Clinical, virological and epidemiological characterization of dengue outbreak in Myanmar, 2015

Abstract

Hospital-based surveillance was conducted at two widely separated regions in Myanmar during the 2015 dengue epidemic. Acute phase serum samples were collected from 332 clinically diagnosed dengue patients during the peak season of dengue cases. Viremia levels were measured by quantitative real-time PCR and plaque assays using FcγRIIA-expressing and non-FcγRIIA-expressing BHK cells to specifically determine the infectious virus particles. By serology and molecular techniques, 280/332 (84·3%) were confirmed as dengue patients. All four serotypes of dengue virus (DENV) were isolated from among 104 laboratory-confirmed patients including two cases infected with two DENV serotypes. High percentage of primary infection was noted among the severe dengue patients. Patients with primary infection or DENV IgM negative demonstrated significantly higher viral loads but there was no significant difference among the severity groups. Viremia levels among dengue patients were notably high for a long period which was assumed to support the spread of the virus by the mosquito vector during epidemic. Phylogenetic analyses of the envelope gene of the epidemic strains revealed close similarity with the strains previously isolated in Myanmar and neighboring countries. DENV-1 dominated the epidemic in 2015 and the serotype (except DENV-3) and genotype distributions were similar in both study sites.

Keywords: 2015 dengue outbreak; Clinical; Myanmar; molecular epidemiology; viremia.

Fig. 1.

Distribution of the occurrence of (a) primary and secondary infection, and (b) disease severity at different levels among patients of different age groups.

Fig. 2.

Comparison of viremia levels by plaque assay (FcγRIIA-expressing and non-expressing BHK cells) and/or by qRT–PCR. (a) DENV IgM-positive vs. DENV IgM-negative patients, (b) according to the day of fever of patients with isolated DENV, (c) day of fever among primary infection cases, and (d) primary vs. secondary infection. *Student's t test was used for analysis.

Fig. 3.

Comparison of viral load levels among patients with isolated DENV and exhibiting different levels of severity of infection. (a) FcγRIIA-expressing BHK cells-based plaque assay, (b) FcγRIIA-non-expressing BHK cells-based plaque assay, and (c) qRT–PCR.

Fig. 4.

DENV-1 phylogenetic tree. Phylogenetic tree was constructed based on the whole nucleotide sequences of the E protein gene of DENV-1 showing the relationship of 82 strains from different sources including 28 strains of DENV-1 isolated during the 2015 epidemic in Myanmar. The representative strains of each genotype obtained from Genbank are named by country origin, strain name, year of isolation and GenBank accession number. *Upper Myanmar; **Lower Myanmar.

Fig. 5.

DENV-2 phylogenetic tree. Phylogenetic tree was constructed based on the whole nucleotide sequences of the E protein gene of DENV-2 showing the relationship of 79 strains from different sources including 24 strains of DENV-2 isolated during the 2015 epidemic in Myanmar. The representative strains of each genotype obtained from Genbank are named by country origin, strain name, year of isolation and GenBank accession number. *Upper Myanmar; **Lower Myanmar.