Spatial mapping on the vector indices of Aedes aegypti in a rural area of Bangalore, India

Introduction

Mosquitoes are common flying insects that live in most parts of the world. Over 3,500 types of mosquitoes can be found worldwide. Some of these mosquitoes can be vectors. A vector is defined as the carrier (animal, insect, or tick) that spreads pathogens to people and animals causing diseases, known as vector-borne diseases.,^1,2 Vector-borne diseases (VBD) cause significant morbidity and death around the world. Mosquitoes are considered as one of the most perilous species on the planet because they can spread many deadly diseases. Almost 700 million people contract mosquito-borne illnesses like malaria and dengue every year resulting in more than one million deaths.³

These diseases have constituted huge economic loss, morbidity, mortality, low productivity, and social discrimination in many developing countries. The dynamics of these mosquito-borne disease transmissions are dependent on the diversity and abundance of mosquito larvae. Mosquitoes are a main group of arthropods in terms of public health importance and have a direct relationship with environmental factors, diversity in habitats, or host preferences. Mosquito species Aedes aegypti and Aedes albopictus are considered to be of public health importance as they carry dengue, chikungunya, and zika. To mitigate the breeding and transmission of these diseases, there is a need to understand the diversity of the breeding habitats in different localities to enable appropriate anti-larval measures. Knowledge of the different stages of the Aedes aegypti’s life will help prevent the breeding and choose the right insecticide.⁴ Biotic and abiotic variables are the main factors affecting mosquito behavior and distribution.⁵

Out of the 30 districts in Karnataka, dengue and chikungunya have been reported in almost all districts in the past decade including Bengaluru. The city has witnessed phenomenal migration in the past decades thereby becoming endemic for the diseases. The rural areas are also facing the burden of industrialization, modernization, globalization, marketization, and administrative institutional power which are considered to be the main driving forces for such rapid urbanization causing population outbursts in the last few decades. The rural spread of Aedes is a comparatively recent occurrence associated with expanding network of rural water supply programs and other development projects without health impact assessments and indiscriminate use of disposable containers, bottles, and improved transport systems. Given the above observations, the present was carried out to assess the Aedes aegypti larval indices and to spatial map the breeding sites.

Materials and Methods

The Aedes aegypti larval survey was conducted in the rural field practice area of a Medical College in Bangalore from July 2020 to October 2020. A household is considered a basic sampling unit for larval surveys. A comprehensive list of 60 villages in the study area was prepared and 30 villages were selected randomly by lottery method. Samples of 320 households were surveyed for Aedes aegypti larvae in water-holding containers based on the feasibility.

A free, mobile, data-gathering platform called Epicollect5 version 4.2.0, developed by the Centre for Genomic Pathogen Surveillance (CGPS) team of the oxford Big Data Institute (BDI) was used in the study to collect the data. The questions of the pretested, semistructured proforma were added to the project and a form was built for data collection. The entries were geotagged by adding locations. The application was later downloaded to the smartphone and the data were collected by accessing the project.

The premise of each household was searched thoroughly both indoors (using a flashlight) and outdoors. Information on the number of households surveyed, number of containers, type of containers inspected (namely cement tanks, tyres, buckets, earthen pots, plastic containers, and miscellaneous containers) and water condition of containers that can be a potential breeding site was examined and recorded. An operational definition of “outdoor” referred to the outside of the building but was confined to its immediate area.

All closed containers holding any volume of water were considered potential breeding sites. Figure 1 illustrates QGIS map which depicts the study area and Figure 2 shows potential breeding sites of the Aedes aegypti larvae 

The Aedes larval indices were calculated using the formulas:

• Container index = No. of positive containers     x 100

                                      No. of containers inspected

• House index = No. of positive houses    x 100

                                No. of houses inspected 

• Breteau index = No. of positive containers      x 100

                                    No. of houses inspected

The study was undertaken after approval from Krishna Institute of Medical Sciences (KIMS), Bangalore Institutional Ethics Committee. The anonymity and confidentiality of the households were ensured at all times. The statistical data were described as proportions and percentages.

Results

A total of 320 households were surveyed and 507 containers were inspected. Among the 507 containers surveyed, 72 containers were positive for Aedes larvae. The most common site for Aedes breeding was identified to be 53 (73.61%) plastic containers, followed by 7 (9.72%) earthen pots, 4 (5.6%) cement tanks, and 1 (1.39%) metal container as depicted in Table-1.

Approximately, 57 (17.81%) households were positive for the presence of Aedes larvae. The container index was found to be 14.20%, the house index was 17.81%, and the breteau index was 22.5% (Table-2).

The spatial mapping of the households with positive larvae is represented in Figure 3. Hotspots of households and neighborhoods positive for Aedes larvae were observed in the study area.

Discussion

Aedes aegypti survey is important for calculating indices to monitor mosquito density and establishing optimal times for the application of larval control measures. A survey on container breeding mosquito larvae by identifying plastic containers, earthen pots, natural containers, tyers, coconut shells, vases, cans, and concrete tanks will provide information on potential breeding sites. Plastic containers show the highest number of larvae followed by clay pots and tyers, metal tins, etc.⁶ All the Aedes larval indices calculated at the time of the survey were above the critical level, indicating that the households are at potential risk for the occurrence of dengue and chikungunya.

Container, house, and breteau index ranged from 12%–58% in studies done in different geographical locations indicating higher transmission of dengue and chikungunya and were similar to the findings in the present study.^7,8,9

Spatial analysis has been vital in mapping the spread of various diseases and assisting in policymaking in recent years. In a study conducted by Jaisankar R et al., the spatial variations in temporal trends of dengue incidences in Tamil Nadu through QGIS were reported.¹⁰ Similarly, in a study conducted by Masthi et al., GPS and spatial mapping were used to depict the coverage of the pentavalent vaccine.¹¹ Mazaheri V et al. showed the mapping of the geographical distribution of rabies in Caspian Sea littoral provinces for effective disease control and surveillance system.¹² Martin C et al. reported the use of a GIS-based malaria information system in South Africa for malaria research, appropriate malaria control measures, and social and economic development.¹³ ) A study conducted in Mumbai reported the prevalence, patterns, and predictors of diarrhea using spatial mapping to evaluate the disease burden.¹⁴ QGIS, GIS, and other software have been used for mapping disease, forecasting disease, and prevention and control measures. Another important component of GIS is the mapping of epidemics is their spatial spread. QGIS mapping of dengue and chikungunya including the vector breeding sites by concerned authorities will help in the improved prevention and control of the disease. Spatial mapping can be used to identify the areas requiring vigorous control measures to prevent transmission of the vectors. ¹⁵

The rapid spread of mosquito breeding sites in the study area was due to the storage of water in plastic containers, especially drums. The present study shows that there is a risk of transmission of dengue and chikungunya diseases in the community. Source reduction is an effective way for the community to manage the populations of different mosquitoes.¹⁶ The elimination of mosquito breeding containers or breeding sites in and around living areas should be the focus area of attention since the presence of water in containers is probably the most important factor in determining the breeding of mosquitoes, especially Aedes sp. ¹⁷ Thus, the management of vector-borne diseases includes an integrated approach of biological, chemical, and personal protection, source reduction, and health education.

Conclusion

The presence and abundance of Aedes aegypti larvae in containers and households of the study area pose a high risk of dengue and chikungunya disease transmission among the residents. Spatial mapping reveals areas that should be prioritized for surveillance and control measures due to a high container infestation of Aedes aegypti larvae. The study also provides details to the concerned authority to enable an effective allocation of resources in mosquito control.

Conflict of Interest

Nil

Source of funding

Nil