Stacy Soh

Refuse storage and collection systems are potential sources of food and harbourage areas for rodents which transmit pathogens. We examined the factors associated with rodent activity in public housing municipal waste collection premises in a highly urbanized city-state. We analysed data from April 2019 to March 2020 in mixed-effects logistic regression models to examine the independent factors associated with rodent activity in central refuse chute rooms (CRCs), individual refuse chute (IRC) bin chambers and bin centres. We accounted for within-year patterns, repeated measures and nested effects. We observed a heterogeneous spatial distribution of rodent activity. Rodent droppings were strongly associated with rodent activity in CRCs (aOR: 6.20, 95% CI: 4.20–9.15), bin centres (aOR: 3.61, 95% CI: 1.70–7.64) and IRC bin chambers (aOR: 90.84, 95% CI: 70.13–117.67). Gnaw marks were positively associated with rodent activity in CRCs (aOR: 5.61, 95% CI: 3.55–8.97) and IRC bin chambers (aOR: 2.05, 95% CI: 1.43–2.95), as were rub marks in CRCs (aOR: 5.04, 95% CI: 3.44–7.37) and IRC bin chambers (aOR: 3.07, 95% CI: 1.74–5.42). Each burrow increased the odds of rodent sightings in bin centres (aOR: 1.03, 95% CI: 1.00–1.06). The odds of rodent sightings in an IRC bin chamber increased with every additional bin chute chamber within the same block (aOR: 1.04, 95% CI: 1.01–1.07). We identified several factors that well predicted rodent activity in waste collection premises. Municipal estate managers with limited resources can adopt a risk-based approach in tailoring the focus of their rodent control interventions.

Dengue is a major vector-borne disease worldwide. Here, we examined the spatial distribution of extreme weekly dengue outbreak risk in Singapore from 2007 to 2020. We divided Singapore into equal-sized hexagons with a circumradius of 165 m and obtained the weekly number of dengue cases and the surface characteristics of each hexagon. We accounted for spatial heterogeneity using max-stable processes. The 5-, 10-, 20-, and 30-year return levels, or the weekly dengue case counts expected to be exceeded once every 5, 10, 20, and 30 years, respectively, were determined for each hexagon conditional on their surface characteristics remaining constant over time. The return levels were higher in the country’s east, with the maximum weekly dengue cases per hexagon expected to exceed 51 at least once in 30 years in many areas. The surface characteristics with the largest impact on outbreak risk were the age of public apartments and the percentage of impervious surfaces, where a 3-year and 10% increase in each characteristic resulted in a 3.8% and 3.3% increase in risk, respectively. Vector control efforts should be prioritized in older residential estates and places with large contiguous masses of built-up environments. Our findings indicate the likely scale of outbreaks in the long term.

The Incompatible Insect Technique (IIT) strategy involves the release of male mosquitoes infected with the bacterium Wolbachia. Regular releases of male Wolbachia-infected mosquitoes can lead to the suppression of mosquito populations, thereby reducing the risk of transmission of vector-borne diseases such as dengue. However, due to imperfect sex-sorting under IIT, fertile Wolbachia-infected female mosquitoes may potentially be unintentionally released into the environment, which may result in replacement and failure to suppress the mosquito populations. As such, mitigating Wolbachia establishment requires a combination of IIT with other strategies. We introduced a simple compartmental model to simulate ex-ante mosquito population dynamics subjected to a Wolbachia-IIT programme. In silico, we explored the risk of replacement, and strategies that could mitigate the establishment of the released Wolbachia strain in the mosquito population. Our results suggest that mitigation may be achieved through the application of a sterile insect technique. Our simulations indicate that these interventions do not override the intended wild type suppression of the IIT approach. These findings will inform policy makers of possible ways to mitigate the potential establishment of Wolbachia using the IIT population control strategy.

The effective reproduction numberR_tis an epidemiological quantity that provides an instantaneous measure of transmission potential of an infectious disease. While dengue is an increasingly important vector-borne disease, few have usedR_tas a measure to inform public health operations and policy for dengue. This study demonstrates the utility ofR_tfor real time dengue surveillance. Using nationally representative, geo-located dengue case data from Singapore over 2010–2020, we estimatedR_tby modifying methods from Bayesian (EpiEstim) and filtering (EpiFilter) approaches, at both the national and local levels. We conducted model assessment ofR_tfrom each proposed method and determined exogenous temporal and spatial drivers forR_tin relation to a wide range of environmental and anthropogenic factors. At the national level, both methods achieved satisfactory model performance (R²_EpiEstim= 0.95, R²_EpiFilter= 0.97), but disparities in performance were large at finer spatial scales when case counts are low (MASE_EpiEstim= 1.23, MASE_EpiFilter= 0.59). Impervious surfaces and vegetation with structure dominated by human management (without tree canopy) were positively associated with increased transmission intensity. Vegetation with structure dominated by human management (with tree canopy), on the other hand, was associated with lower dengue transmission intensity. We showed that dengue outbreaks were preceded by sustained periods of high transmissibility, demonstrating the potential ofR_tas a dengue surveillance tool for detecting large rises in dengue cases. Real time estimation ofR_tat the fine scale can assist public health agencies in identifying high transmission risk areas and facilitating localised outbreak preparedness and response.

The release of Wolbachia-infected mosquitoes is a promising disease intervention strategy that aims to control dengue and other arboviral infections. While early field trials and modelling studies suggest promising epidemiological and entomological outcomes, the overall cost effectiveness of the technology is not well studied in a resource rich setting nor under the suppression approach that aims to suppress the wild-type mosquito population through the release of Wolbachia-infected males. We used economical and epidemiological data from 2010 to 2020 to first ascertain the economic and health costs of dengue in Singapore, a high income nation where dengue is hyper-endemic. The hypothetical cost effectiveness of a national Wolbachia suppression program was then evaluated historically from 2010 to 2020. We estimated that the average economic impact of dengue in Singapore from 2010 to 2020 in constant 2010US$ ranged from $1.014 to $2.265 Billion. Using empirically derived disability weights, we estimated a disease burden of 7,645–21,262 DALYs from 2010–2020. Under an assumed steady-state running cost of a national Wolbachia suppression program in Singapore, we conservatively estimate that Wolbachia would cost an estimated $50,453–$100,907 per DALYs averted and would lead to an estimated $329.40 Million saved in economic costs over 2010 to 2020 under 40% intervention efficacy. Wolbachia releases in Singapore are expected to be highly cost-effective and its rollout must be prioritised to reduce the onward spread of dengue.