Since the first outbreak of DF in 1978, the record number of dengue outbreaks were observed in Zhejiang, Beijing, Guangxi, Fujian, Yunnan, Henan, Hubei, and Jiangsu. DENV infections have become a public health concern in China[19, 25, 27]. Located in southwestern China,Yunnan Province neighbors the southeast Asian countries, such as Myanmar, Laos and Vietnam, which are all dengue-endemic areas. Therefore, imported cases from Laos and Myanmar are the main reason of DENV epidemic in Yunnan Province [23, 24, 28]. Xishuangbanna has abundant tourism resources and economic development due to trade with southeast Asian countries. As a result, Xishuangbanna appeal to millions of tourists worldwide every year, increasing the risk of DENV infections in the local population. Since the first outbreak of DF in Yunnan in 2008[22], the number of DENV cases has been increasing in Xishuangbanna. There were 1319 cases confirmed DENV–3 in 2013, 1132 cases confirmed DENV–2 in 2015 and 1384 cases confirmed DENV–1 in 2017[25, 29–31]. Therefore, it is important to continuously monitor the DF epidemic in Xishuangbanna.
In our study, a total of 87 serum samples collected from febrile patients at Xishuangbanna. However, 52 of these samples were successfully to amplified by PCR and sequenced, this reduction may be due to in sample collection time points.In this study , phylogenetic analysis was conducted based on the structural protein genes (C/prM/E), which are critical for viral adsorption, penetration, replication, and release[32–34], and they were compared with reference sequences from all over the world. Our results reveal that all 52 strains were DENV–1 and can be divided into two clusters and most of the epidemic strains of Xishuangbanna were evolutionarily close to the Fujian and Singapore strains, and two of epidemic strains of Xishuangbanna were evolutionarily close to the Myanmar 2017 strain. Previous studies have reported that the imported cases in China were primarily from Asian countries and Laos and Myanmar were the main cause of the DF epidemic in the Yunnan Province[23, 24, 35, 36]. And our previous study show that the first DENV–1 outbreak in Xishuangbanna in 2017, which contain imported cases from Myanmar, were evolutionarily close to the domectic epidemic strains(Guangzhou, JQ048541 and Hubei, KP772252) and Southeast Asia epidemic strain (Laos, KC172834). Compared with the DENV–1 outbreak in 2017, it is a small-scale DENV epidemic in xishuangbanna in 2018, but the viral strains also close to the domectic epidemic strains(Fujian, DQ193572) and Southeast Asia epidemic strain (Singapore, MF314188 and Myanmar, MG679801)). This suggests that the outbreak of DF may be due to imported cases from neighboring countries of Southeast Asia or the domectic epidemic strains circuating. Genetic recombination is an vital mechanism for virus to evolve, as the virus can gain genetic variation through gene recombination, and the speed of gene recombination is higher than mutation and natural selection alone[37, 38]. To reveal the role of genetic ecombination, the structural proteins of Xishuangbanna 2018 strains (MN123849 and MN123854) were analyzed by homologous recombination compared with DENV–1SS. The results indicated that no recombinant signals were found in the two strains of DENV.
Although DENV have spread rapidly worldwide and lead to millions of people infected every year, licensed vaccine or effective antiviral treatment are not available to control DENV. Therefore, it is urgent to develop a safe and effective vaccine against DENV infection. Structural proteins, especially E protein, serve as the main target to develop subunit vaccine candidates[39]. The E protein has three domains(I-III). Domain III contains serotype-specific epitopes that can elicit neutralizing antibodies and the host cell receptor recognition site that subserve viral attach and entry into host cell[40–42], EDIII is widely used in vaccine development for it can induce antibodies against DENV infection[42–44]. Kawano et al. study found that the amino acid residue located in E155(DI region) of DENV–4 converted from T to I change virulence of the virus[45], our previous study also find the E155 amino acid of DENV–1 change from I to T may cause a weak in viral virulence[25]. However, in our this study, we found the amino acid residue of E155 (position: 435) in strains from Xishuangbanna was mutation from T to S, this may also affect the virulence of the endemic virus.
Due to geographical location and national economic policies, there have been DF outbreaks in most southeast countries, and therefore it is hard to find the exact origin of the pandemic strain. Thus, this study can serve as potential reference for further investigation of molecular epidemiology, infection, pathogenicity and the vaccine development of DENV.