Gravitrap deployment for adult Aedes aegypti surveillance and its impact on dengue cases

Abstract

House Index, Container Index, and Breteau Index are the most commonly used indices for dengue vector surveillance. However, these larval indices are a poor proxy for measuring the adult population-which is responsible for disease transmission. Information on the adult distribution and density are important for assessing transmission risk as well as for developing effective control strategies. This study introduces a new entomological index, Gravitrap aegypti index (GAI), which estimates the adult female Aedes aegypti population in the community and presents its association with dengue cases. Gravitraps were deployed across 34 treatment sites in Singapore from September 2013 to September 2016. The GAI, derived from the Gravitrap surveillance data, was analysed to investigate the spatio-temporal patterns of the Ae. aegypti population in Singapore. The index was further categorised into low, moderate, and high-risk groups and its association with dengue cases were examined. A Before-After Control Impact analysis was performed to evaluate the epidemiology impact of Gravitrap system on dengue transmission. The Ae. aegypti population exhibits a seasonal pattern, and spatial heterogeneity in Ae. aegypti abundance was observed among treatment sites. The Ae. aegypti population was also found to be unevenly distributed among floors of an apartment block, with low floors (floors 1-4) having a higher abundance of mosquitoes trapped than mid (floors 5-8) and high (floors �9) floors. Areas with high GAI were shown to have higher dengue case count. Gravitrap has also demonstrated to be a good dengue control tool. The contribution of cases by treatment sites to the national numbers was lower after Gravitraps deployment. The GAI, which is of better relevance to dengue transmission risk, could be recommended as an indicator for decision making in vector control efforts, and to monitor the spatio-temporal variability of the adult Aedes population in the country. In addition, findings from this study indicate that Gravitraps can be used as a dengue control tool to reduce dengue transmission.

Spatio-temporal analysis of the main dengue vector populations in Singapore

“…Aedes mosquito data were collected fortnightly for each of the 552 sites from 2017–2018 [ 19 ], with odd-numbered blocks inspected 1 week and even-numbered blocks the next. Fortnightly collections were then halved to obtain the weekly numbers of Ae.…”

“…Vector control activities were also ramped up in dengue cluster areas, with space sprays used for adulticiding. To monitor the spatio-temporal trend of the adult Aedes abundance in Singapore, the National Environment Agency also established an islandwide Gravitrap surveillance system in 2017, with over 50,000 Gravitraps deployed in the public housing estates across the island [ 18 , 19 ], which accommodate ~ 80% of the resident population [ 20 ]. The weekly mean catch per trap for each species provides an indication of the Aedes abundance around each specific residential location and each time point, which is presumed to be closely associated with an individual’s risk of exposure to mosquito bites inside or around homes and also much less susceptible to the measurement bias encountered in non-systematic breeding sites inspection [ 21 ].…”

Short Report: Adult Aedes abundance and risk of dengue transmission

“…two Gravitraps on each of three floors per apartment block: lower floor(2 nd ), mid floor (5 th or 6 th ) and high floor (10 th or 11 th ). Mosquitoes were collected from the Gravitraps on a weekly basis and are entomologically identified in the laboratory using mosquito identification keys [ 21 , 22 ]. More than 50,000 Gravitraps were deployed as of 2019 [ 23 ].…”

Reducing mosquito-borne disease transmission to humans: A systematic review of cluster randomised controlled studies that assess interventions other than non-targeted insecticide

“…When mosquito-borne disease rates show considerable year-to-year variation,[ 16 ] simply applying an intervention to the entire study area and comparing to pre-baseline rates would not provide evidence of the intervention’s effectiveness, although this approach has demonstrated the effectiveness of gravitraps in reducing dengue incidence in Singapore by focussing on multiple sites. [ 29 ] Furthermore, a cRCT study design has the advantage of controlling for the movements and behaviours of study area residents, therefore reducing the impact of contamination bias on outcome measures. Due to the limited size of the proposed study area, it may not be possible to incorporate buffer zones between control and intervention clusters to reduce contamination from mosquito movement without compromising statistical power.…”

“…However, mosquito trapping has been effective in reducing mosquito abundance in other studies. [ 29 , 35 , 36 ] The study design should maximise participant compliance with the intervention and assess compliance levels. Measures of self-reported compliance can be complemented by other investigations, such as unannounced checks.…”