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. 2016 Jul 26;10(7):e0004842.
doi: 10.1371/journal.pntd.0004842. eCollection 2016 Jul.

A Sequence of Flushing and Drying of Breeding Habitats of Aedes aegypti (L.) Prior to the Low Dengue Season in Singapore

Osama M E Seidahmed  1 , Elfatih A B Eltahir  1
Affiliations

A Sequence of Flushing and Drying of Breeding Habitats of Aedes aegypti (L.) Prior to the Low Dengue Season in Singapore

Osama M E Seidahmed et al. PLoS Negl Trop Dis. .

Abstract

In dengue-endemic areas, transmission shows both a seasonal and interannual variability. To investigate how rainfall impacts dengue seasonality in Singapore, we carried out a longitudinal survey in the Geylang neighborhood from August 2014 to August 2015. The survey comprised of twice-weekly random inspections to outdoor breeding habitats and continuous monitoring for positive ones. In addition, observations of rainstorms were collected. Out of 6824 inspected habitats, 67 contained Aedes aegypti, 11 contained Aedes albopictus and 24 contained Culex spp. The main outdoors habitat of Aedes aegypti was storm drains (54/67). We found that 80% of breeding sites in drains (43/54) were lost after intense rainstorms related to the wet phase of the Northeast monsoon (NE) between November 2014 and early January 2015. Subsequently, 95% (41/43) of these flushed drains had dried out during the dry phase of the NE in late January-February 2015. A return in the outdoor breeding of Aedes aegypti was observed after the onset of Southwest monsoon (SW) between May and August 2015. There was also a reduction in productivity of breeding habitats for larvae and pupae after the onset of the NE. In wet equatorial regions like Singapore, rainfall varies with the monsoons. A monsoon-driven sequence of flushing and drying shapes the outdoor seasonal abundance of Aedes aegypti. This finding can be used to optimize vector control strategies and better understand dengue in the context of climate change.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Seasonality of dengue in Singapore: boxplot of monthly percent of cases (1983–2015).
Fig 2
Fig 2
A) Mean daily intensity of rainfall in Singapore per a rainy day (1983–2011). The intensity is calculated by dividing amount of rainstorms by rainy days of a month. B) Mean duration of a dry period (i.e. sum of hourly stretches without rainfall) without a rainstorm. C) Monthly average numbers of rainy days; notice February is the driest month with only eight rainy days. Arrows indicate the Northeast (i.e. the wet and dry phases, NE1 and NE2, respectively) and Southwest (SW) monsoons. Data source: Changi station—National Environmental Agency (NEA).
Fig 3
Fig 3. A descriptive sketch for the rainfall flushing mechanism shows how intense rainstorms during the monsoon results in washing breeding of dengue vector from stagnant drains.
Fig 4
Fig 4. The study area: Geylang neighborhood, Singapore.
The figure shows locations of positive breeding sites for Aedes aegypti, Ae albopictus and Culex spp. Breeding drains of Ae. aegypti are given serial numbers that also used in Fig 6.
Fig 5
Fig 5. Monthly ratio of wet/dry habitats in Geylang (August 2014–August 2015).
Total numbers of inspected habitats are shown above bars. Arrows indicate periods of the wet and dry phases of the Northeast monsoon (NE1 and NE2, respectively) and Southwest monsoon (SW). Note: no inspections were carried out between 2/21/15 and 3/10/15.
Fig 6
Fig 6. Timeline of the breeding drains of Aedes aegypti in Geylang, Singapore: August 2014- August 2015.
(SP: stagnant and positive, DN: Dry and Negative, SN: Stagnant and Negative, FN: Flushed and Negative). Grids along the x-axis represent the twice-weekly follow-ups. White grids indicate no inspections were carried out in these drains (two weeks between 2/21/2015 and 3/10/2015). Locations of the breeding drains are shown in Fig 4.
Fig 7
Fig 7
A. Intensity of daily rainstorms in Geylang (August 2014–August 2015). B. Increase of water level in a drain in Geylang (August 2014–August 2015). C. larval density of Aedes aegypti per an outdoor breeding habitat in Geylang per semiweekly survey. D. Pupal-productivity of drains and non-drains for Ae. aegypti per semiweekly survey. Note: no monitoring was carried out between 2/21/2015 and 3/10/2015.
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Grants and funding

This research was supported by the National Research Foundation Singapore through the Singapore-MIT Alliance for Research and Technology's Center for Environmental Sensing and Modeling interdisciplinary research program. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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