Scientific evidence emerging from an ongoing epidemiological study on dengue fever in Kerala has pointed to the need for better vector surveillance and control strategies which will help the State develop accurate dengue outbreak prediction models.
The study, being conducted by the Community Medicine Department of Thiruvananthapuram Government Medical College (TMC), with WHO’s technical support, is the first ever to explore the epidemiology of dengue fever in Kerala.
It was launched by the government in 2017 in the aftermath of the worst ever dengue outbreak faced by Kerala, which had resulted in high case burden and mortality.
Unreliable indices
Vector control remains at the forefront as the most important tool for dengue control.
Yet, epidemiologists are more in agreement that the entomological indices that are currently relied on to estimate dengue transmission risk and predict an impending outbreak, such as Breteau Index (BI), House Index (HI) or Container Index (CI), may be unreliable and that there is little evidence of any quantifiable association between these indices and dengue outbreaks.
“For long we have been relying on BI and HI to predict dengue outbreaks because these are easy to do. But increasing evidence points to adult mosquito surveillance being the crucial factor determining the severity of an outbreak. However, devising surveillance and control measures to check the population of adult mosquitoes at the community level is not easy because both Aedes aegypti and Aedes albopictus are endemic to the State and the control strategies for both are different,” says T.S. Anish, Associate Professor, Community Medicine, TMC.
If the universal reliance on source reduction, which focusses on reduction of mosquito larval sources, has had limited successes, fogging with malathion, another popular method being used to kill adult mosquitoes, has not been yielding any results, other than rendering the air toxic, increasing insecticide resistance, and killing butterflies.
Shift in focus
“We need to shift the focus of vector control from these strategies to those which target adult mosquitoes. A.aegypti prefers to be indoors -- dark, cool corners, underneath tables or cots -- and rarely moves away from its resting places if it finds breeding facilities also. New evidence tells us that techniques such as indoor residual spraying on the lower part of walls and indoor space spraying in homes and workplaces, along with the use of gravi traps and ovi traps, could be more effective vector control tools,” Dr. Anish says.
As part of the study, researchers are now studying how effective vector traps are, to generate evidence as to its widespread use.
“Despite the widespread presence of albopictus, we have never isolated dengue virus in this mosquito in Kerala. We have to believe in the efficiency of aegypti as a dengue vector as in our study, wherever aegypti density was high, dengue sero prevalence in the community was also high. Caseload was low in rural areas where albopictus density was high,” says Dr. Anish
