An epidemic of dengue-1 in a remote village in rural Laos

Abstract

In the Lao PDR (Laos), urban dengue is an increasingly recognised public health problem. We describe a dengue-1 virus outbreak in a rural northwestern Lao forest village during the cool season of 2008. The isolated strain was genotypically "endemic" and not "sylvatic," belonging to the genotype 1, Asia 3 clade. Phylogenetic analyses of 37 other dengue-1 sequences from diverse areas of Laos between 2007 and 2010 showed that the geographic distribution of some strains remained focal overtime while others were dispersed throughout the country. Evidence that dengue viruses have broad circulation in the region, crossing country borders, was also obtained. Whether the outbreak arose from dengue importation from an urban centre into a dengue-naïve community or crossed into the village from a forest cycle is unknown. More epidemiological and entomological investigations are required to understand dengue epidemiology and the importance of rural and forest dengue dynamics in Laos.

Figure 1. Map of Lao PDR.

Locations of the hometowns of the patients whose dengue viruses were sequenced are in grey (Luang Namtha, Latsavang (indicated by a star), Vientiane and Salavan).

Figure 2. Neighbour-Joining tree of 2,199 dengue 1 envelope gene sequences.

Tree produced using Mega 5.05 software with Kimura-2 model. Bootstrap values (in percentage), generated by using 500 replicates, are only indicated for the nodes that define the genotypes, and for clades inside genotype 1. The sequences from the Latsavang outbreak are indicated by red dots. The other Lao strains from this study are indicated by light blue dots while a dark blue dot is used for the 1996 Lao strain. The sequences from Vientiane are indicated by ‘VTE’, the ones from Luang Namtha by ‘LNT’, and the ones from Salavan by ‘SV’; all followed by year of collection.

Figure 3. Genotype 1 subtree with genome sequences.

Genotype 1 subtree, 843 sequences, from tree performed with the 39 Lao dengue 1 envelope sequences produced in this study aligned with the 1,318 DENV-1 almost complete sequences (>10,000 bases) downloaded from HFVdb. Evolutionary branches that do not include 2007–2010 Lao DENV-1 strains are not shown to increase the legibility. Origin, country (ISO 3166 code) and year of strains within these branches are indicated. The sequences from the Latsavang outbreak (XB) are indicated by red dots. The sequences from Luang Namtha (LNT) are indicated by squares, the one from Salavan (SV) by triangles and the ones from Vientiane (UI) by lozenges. Sequences from 2007 are in yellow, the ones from 2008 (except the ones from Latsavang) are in purple, the ones from 2009 in blue and the ones from 2010 in green. Groups of sequences supported by a high bootstrap value (>90) that contain at least one of Lao sequence are designated as cluster (1 to 7).