Close contact – analysis of dengue diversity reveals outbreaks may have a local structure

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New research has suggested that outbreaks of dengue virus have a local structure – with a majority of related cases occurring in individuals living less than 200m apart. These findings offer new insights into disease transmission and could lead to more targeted and effective control programs.

Dengue infects over 300 million individuals every year and, although only a small proportion develop severe dengue hemorrhagic fever, this can prove fatal. This study, led by John Hopkins Bloomberg School of Public Health (MD, USA) and the University of Florida (FL, USA), hoped to characterize the diversity of dengue viruses on a spatial and temporal scale.

The researchers carried out genetic sequencing on viruses obtained from 640 dengue infections  occurring between 1994 and 2010. These samples were taken both from densely-populated areas of Bangkok (Thailand) also in addition to more rural regions outside the capital and the sequencing data was overlaid with information about the location of infected individuals.

The team demonstrated that, in individuals living fewer than 200m apart, 60% of the dengue cases were derived from same transmission strain compared with individuals separated by 1–5km, where only 3% of cases came from the same transmission chain.

In addition, the scientists investigated the diversity of dengue viruses across Bangkok, estimating that there were approximately 160 distinct chains of transmission co-circulating the city within each season.  They discovered that larger populations supported a larger diversity of dengue virus strains; however, in the most densely populated areas the team observed less diversity than they had expected.

Senior author Derek Cummings, from the University of Florida, explained: “Our findings suggest that large urban centers provide a source of dengue diversity that could possibly be dispersed to other areas of the country and the world.

“But the fact that diversity saturates at the large population densities also suggests that these areas might be areas where intense competition is occurring between dengue viruses.”

The authors suggest their findings have elucidated more about the disease spread of dengue virus, and hope that interventions such as mosquito control and vaccination could potentially be better targeted based on improved understanding of transmission patterns.

Author Justin Lessler, from John Hopkins University, concluded: “What is exciting about this is that we are using new scientific tools to allow us to look inside the black box of disease transmission that we haven’t before been able to penetrate on this scale. Understanding the patterns of how infections are spread might help us start to appreciate why certain interventions aren’t working, how some could work better and what we can do to protect more people from what can be a devastating illness.”

Sources: Salije H, Lessler J, Berry IM et al. Dengue diversity across spatial and temporal scales: Local structure and the effect of host population size. Science. www.jhsph.edu/news/news-releases/2017/most-dengue-infections-transmitted-in-and-around-home.html

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