New research modelling the epidemiology of dengue haemorrhagic fever (DHF) in Thailand has uncovered a 3-year periodicity in the magnitude of disease incidence, and a travelling wave of infection that spreads outwards from urban centres.

Of the 50–100 million people each year that are infected with mosquito-borne Dengue virus, up to 500,000 develop the potentially fatal DHF. Because the incidence of DHF can fluctuate tenfold from one year to the next, epidemics can overwhelm healthcare providers.

Using the statistical tool empirical mode decomposition (EMD) to analyse the monthly incidence of DHF in 72 provinces of Thailand over a 14-year period, the authors found a 3-year periodicity of outbreaks that was masked in the raw data. This periodicity is reminiscent of host–pathogen and predator–prey relationships, but hasn't been observed before for a vector-borne disease.

By correlating the periodicity with the distance of provinces from Bangkok, a clear pattern emerged — 68 provinces had a pattern that was synchronized with, but lagged behind, Bangkok. DHF emanated out from Bangkok at 148 km per month as a travelling wave, which had a broad range affecting almost the whole country. Border regions conflicted with the pattern and the authors speculate that other urban areas might affect spread as well.

Why does a wave emanate from Bangkok? One reason might be complex interactions between the four different serotypes of dengue virus and a large urban host population.

There are few tools other than vector control to prevent dengue virus transmission, so this model could be valuable in reducing the burden of this disease.