The Mosquito, the Virus, the Climate: An Unforeseen Réunion in 2018

Abstract

The 2018 outbreak of dengue in the French overseas department of Réunion was unprecedented in size and spread across the island. This research focuses on the cause of the outbreak, asserting that climate played a large role in the proliferation of the Aedes albopictus mosquitoes, which transmitted the disease, and led to the dengue outbreak in early 2018. A stage-structured model was run using observed temperature and rainfall data to simulate the life cycle and abundance of the Ae. albopictus mosquito. Further, the model was forced with bias-corrected subseasonal forecasts to determine if the event could have been forecast up to 4 weeks in advance. With unseasonably warm temperatures remaining above 25°C, along with large tropical-cyclone-related rainfall events accumulating 10-15 mm per event, the modeled Ae. albopictus mosquito abundance did not decrease during the second half of 2017, contrary to the normal behavior, likely contributing to the large dengue outbreak in early 2018. Although subseasonal forecasts of rainfall for the December-January period in Réunion are skillful up to 4 weeks in advance, the outbreak could only have been forecast 2 weeks in advance, which along with seasonal forecast information could have provided enough time to enhance preparedness measures. Our research demonstrates the potential of using state-of-the-art subseasonal climate forecasts to produce actionable subseasonal dengue predictions. To the best of the authors' knowledge, this is the first time subseasonal forecasts have been used this way.

Keywords: Aedes albopictus; Réunion; dengue; outbreak; subseasonal forecast.

Figure 1

The stage‐structured vector model performs well in comparison to Réunion Breteau index between 2007 and 2012 (Boyer et al., 2014). (a) Time series of precipitation data. (b) Time series of temperature data. (c) Time series of normalized Breteau index and normalized modeled larva abundance, where annotations indicate correlation between the two and that a sliding mean of 4 weeks was applied to the larvae abundance. (d) A linear regression model of the modeled larvae abundance against the Breteau index shows an adjusted R‐squared of 0.32.

Figure 2

(main) Simulated weekly standardized abundance of Aedes albopictus adult population in Réunion (blue curve, units in standard deviations, σ), overlaid with weekly number of reported dengue cases by ECDC (purple curve). (a) As in the main panel, but for the period highlighted in gray: 1 April 2017 to 26 March 2018. Vertical scale for dengue cases ranges from 0 to 40 to better visualize variations before December 2017. (b) Weekly rainfall amounts (green curve) and temperature values (red curve) for the period highlighted in gray; horizontal red lines indicate suitable temperatures for the vector: 20–30°C. Periods highlighted in yellow in panels (a) and (b) indicate the timing and duration of each one of the seven tropical storms/cyclones referred to in the main text. The base period for standardization is 2010–2016. Shading represents the uncertainty envelope of each curve: For standardized vector abundance, it corresponds to ±1𝜎, while for dengue cases, rainfall, and temperature, it is defined by the observed difference in weekly maxima and minima.

Figure 3

Forecast skill, measured by the Spearman rank correlation, for lead times (a) 1, (b) 2, (c) 3, and d) 4 weeks preceding the target week starting on 8 January 2018. A green square shows the location of Réunion.

Figure 4

(a) Observed and forecast rainfall anomaly (mm/day) produced (b) 1, (c) 2, (d) 3, and (e) 4 weeks preceding the target week of 8 January 2018.

Figure 5

Forecast (dashed, with standard deviation shading) and reference (simulation used observed climate data; solid) adult (red) and larvae (blue) mosquito abundance, produced (a) 1, (b) 2, (c) 3, and (d) 4 weeks preceding the target week of 8 January 2018. Each forecast was produced forcing the vector model with output from the ECMWF model (51‐member ensemble mean) and has a horizon of 45 days. The analysis focuses on the week starting on 8 January 2018 (gray shading).