The spatial structure of dengue transmission in Kamphaeng Phet Thailand

Dengue virus (DENV) is a mosquito transmitted arbovirus that represents a severe threat to global populations. The four serotypes of DENV often circulate endemically resulting in many discrete but overlapping transmission chains in space and time, effectively hiding the underlying patterns of transmission, and longer term consequences on local population immunity. Here we combined serotyped case data with sequence data from a single setting over 25 years to characterise the spatial dependence between cases, the number of distinct transmission chains circulating at any time and whether community transmission was followed by serotype-specific local herd immunity.

In this study, we combined geolocated case data (N = 7,487) from individuals that presented at a provincial hospital in Kamphaeng Phet Province, Thailand alongside a subset of the sequences of the viruses (N = 302) that infected them. We estimated the spatial dependence between case pairs over different temporal and spatial windows, using both serotype and sequence data, and used logistic regression to identify whether host population size or epidemic size were correlated with viral diversity. We found there was spatial dependence between cases at up to 5 km, resulting in localised serotype-specific patterns of immunity in subsequent years. Within a 78.5 km2 area there were 9.6 distinct transmission chains at any time and that each 100,000 increase in population size was associated with 9.8 additional chains. While local viral diversity was also correlated with the size of the epidemic within any month, we found diversity only had moderate predictive power to accurately recover epidemic dynamics.

These findings highlight the nuanced and spatially variable ecology of a highly endemic pathogen with substantial immunity. This study also illustrates the strengths of combining multiple data sources to shed light on the nature of DENV transmission.