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Malaria is an infectious parasitic disease transmitted through the bite of Anopheles mosquitoes, which seriously endangers human health (1). Although significant progress has been made on malaria control in several countries, the infection is still on the list of top health threats to humans, causing 409,000 deaths worldwide in 2019 [World Health Organization (WHO), World Malaria Report 2020]. Historically, high malaria incidence rates have been reported from 24 provincial-level administrative divisions (PLADs) of China (2). Following several decades of prevention and control, the Central Government of China with strong political commitment in malaria elimination since 2010, endorsed the National Action Plan for Malaria Elimination (2010–2020) with the goal of malaria-free nationwide by 2020 (3). The last indigenous case of malaria in China was reported in April 2016 in Yunnan Province; since then, zero indigenous case has been reported in the country (4). In order to further strengthen the surveillance capacity for post-elimination phase and meet the requirement of malaria certification. Surveillance has been carried out in areas at high risk of malaria transmission based on various indicators, such as the distribution of Anopheles spp., the number of imported cases, and the risk of re-establishment of malaria transmission in China. This study aims to describe the population distribution, density, and seasonal fluctuations of Anopheles spp. at the sentinel sites (n=18) for malaria vector surveillance from 2018 to 2020. The results showed that same malaria vectors were found in the surveillance regions, but with significant discrepancy of species distribution and seasonal fluctuation. The study will help advance our knowledge in providing baseline data for the prevention and control of imported malaria cases to consolidate the achievements of malaria elimination in China.
The statistical data of this study were achieved using information from the malaria vector surveillance system in the national malaria zero indigenous case reporting phase, 2018–2020. All 18 national malaria sentinel sites in 12 PLADs of China were enrolled for malaria vectors surveillance during the malaria transmission phase (Figure 1). According to the strategy in the “National working program for malaria vector surveillance leading for malaria elimination certification in China”, lamp trapping and human-baited tent trap methods were used to monitor the population distribution, density, and seasonal fluctuation of Anopheles vectors, and quality control was carried out by Provincial and National CDCs. Microsoft Excel software (version 2016, Microsoft, USA) was used to analyze the surveillance data.
From 2018 to 2020, a total of 33,727 Anopheles spp. mosquitos were captured by the lamp trapping method in 18 surveillance sites. From these, numbers of 31,126 An. sinensis, 1,520 An. anthropophagus, 111 An. minimus, and 970 other Anopheles spp. were identified by morphological analysis (Figure 2). An. dirus was not identified. The distribution and density of Anopheles spp. in different areas was significantly different. The proportions of An. sinensis, An. anthropophagus, and An. minimus in the total number of Anopheles spp. were 92.29%, 4.51%, and 0.33%, respectively. An. sinensis was the most widely distributed and was detected in all areas except Motuo County, Xizang (Tibet) Autonomous Region. An. anthropophagus was only detected in Donggang City, Liaoning Province. An. minimus was found distributed in Yunnan and Guangdong Provinces.
Figure 2.The number of malaria vector captured by lamp trapping method in 12 monitoring PLADs, 2018–2020.
Abbreviations: PLAD=provincial-level administrative divisions; An. minimus=Anopheles minimus; An. dirus=Anopheles dirus; An. anthropophagus=Anopheles anthropophagus; An. sinensis=Anopheles sinensis.Density and seasonal fluctuations of malaria vectors were monitored by human bait trapping methods in all surveillance sites. However, Motuo County in Xizang (Tibet) Autonomous Region was not monitored as required and was not included in the analysis. The monitoring results of the average density of malaria vectors in the surveillance sites from 2018 to 2020 were included in the density monitoring analysis and are shown in Table 1. A total of 5,481 Anopheles were captured by human bait trapping method in 17 surveillance sites, including 5,326 An. sinensis, one An. anthropophagus, 85 An. dirus, 24 An. minimus and 45 other Anopheles spp., which were different from those captured by lamp trapping method. An. anthropophagus was only detected in Congjiang County, Guizhou Province; An. dirus was detected in Wuzhishan City, Hainan Province; and An. minimus was detected in Hainan, Yunnan, and Guangdong Provinces. From 2018 to 2020, the average density of captured malaria vectors was significantly different among the surveillance sites, with the distribution range ranging from 0 to 9.15 (numbers per man per hour). Anhui and Henan provinces had the highest density distribution of Anopheles spp. Among 17 surveillance sites in 11 PLADs from 2018 to 2020, Zhejiang Province had only 1 An. sinensis, Wuzhishan City of Hainan Province had the peak malaria vectors density in May, Tengchong City and Yingjiang County of Yunnan Province, Tianya District of Hainan Province, Donggang City of Liaoning Province, and Congjiang County of Guizhou Province had peak malaria vector abundance in August, and the remaining 10 surveillance sites had a peak in Anopheles abundance between June and July.
PLADs Surveillance site Average density (number/person•hour) Early May Late May Early Jun Late Jun Early Jul Late Jul Early Aug Late Aug Early Sep Late Sep Early Oct Late Oct Anhui Feidong 1.53 1.80 4.87 6.83 4.32 5.65 2.90 1.98 2.40 1.50 0.70 0.99 Shucheng 0.00 0.01 0.33 0.35 1.64 1.67 1.25 0.52 0.14 0.04 0.06 0.03 Yunnan Tengchong 0.00 0.00 0.00 0.00 0.00 0.04 0.08 0.17 0.04 0.08 0.09 0.13 Cangyuan 0.42 0.22 0.47 0.17 0.47 1.39 0.19 0.31 0.20 0.17 0.00 0.08 Yingjiang 0.11 0.31 0.45 0.06 0.17 0.31 0.86 0.50 0.36 0.03 0.00 0.03 Hainan Tianya District 0.30 0.47 0.43 0.53 0.50 0.50 0.83 0.93 0.83 0.90 0.37 0.23 Wuzhishan 0.73 0.29 0.48 0.31 0.40 0.21 Henan Yongchen 0.00 0.17 0.14 0.06 1.64 0.00 0.00 0.00 0.03 0.00 0.00 0.00 Shihe − − 2.75 9.15 3.85 6.00 2.50 1.90 2.55 1.30 0.20 0.00 Puyang 0.00 0.00 0.17 3.03 6.03 7.31 4.75 2.59 0.72 0.50 0.00 0.00 Guizhou Congjiang − 0.34 0.31 0.39 0.07 0.01 Jiangsu Jintan 0.00 0.00 1.11 1.63 2.81 2.33 0.75 1.21 1.07 0.60 0.38 0.07 Hubei Zaoyang − − 2.90 2.97 2.13 0.90 1.20 0.87 0.20 0.07 0.00 0.00 Guangdong Potou 0.00 0.07 0.00 0.04 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 Guangxi Shanglin − 0.31 0.11 0.13 0.06 0.03 Zhejiang Yiwu 0.00 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Liaoning Donggang 0.00 0.00 0.00 0.00 0.00 0.05 0.16 0.45 0.00 0.00 0.00 0.00 Note: − : Not monitored
Abbreviation: PLAD=provincial-level administrative division.Table 1. Results of malaria vector average density surveillance by the human bait trapping methods in 17 sentinel sites, 11 PLADs, China, 2018–2020.
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