Larval survey of Aedes aegyptifor predicting Dengue transmission

Introduction

Dengue is a mosquito-borne emerging viral infection that has spread throughout the tropical world over the past 60 years and now affects over half the world’s population.¹ The geographical range of dengue is expected to further expand due to ongoing global phenomena including climate change and urbanization.² WHO has estimated >100 million dengue infections occur annually, while a recent study calculated that the figure may be closer to 300 to 400 million.^3,4 Dengue has been confirmed in 128 countries worldwide with major social and economic consequences and is endemic throughout the tropics, and almost 50% of these at-risk individuals live in dengue endemic countries in South-East Asia.⁵ Dengue is transmitted by Aedes mosquitoes, primarily highly urban-adapted vector, which thrives in the man-made urban environment, particularly in deprived communities, where water storage is routine, sanitation is poor and non-biodegradable containers accumulate.⁶ These mosquitoes also spread Chikungunya and Zika viruses.

The outbreaks of DF/DHF are most likely to occur in post-monsoon period when the breeding of the mosquitoes is highest.⁷ Therefore, the key to control vector is a comprehensive approach by way of regular vector surveillance and integrated management of the Aedes mosquitoes.⁸ Breaking the human-to-mosquitoto-human cycle by controlling the Aedes aegypti population reduces dengue incidence.⁹ Vector surveillance is recommended by WHO, as a routine practice in dengue-endemic countries to provide a quantifiable measure of fluctuations in magnitude and geographical distribution of dengue vector populations, ultimately with the purpose of predicting outbreaks and evaluating control. One of the most commonly used surveillance method is the larval surveys.¹⁰

Larval surveillance during the pre-monsoon and monsoon is important to find out the extent of prevalence of vectors in high risk localities/ areas, usually urban areas having highpopulation density, poor sanitation and large number of coolers, flower vases, coconut shells, construction sites, overhead tanks, discarded buckets, tyres, utensils, etc. which promote mosquito breeding.¹¹

In larval surveys, the basic sampling unit is the house/ premise, which is systematically searched for water holding containers and examined for the presence of mosquito larvae. The indices include House Index (HI), Container Index(CI)&Breteau Index(BI) and the epidemiological interpretation of various entomological indices for risk of disease transmission is HI>1, CI >1& BI>5 respectively.¹²

In this background, the present study was conducted to determine the larval indices of Aedes aegypti mosquitoes in an urban poor locality in Bangalore, with the purpose of predicting the risk of disease transmission. Therefore the present study was conducted to assess the larval indices of Aedes aegypti mosquito in the urban poor locality and to provide health education regarding source reduction and disease prevention.

Materials and Methods

The study was initiated after getting the clearance from Institutional ethics committee, Kempegowda Institute of Medical Sciences (KIMS), Bangalore. It was an ICMR approved student project.

A cross-sectional study was conducted in an urban poor locality, which belongs to field practice area of Kempegowda Institute of Medical Sciences, Bangalore. The rainy season in the study area is usually from June to October and the dengue transmission commonly occurs from August to October. Therefore, the study was conducted prior to the transmission i.e., from June to August, 2018.

A door to door survey was conducted among all the 1206 houses in the study area after obtaining informed consent; to find out the presence of larvae of Aedes mosquitoes in different water containers lying in domestic or peri-domestic situations. All types of water container were thoroughly checked for Aedes breeding with the help of torch light. The larvae were observed by throwing light with the use of a torch at the rim of the container and are confirmed by their oscillatory movement.

Then, all the three larval indices of Aedes aegypti were calculated.

i) House index (Hl): Percentage of houses infected with larvae

HI = Number of Houses infectedX100

Number of Houses inspected

ii) Container Index (Cl): Percentage of water holding containers infected with larvae

Cl= Number of positive containersX100

Number of containers inspected

iii) Breteau Index (Bl): Number of positive containers per 100 houses inspected

Bl = Number of positive containersX100

Number of houses inspected

Subsequently, all the households were given health education regarding the mosquito breeding source and its reduction, disease prevention by personal protective measures and the importance of various vector control measures by using the power point presentation on the laptop.

Results

The present study showed that, among the 1206 houses surveyed, 74 houses had breeding places for Aedes aegypti i.e., larvae was present in the containers, giving the house hold index (HI) of 6.14%; similarly, 89 out the 2510 containers surveyed had larvae, showing the container index (CI) of 3.54% and 89 houses had containers having Aedes aegypti larvae, giving the Breteau index (BI) of 7.37% (Table 1).

All the larval indices of Aedes aegypti were above the threshold levels of HI> 1, CI > 1 & BI > 5 for predicting the risk of dengue transmission.

In this regard, all the households were given health education regarding the disease prevention and the importance of various vector control measures such as elimination of vector breeding sites by using the power point slides on a laptop before the usual occurrence of dengue outbreak. The slides included the real pictures of the problematic areas & the preventive measures which have to be taken-up by the households & the community; such as clearing domestic and peri-domestic areas of unused containers, tyres, coconut shells, broken glassware etc. which can collect water and also regarding their personal protection.

Discussion

Dengue fever is the important mosquito spread viral disease and a major international public health concern.¹³ The disease is found in tropical and sub-tropical regions around the world, predominantly in urban and semiurbanareas.¹⁴ Demographic and societal changes leading to unplanned and uncontrolled urbanization have resulted in creation of urban slums, likewise, concurrent population growth has put severe constraints on civic amenities, particularly water supply and solid waste disposal; thereby, increasing the breeding potential of disease vectors and the resulting dengue outbreaks.¹⁵

Aedes aegypti, the primary vector of dengue, is a ‘hydrophilic species’ i.e. humidity loving. Therefore, it has adapted to breeding in waterstorage containers in domestic habitation. During the rainy season, when temperatures come down and humidity increases, the species invades peri-domestic areas and breeds profusely in any natural or manmade container holding rainwater, building up a very high density.^16,17

In the absence of antiviral drugs and vaccines, vector control is the only option against dengue. Epidemics of dengue result in human suffering, strained health services and massive economic losses. The high disease burdens and unexpected surges in cases is a challenge to health systems in the country, which is the need of the time. The global strategy for dengue prevention and control, 2012–2020aims to address this need. The specific objectives are to reduce mortality and morbidity from dengue by 2020 to at least 50%, which can be achieved by applying existing knowledge and appropriate technology.³

Dengue morbidity can be reduced by implementing improved outbreak prediction and detection through co-ordinated epidemiological and entomological surveillance; promoting the principles of integrated vector management and deploying locally adapted vector control measures including effective urban and household water management.¹⁸

The present study showed that the HI, CI and BI were 6.13 %, 3.54%, 7.37% respectively. All the larval indices for Aedes aegypti were above the risk of transmission level of >1, >1 and > 5 for HI, CI and BI respectively, thereby predicting the potential risk of dengue transmission. Likewise, a dengue vector surveillance conducted at Thiruvananthapuram, also showed that, the HI, CI&BI were 11.5, 9.9& 5.19 respectively and the study concluded that there would be high chances of dengue outbreak.¹⁹

Similarly, a study conducted in Bangalore on measuring inter epidemic risk in a dengue endemic rural area using Aedes larval indices showed that, the Container index was 6.72%, House Index was 12% and the Breteau Index was 13.64%.²⁰ Another study from Pune on entomological survey for dengue vector to determine the potentiality of dengue outbreak showed that, after inspecting a total of 311 containers and 109 houses in the larval survey, the HI, CI, and BI were found to be 7.3%, 3.9% and 6.2 per 100houses, respectively.²¹A study on entomological surveillance and its significance during a dengue outbreak in the District of Tirunelveli in Tamil Nadu, showed that, the HI, CI, and BI was 48.2%, 28.6%, &48.2% respectively. The study concluded that entomological surveillance and its significance can be used to halt the outbreak of dengue.²²

All the studies showed that, the larval indices of Aedes aegypti were higher than the level of risk of transmission. Therefore, the key to control dengue is adoption of a comprehensive approach by way of regular vector surveillance and integrated management of the Aedes mosquitoes through biological and chemical control that are safe, cost effective; and environmental management, legislations as well as action at household and community levels.²³

Involvement of household and the community for Aedes mosquito control is of paramount importance as the problem revolves mainly around man and his environment. Community should be involved in the task of elimination of Aedes breeding in and around their houses for keeping houses free of larval breeding and reduction/elimination of adult mosquitoes.²⁴

Intense and persistent behaviour change communication (BCC) campaign is crucial, the community must be assured that, this is a preventable disease and empowered with the knowledge about mode of transmission, vector control options, availability of services in addition to correct treatment, so that timely and appropriate action is taken. Special campaigns must be carried out through mass media including newspapers/magazines, radio and TV, especially using local cable networks as well as outdoor publicity like hoardings, rallies and health education materials should be developed and widely disseminated in the form of posters, pamphlets and handbills.²⁵

In conclusion, the present study showed that, the larval indices are high in the study area and predicts the chance of dengue transmission. Therefore, immediate & integrated vector control measures have to be taken to prevent the outbreak of the disease.

Conflict of Interest: None

Source of Funding: Nil.