Distribution of Mosquito Larval Breeding Sites in the Urban Slums ofBengaluru, Karnataka: An Observational Study

Introduction

Around 17% of infectious diseases are spread by vectors across the globe.¹ Mosquitoes act as vectors of deadly diseases like Dengue, Chikungunya and Malaria. Mosquito borne diseases account for more than 7,00,000 deaths worldwide.² Lifecycle of mosquito consists of an egg stage, larval and pupal stage and adult stage. Adult female mosquitoes act as vectors of most of the diseases. Aedes aegypti, the most common mosquito species responsible for transmitting dengue fever breeds entirely in man-made water reservoirs found in and around households, factories and construction sites.³

Periodic entomological surveillance must be done to identify changes in the distribution and density of vectors, evaluate control programmes and to facilitate timely decisions regarding interventions. Several entomological indices have been proposed to monitor Aedes spp. population and the most commonly used among them for the immature stages include House Index, Container Index and Breteau Index.

Burden of diseases transmitted by mosquitoes is greater in areas inhabited by the poor and vulnerable sections of the society like tribal settlements and urban slums owing to the lack of knowledge about these diseases or the potential breeding sites of vectors. Hence the present study was undertaken to identify the distribution of mosquito larval breeding sites in the urban slums of Bengaluru, a metropolitan city in Karnataka, South India.

Materials and Methods

An observational study was conducted during August 2021 in the urban field practice area of a government medical college in Bengaluru, Karnataka. A team consisting of ASHA workers and health inspectors did a house-to-house survey in the catchment area of a UFWC located in JC road, Bengaluru to look for indoor and outdoor breeding sites of mosquito larvae. All possible water holding containers in and around the houses were carefully inspected with the help of torch light for the existence of mosquito larvae. Probable breeding sites like cisterns/tanks, earthen pots, barrels or drums, solid waste, grinding stones or tap pits and unused tyres were thoroughly examined to rule out the presence or absence of larvae. Breeding sites with the presence of larvae were taken as positive. Entomological indices like House Index (HI), Container Index (CI) and Breteau Index (BI) were calculated based on the number of houses and containers positive for Aedes larvae.⁴ Data was entered into MS Excel and analyzed using SPSS Version 20.0.

Results

A total of 2592 houses were surveyed during the month of August, among which 20 were positive for Aedes larvae. Thus, the House Index (percentage of houses positive for Aedes larvae) was calculated to be 0.73%.

The total number of containers surveyed during the month (both indoor and outdoor) were 819 and the number of positive containers were 123. The container index (percentage of containers positive for Aedes larvae) was calculated to be 15.01%. Breteau Index (number of wet containers positive for 100 houses) was calculated to be 4.75.

Among the indoor breeding sites, cement tanks and cisterns were the most common ones (87.8%) followed by barrels and drums (12.19%). Cement tanks and cisterns also formed the most common outdoor breeding sites (67.07%), followed by other containers like grinding stones and tap pits, unused tyres etc (19.51%). Barrels and drums constituted 9.76% and pots or earthen pots constituted 3.65% of outdoor breeding sites.Source reduction was done for 54 containers and 10 containers were treated with larvicides.

Discussion

On the inner, wet walls of containers female mosquitoes lay eggs.⁵ Even a small amount of water attracts mosquitoes. Larvae, that emerge from the eggs feed on microorganisms present in water. Environmental management methods are important to control the immature stages of mosquito and anti-larval measures form the mainstay of Integrated Vector Management. Incidental water collections, mostly man made are the most common breeding sites of Aedes spp. mosquitoes. Identifying them and source reduction would play a major role in control of disease transmission. Overcrowding, improper waste disposal and lack of proper drainage system lead to proliferation of vectors in urban areas. Hence the present study was undertaken to identify breeding sites and distribution of mosquito larvae in the urban slums of Bengaluru. Entomological indices were calculated from the number of houses and containers positive for Aedes larvae for prediction and preparedness for an outbreak of dengue fever or other mosquito borne diseases in the area. A similar study on Entomological Surveillance during a dengue outbreak was conducted in Tirunelveli, Tamil Nadu. Entomological indices like HI, CI and BI were calculated to be 48.2%, 28.6% and 48.2 respectively.⁶ The indices are higher than that in the present study, which may be due to the fact that it was conducted during an outbreak of dengue fever in the area.

A considerable reduction in all the indices was noticed after anti-larval work. Entomological indices were also compared with the number of cases of dengue fever.

A ten-year study was conducted in two districts of Sri Lanka to predict dengue epidemics from entomological indices. Monthly vector indices and data on reported dengue cases were collected from ten dengue high risk areas in the two districts. Discriminative power of HI, CI and BI in identifying dengue epidemics were found by plotting ROC curves. It was found that HI and BI were better predictors of epidemics and average threshold values for these indices were defined.⁷ The study showed a difference in all the indices from the present study as the area involved was different and it also had a long study period.

Conclusion

The findings of the study point out the need for conducting periodic larval surveillance in areas with high risk of dengue epidemics. The present study also warrants the need for longer duration studies to correlate the number of cases of mosquito borne diseases like dengue fever and entomological indices and the need for developing entomological thresholds to predict outbreaks of dengue and other diseases in the urban areas.

Limitations of the study

The study period was very short which would have affected the results. Activities of the survey staff were not supervised by entomologists in the field which might have led to missed identification or false identification of the presence of larvae. Lack of supply of instruments like enamel bowl or needle, pipettes or larval nets would have resulted in unscientific larval collection. Predictive power of indices with dengue epidemics was not done.

Recommendations

Capacity building to be done at the primary health care level for identification of larvae in the field. Grass root level workers are to be trained in scientific larval collection and preparation and usage of insecticides and larvicides. Anti-adult and anti-larval measures and larval surveys should be conducted simultaneously to ensure community participation in Integrated Vector Management. IEC materials to be supplied and health education at the household level regarding the breeding sites, biting habits of mosquitoes, symptoms and treatment facilities available for mosquito borne diseases to be given by the field staff.

Conflicts of interest

The authors have no conflicts of interest associated with the material presented in this paper.  

Acknowledgements

We wholeheartedly thank all the field staff like ASHA workers, ANMs and health inspectors for their active participation in the survey and accurate reporting of field level data.