Over the last century, DENV-2 epidemics were reported in 1986, 1987, 1988, and 1993 in Guangzhou [14, 15], thereafter, they have subsided. However, our findings indicate that DENV-2 has been spreading throughout Guangzhou from 2001 to 2020. Although no DENV-2 infection was detected prior to 2005, the number of annual DENV-2 cases continued to increase between 2010 and 2018. We also determined that the percentage of DENV-2 domestic cases increased from 80.95 to 95.31% between 2015 and 2018, reaching a peak in both number and percentage in 2018. Thereafter, the number and percentage of domestic cases declined slightly in 2019 but remained the second highest compared to those in previous years. Moreover, a sharp decrease in DENV-2 cases occurred in 2020, which might be related to the quarantine imposed due to the coronavirus disease 2019 outbreak [16]. That is, limiting imported cases with the quarantine may have controlled the local dengue epidemic, highlighting the relevance of monitoring imported cases for local control.
The DENV-2 epidemic in Guangzhou is greatly impacted by Southeast Asian countries. In fact, our epidemiological investigation revealed that 83.72% of DENV-2 imported cases, most of which were returning travellers, originated from Southeast Asian countries. The results of a BLAST search in GenBank and the phylogenetic tree (Fig. 1) further revealed that the DENV-2 strains sequenced in Guangzhou were closely related to those in Southeast Asian countries, which was similar to the characteristic of dengue epidemics involving the other three serotypes in Guangzhou [4, 17, 18]. Indeed, the World Health Organization statistics revealed that infections in Southeast Asian countries account for half of the global dengue burden. Meanwhile, from 2015 to 2019, dengue cases in Southeast Asia increased by 46% (from 451,442 to 658,301) [19]. China not only contiguous with Southeast Asian countries, but also is farther connected by economic ties. Moreover, with the opening of private travel abroad, the number of travellers to Southeast Asian countries has steadily increased [20,21,22]. This situation is not unique to China, as the spread of dengue viruses by travellers has become a global issue [23]. Therefore, better preparation is needed, with strict regulations, to prevent the spread of infection when travelling in endemic areas. For example, a convenient and rapid method for screening viremia that can be applied at customs may help curb the import and spread of DENV.
The Malaysia/Indian subcontinent genotype was responsible for the epidemic in Guangzhou. As displayed in the phylogenetic tree in Fig. 1, this genotype, shown in yellow, constitutes 95.95% (142 sequences) of the total 148 DENV-2 sequences. Meanwhile only six sequences of the Southeast Asia genotype were detected. Moreover, no genotype shift in the Malaysia/Indian subcontinent genotype was observed over the 20-year study period. The most recent common ancestor of all Malaysia/Indian subcontinent strains was estimated to have appeared in 1955. However, when dividing the Malaysia/Indian subcontinent genotype into its different lineages, a shift from lineage GZ4 to GZ5, was observed between 2013 and 2014, which coincided with a rising number of DENV-2 cases since 2014. Although there is no clear relationship between lineage and virulence in DENV, outbreaks, limited circulation, and spreading related to shifts in lineages have been reported [24,25,26,27]. Substitutions in the envelope gene that may result in maturation and activation of macrophages, with consequent enhancement of the immune response characterised by increased production of cytokines, are considered to be likely causes of the prevailing differences among lineages. Meanwhile, positive selection analysis of the GZ5 lineage by MEME exhibited signs of directional selection, which may contribute to the prevalence of GZ5. However, further research is needed to confirm whether this lineage shift is responsible for the rise in cases.
With the prevalence of the Malaysia/Indian subcontinent genotype in Guangzhou, the relatively low rate of DHF/DSS suggests that this genotype may be less virulent. Secondary infection with DENV-2 after infection with heterotypic DENV is believed to be associated with an increased risk of DHF/DSS [6]. However, with the DENV-1epidemic in Guangzhou persisting for more than 20 years and the rising number of DENV-2 cases [4, 5], the incidence of DHF/DSS remained relatively low compared with the global incidence and that of Southeast Asian countries [28,29,30,31]. Another study revealed that secondary infection with the American genotype of DENV-2 failed to cause DHF/DSS [32], whereas other extensive studies indicated that the Southeast Asian genotype was more efficient at infection and was also more likely to cause DHF/DSS [7, 8, 32, 33]. Of the 148 sequences detected in Guangzhou, only six (4.05%) belonged to the Southeast Asia genotype, four of which were identified from imported cases. This revealed that the Southeast Asia genotype was rare in Guangzhou, which may explain the low incidence of DHF/DSS. Meanwhile, the prevailing Malaysia/Indian subcontinent genotype has a limited capacity for leading to DHF/DSS. However, further studies are needed to determine whether the incidence of DHF/DSS is low in other areas, with an epidemic dominated by the Malaysia/Indian subcontinent genotype. Determining the critical differences between genotypes and host immune mechanisms that may enhance the pathogenesis of genotypes might provide new insights to advance the current understanding regarding the mechanism of DHF/DSS.
The DENV-2 epidemic in Guangzhou was complex. Specifically, the MCC tree in Fig. 2 revealed that the Guangzhou strains originated from different time periods, indicating various evolution and dissemination paths. Strains that clustered into lineage GZ5 shared the eldest ancestor in 1995, whereas strains belonging to lineage GZ1 emerged in 2016. Meanwhile, the domestic and foreign strains of lineage GZ2 detected in 2014 and 2016 shared the same ancestor in 2012. The strain then evolved during 2014 and 2016 in Guangzhou and spread not only in China but also throughout Thailand and Vietnam. However, within the same year, the strains may have also be distributed in different lineages; for example, the sequences from isolates obtained in 2018 were distributed among lineages GZ1, GZ3, GZ4 and GZ5. This co-epidemic of different lineages showing different origins has complicated the epidemic situation in Guangzhou.