Wednesday, 22 January 2014

Dengue in Viet Nam
In some, maybe the majority of cases, economic development tends to improve a country's situation regarding infectious disease; a proper sewerage system, for example, may decrease the incidence of diseases associated with the contamination of water sources. Where dengue virus is concerned this is not the case. Dengue is a human virus spread by mosquitoes of the genus Aedes (in particular A. aegypti), which happily breed in dirty water. Economic development tends to result in increased urbanisation and, as a result, ideal breeding conditions are generated for the mosquitoes (tin cans, old tyres etc.). Together, the result is a dense population of humans in the same location as the mosquitoes: in the absence of a vaccine or antivirals dengue has thus thrived. Whilst the conditions are favourable for dengue in general, there are inevitably more specific drivers of transmission and outbreaks.

A recently published study in PLOS Neglected Tropical Diseaes by Rabaa et al looked into what the drivers are for dengue in Viet Nam. The situation in Viet Nam can broadly be regarded as the south (tropical) region being endemic, whilst the north (sub-tropical) is not endemic, but experiences frequent introductions. In central Viet Nam the virus can persist for more extended periods of time, perhaps due to more favourable conditions for transmission and, ultimately, a higher level of immunity. As an illustration as to the impact of urbanisation, Ho Chi Minh city in the south is highly endemic and represents a large source of viruses for the rest of Viet Nam.

The authors compared the dengue serotype 1 (DENV-1) sequences of the envelope (E) gene.
Using a maximum likelihood approach to get an additional grasp of geographical relationships, they found that, perhaps unsurprisingly, all of the sequences belonged to the Southeast Asia subtype of Genotype I.

Phylogeography of DENV-1 genotype 1 in Southeast Asia, 1998-2009. A) Map of north (red) central (yellow) and south (blue) Viet Nam. The colours match in the graphs of mean min/max temperature (B, top graph) and mean precipitation (B, bottom graph), and branches in the maximum clade credibility phylogenetic tree (C). Purple branches represent Singapore sequences.

On the whole, DENV-1 seems to invade subtropical northern Viet Nam regularly, but never seems to become endemic - most likely due to the cold winter temperatures resulting in conditions that are refractory to continued transmission. Such invasions also occur in the central regions, although these persist for longer.
Interestingly, although (as may be expected) within a particular region the diversity among viruses was limited,  on a broader scale Ho Chi Minh City in the south was found to act as a source of virus throughout Viet Nam.

On the other hand, despite local diversity being low, it's interesting that geographically long distance movements were observed in a time-scale that precludes the hypothesis that it's merely natural spread via vectors. Instead, it appears that the movement of infected humans is responsible for seeding at least some of the regions. This is one route by which the north can be seeded. However, because the north is sub-tropical, there comes a time in the year when the vectors die off and transmission is reduced; a familiar scenario with non-tropical arboviruses. 

As interesting a piece of work as this is in itself, it arguably demonstrates something important at a more global level. Clearly DENV can be seeded in different regions by people moving around Viet Nam; if this can happen within Viet Nam, then it's not a massive step to extend Viet Nam to the world.

Maia A. Rabaa, Cameron P. Simmons, Annette Fox, Mai Quynh Le, Thuy Thi Thu Nguyen, Hai Yen Le, Robert V. Gibbons, Xuyen Thanh Nguyen, Edward C. Holmes, John G. Aaskov (2013). Dengue Virus in Sub-tropical Northern and Central Viet Nam: Population Immunity and Climate Shape Patterns of Viral Invasion and Maintenance PLOS Neglected Tropical Diseases DOI: 10.1371/journal.pntd.0002581

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