Nan15]. The WHO concluded that children under 5 years of age and pregnant women were particularly susceptible to infection, illness and death, especially in high transmission areas [1]. The susceptibility to malaria among the population was mostly related with the types of transmission (continuous, seasonal or short time). In permanent transmission areas (most African regions), adults developed the immunity and children were more susceptible. While in seasonal transmission areas, the level of exposure was not enough to build a consistent natural immunisation in adults, so the whole population expressed comparable susceptibility. In China, all age groups showed a similar susceptibility to malaria (Supplementary Fig. S4) and population at highest risk was exposed to risk factors related to work activities and location of residence [Reference Wang20].

P. vivax is the dominant malaria parasite in most countries outside of sub-Saharan Africa and that P. falciparum is responsible for most malaria-related deaths globally [1, 3]. In China, P. vivax accounted for 76.3% (185 013 cases) of malaria incidence cases and 8.5% (207 701 cases) by P. falciparum between 2004 and 2013. The proportion of malaria deaths attributed to P. falciparum was 87.7% and 6.8% by P. vivax. These results were consistent with other reports [Reference Kong19, Reference Lin21–Reference Xiao-Dong23].

China has initiated many efforts to prevent malaria and reduce the disease burden. Case reporting from the private sector was mandatory in the 1950s. Since then, many policies and strategies for intervention, diagnosis, treatment and surveillance have been adopted [24]. The global fund to fight AIDS, tuberculosis and malaria has supported China between 2003 and 2012 [Reference Hu7]. The Chinese government has covered the investment gaps since 2012 [Reference Wang20]. The national malaria elimination action was initiated in 2010 [25]. The successful control of malaria in China was mainly attributed to the country leadership efforts (national elimination strategy, community mobilisation, case-based surveillance and reference laboratory systems) and the country-led efforts (regional and inter-sectoral collaboration, sustained financial support) [Reference Feng26]. The technical ‘1–3–7' strategy, which specifies case reporting within one day, case investigation within three days and focuses investigation and action within seven days, was also rolled out and widely disseminated [Reference Cao6]. The timeliness of the implementation of malaria surveillance and response largely improved after the strategy was integrated into the CISDCP system. Other countries with high malaria burden could learn that elimination of disease should involve country leadership and multi-sectoral cooperation.

China is listed as one of the countries that were on course for malaria elimination by 2020 [4, 8]. The number of malaria cases and deaths between actual and prediction were very close. In 2017, there were 2 697 malaria cases reported, among which six patients died [27]. The predicted number of malaria cases was 2 914 for 2017 and 2 562 for 2020. The predicted number of deaths would decrease from 13 in 2017 to 10 in 2020. Among those cases, most of which were imported cases and became the major source of disease burden. There will be none indigenous cases during 2016 and 2020 by the prediction compared to the actual three indigenous cases from areas bordering Yunnan and Tibet in 2016, and zero in 2017 [8]. In short, the goal of eliminating malaria by 2020 will be highly probable in China.

Along with the vector controlling (insecticide-treated mosquito nets and indoor residual spraying) nationwide [1], China should give more attention to cross-border areas and imported malaria cases recently [Reference Feng28]. Yunnan and the Tibet Autonomous Region were the only regions that reported locally acquired malaria cases in 2014 [Reference Hu7]. Several high-risk counties in the long international land border still face a persistent risk of malaria introduction and transmission [Reference Zhang22]. The imported malaria cases are the key challenges to achieve malaria elimination in China [Reference Lai29]. Since the majority of malaria infections are asymptomatic, active case detection methods in community should also be conducted to overcome the limitations of passive surveillance [Reference Sturrock30]. A recent study revealed that African ape parasites in the wild are 10-fold more diverse than human P. vivax and exhibit no evidence of species specificity, which means that P. vivax will never be eradicated unless to eliminate malaria from apes, or it can cross the border at any time and cause new malaria outbreaks [Reference Loy31]. Resurgence of malaria may occur if control and surveillance measures are scaled back too early [Reference Lai32]. Historically, a marked increase in malaria incidence worldwide occurred as the funding was cut further [3]. For China, a practical strategy should focus on the investigation and disposal of malaria case and foci in the specific areas and among the high-risk population [Reference Yang and Zhou33].