Epidemiological and molecular characteristics of emergent dengue virus in Yunnan Province near the China-Myanmar-Laos border, 2013-2015

Abstract

Background: Yunnan Province is located in southwestern China and neighbors the Southeast Asian countries, all of which are dengue-endemic areas. In 2000-2013, sporadic imported cases of dengue fever (DF) were reported almost annually in Yunnan Province. During 2013-2015, we confirmed that a large-scale indigenous DF outbreak emerged in cities of Yunnan Province near the China-Myanmar-Laos border.

Methods: Epidemiological characteristics of DF in Yunnan Province during 2013-2015 were evaluated by retrospective analysis. A total of 232 dengue virus (DENV)-positive sera were randomly collected for sequence analysis of the capsid/premembrane region of DENV from patients with DF in Yunnan Province. The envelope gene of DENV isolates was also amplified and sequenced. Phylogenetic analyses were performed using the neighbor-joining method with the Tajima-Nei model.

Results: Phylogenetically, all DENV-positive samples could be classified into DENV-1 genotype I and DENV-2 Asian I genotype during 2013-2015 and DENV-4 genotype I in 2015 from Ruili City; and DENV-3 genotype II in 2013 and DENV-2 Cosmopolitan genotype in 2015 from Xishuangbanna Prefecture.

Conclusions: Our results indicated that imported DF from patients from Laos and Myanmar was the primary cause of the DF epidemic in Yunnan Province. Additionally, DENV strains of all four serotypes were identified in indigenous cases in Yunnan Province during the same time period, while the dengue epidemic pattern observed in southwestern Yunnan showed characteristics of a hypoendemic nature: circulation of DENV-1 and DENV-2 over consecutive years.

Keywords: Dengue fever; Dengue virus; E gene; Homology; Molecular epidemiology; Phylogenetic analysis; Public health.

Fig. 1

Regional distribution of dengue fever cases in Yunnan Province, China, 2013–2015. The green region is Yunnan Province, which comprises 16 prefectures and is located in Southwest China. The cities with large-scale dengue outbreaks are highlighted using red triangles (including indigenous and imported cases), and sporadic imported cases of dengue fever evaluated in this paper are highlighted using blue pentagrams. The maps were generated and modified with Photoshop CS 8.0.1

Fig. 2

Monthly distribution of dengue fever cases in Yunnan Province, China, 2013–2015. a Monthly distribution of all indigenous and imported dengue fever cases in Ruili City, 2013–2015. b–d. Monthly distribution of indigenous and imported dengue fever cases in Ruili City in 2013 b, 2014 c, and 2015 d. e Monthly distribution of indigenous dengue fever cases in Jinghong City, 2013 and 2015. *There were more indigenous cases than imported cases in Jinghong, thus analysis of monthly distribution only included indigenous dengue fever cases. f Monthly distribution of indigenous dengue fever cases in Gengma County, 2015

Fig. 3

Age distribution of dengue fever cases in Yunnan Province of China, 2013–2015. Age distribution a and case constituent ratio b of age composition of dengue fever cases in Ruili City, 2013–2015. Age distribution c and case constituent ratio d of age composition of dengue fever cases in Jinghong City, 2013–2015

Fig. 4

Phylogenetic analysis of isolated DENV-1 E gene sequences with homologous strains reported in Yunnan Province. The phylogenetic tree was generated in MEGA version 6 (www.megasoftware.net) using the neighbor-joining method with 1000 bootstrap replicates. Viruses isolated from indigenous and imported cases evaluated in this paper are highlighted using red branches (isolates from imported cases are labeled with blue triangles). The isolates RL16–RL58, DGRL6–DGRL283, and 15DGR5–15DGR410 were isolated from Ruili City in 2013, 2014, and 2015, respectively. The isolates GM1502–GM1510 and LC1502 were isolated from Gengma County and Linxiang County of Lincang Prefecture in 2015. The isolates YNH22 andYNH12 were isolated from Kunming Prefecture in 2013

Fig. 5

Phylogenetic analysis of isolated DENV-2 E gene sequences with homologous strains reported in Yunnan Province. The phylogenetic tree was generated in MEGA version 6 using the neighbor-joining method with 1000 bootstrap replicates. Viruses isolated from indigenous and imported cases evaluated in this paper are highlighted using red branches (isolates from imported cases are labeled with blue triangles). The isolates RL16–RL54 and 15DGR8–15DGR641 were isolated from Ruili City of Dehong Prefecture in 2013 and 2015, respectively. The isolates JH1502–JH15–207 were isolated from Jinghong City of Xishuangbanna Prefecture in 2015

Fig. 6

Phylogenetic analysis of isolated DENV E gene sequences with homologous strains reported in Yunnan Province. The phylogenetic tree was generated in MEGA version 6 using the neighbor-joining method with 1000 bootstrap replicates. Viruses isolated from indigenous and imported cases evaluated in this paper are highlighted using red branches (isolates from imported cases are labeled with blue triangles). The isolates JH88–JH931 and MN1302–MN1308 were isolated from Jinghong City and Mengla County of Xishuangbanna Prefecture in 2013, respectively; the isolates 15DGR9–15DGR394 were isolated from Ruili City of Dehong Prefecture in 2015. a DENV-3, b DENV-4