Empirical optimization of risk thresholds for dengue: an approach towards entomological management of Aedes mosquitoes based on larval indices in the Kandy District of Sri Lanka

Lahiru Udayanga,Kusumawathie Pahalagedara,Mohamed Cassim Mohamed Iqbal,Wimaladharma Abeyewickreme,Mohamed Mujithaba Mohamed Najim,Nayana Gunathilaka

doi:10.1186/s13071-018-2961-y

Lahiru Udayanga, Kusumawathie Pahalagedara + Show 4 more

Open Access

https://doi.org/10.1186/s13071-018-2961-y

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Abstract

BackgroundLarval indices such as Premise Index (PI), Breteau Index (BI) and Container Index (CI) are widely used to interpret the density of dengue vectors in surveillance programmes. These indices may be useful for forecasting disease outbreaks in an area. However, use of the values of these indices as alarm signals is rarely considered in control programmes. Therefore, the current study aims to propose threshold values for vector indices based on an empirical modeling approach for the Kandy District of Sri Lanka.MethodsMonthly vector indices, viz PI, BI and CI, for Aedes aegypti and Aedes albopictus, of four selected dengue high risk Medical Officer of Health (MOH) areas in the Kandy District from January 2010 to August 2017, were used in the study. Gumbel frequency analysis was used to calculate the exceedance probability of quantitative values for each individual larval index within the relevant MOH area, individually and to set up the threshold values for the entomological management of dengue vectors.ResultsAmong the study MOH areas, Akurana indicated a relatively high density of both Ae. aegypti and Ae. albopictus, while Gangawata Korale MOH area had the lowest. Based on Ae. aegypti, threshold values were defined for Kandy as low risk (BIagp > 1.77), risk (BIagp > 3.23), moderate risk (BIagp > 4.47) and high risk (BIagp > 6.23). In addition, PI > 6.75 was defined as low risk, while PI > 9.43 and PI>12.82 were defined as moderate and high risk, respectively as an average.ConclusionsThreshold values recommended for Ae. aegypti (primary vector for dengue) along with cut-off values for PI (for Ae. aegypti and Ae. albopictus), could be suggested as indicators for decision making in vector control efforts. This may also facilitate the rational use of financial allocations, technical and human resources for vector control approaches in Sri Lanka in a fruitful manner.

Highlights

Larval indices such as Premise Index (PI), Breteau Index (BI) and Container Index (CI) are widely used to interpret the density of dengue vectors in surveillance programmes
Breteau Index The highest value of maximum Breteau Index for Aedes aegypti (BIagp) value (15.62) was recorded in the Kandy Municipal Council (KMC) and Gangawata Korale (GK) Medical Officer of Health (MOH) areas (Fig. 2), while Akurana had the lowest value of maximum BIagp value as 12.14
PI > 6.75% was defined as low risk, while PI > 9.43 and PI > 12.82 were defined as moderate and high risk, respectively, as an average

Summary

Introduction

Larval indices such as Premise Index (PI), Breteau Index (BI) and Container Index (CI) are widely used to interpret the density of dengue vectors in surveillance programmes. These indices may be useful for forecasting disease outbreaks in an area. During the peak of an epidemic, chemical fogging is conducted based on the Breteau Index (BI) in dengue high risk areas. The selection of high risk areas to initiate vector control activities and post-monitoring of the effectiveness of ongoing vector control activities are achieved based on the entomological indicators, larval indices in Sri Lanka [6]

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