Way Forward for Seasonal Planning of Vector Control of Aedes aegypti and Aedes albopictus in a Highly Dengue Endemic Area in India

    

    

    Figure 6:  Season wise preference of breeding of Aedes mosquitoes in
Indoor, Outdoor and Peri-domestic areas in Delhi during 2013 & 2014.
    
    

Autumn season: During this season, there was a declining trend of average breeding indices as compared to rainy season (Table 3). Pupal Index was recorded as 1.9 while PPC was 0.81. During this season in both years number of positive containers presents more in peri-domestic area. Larval and pupal production was found maximum in syntax tank (Table 4). However, total of 55 positive containers with immatures, pupae were present only in 14 containers. Pupae production was higher in sintex tanks, limited to Percent proportion of pupal production was also found higher as compared to other season.

Number of containers with immature and pupae were significantly more during summer and rainy months. However, percent proportion of containers with pupae productivity was found high during spring and autumn seasons (Table 4).

Seasonal variation of breeding in Indoor (ID), Peri Domestic (PD) and Outdoor (OD)

During the year 2013 & 2014, total of 700 positive containers with Aedes breeding, 504 numbers of containers (72%) were in peridomestic area followed by 15% in Indoor and 13% outdoor area.

(Figure 6) shows that maximum number of containers (331) was found with Aedes breeding during the rainy season followed by summer (252), autumn (57), winter and spring, respectively. In all five season dengue vector preferred to breed in peri-domestic area with peaks during summer and monsoon. Breeding in outdoor containers increased during rainy season. However, breeding in indoor containers was found maximum during summer; it might be due to hot climatic condition of Delhi. During winter and spring season not a single container was found positive in outdoor area except one positive container recorded in late winter it might be due to low outdoor temperature.

PWSC, coolers, planted pots and earthen pots contributed more than 80% of breeding in peri-domestic area. Therefore, it needs to pay attention and control breeding. Adult mosquito emerged from these breeding sites may bite indoor and in peri-domestic sites as mostly human activities are concentrated in this area. Breeding in ID and PD areas was found throughout the year in all seasons and maximum percentage positivity for indoor containers was recorded in the month of July and for peri-domestic area.

Discussion

Present study revealed that Ae. aegypti was found the most predominant Aedes mosquitoes (91.15%) followed by Ae. albopictus (6.52%) and Aedes vittatus (2.3%) in Delhi. However, during 2011- 12, the proportion of Ae. aegypti was 86.89% and Ae. albopictus was 11.5% in Delhi [10]. During 2013-14, PWSC, cement tanks, coolers and sintex tanks contributed 83% of total Aedes breeding of immatures and these containers also contributed 89% of total pupal production. These containers were found to be perennial and key containers for production of larvae as well as pupae. Identification of key containers breeding of Aedes is important to plan for priorities for implementation of control interventions in those containers.

Our studies show that Aedes breeding in various plastic water storage containers have been significantly increased from 27% (2008- 09) to 50% (2013-14), thus, preference of breeding has been shifted in PWSC, while in earlier studies, breeding was more in cement tanks and desert coolers. PWSC are most common containers used by the residents of Delhi to store water for domestic purpose due to shortage of water supply in the colonies. Pipe water supply does not fulfil the requirement of influx huge population growth in the city, unplanned development activities and extension of colonies. Plastic containers such as cans, buckets and drums are found most abundant containers, cheaper and easily available to store water, most of them don’t have lids and residents pour water without emptying completely, these containers constituted the major breeding sources of Ae. aegypti and Ae. albopictus in Delhi. Similarly, it was reported earlier from Tirunelveli, Tamil Nadu [18] that the most common storage containers used by the residents were plastic drums and other plastic containers and aluminium utensils that were found major breeding resources. Singh et al., 2013 [19] reported that in all the localities surveyed the plastic containers were maximum followed by tin containers, earthen pots, desert coolers and cement tanks. The plastic drums and plastic containers were identified a key sites (50.7%) for breeding of Ae. aegypti in Philippines also [20]. Seng et al.[21] reported that 15.2% containers were infested with Aedes immatures stages and the majority of these were water storage jars (64.2%) followed by concrete tanks, small pots and tyre in Cambodia and Aedes pupae were also collected maximum from water storage jar (76.1%). However, pupae production in solid waste was found less in Delhi and recorded only during autumn and rainy season (Table 4). The relative importance of container categories, plastic items and cement tanks were the categories contributing the most to the risk of dengue transmission in the capital.

During all seasons in Delhi, Aedes breeding was predominantly recorded in different water storage containers particularly those were kept in and around houses (ID/OD) for domestic use. These are rarely cleaned and dried, thus resulting in high Aedes breeding, thus water storage habits were found as one of the most important factor in determining the breeding of Aedes. Similar observation was recorded by Balakrishnan et al. [22] from Tamil Nadu. Rajesh et al. [23] also reported that rapid spread of Aedes specie was due to the storage of water in plastic containers and cement tank. It seems from the published reports from India [18], 2014; from Thailand [24], Etiopia [25], in Philippines [20] as well as from our studies that water storage practices are favourable to give rise breeding of Ae. aegypti and these storage containers were found closely associated to the spread of dengue infection. Therefore, there is a need to create awareness in the community to cover store water containers with proper lid and storage containers should be cleaned, empty and dried regularly on weekly basis. Proper surveillance followed by health education is important for effectively checking rise in Aedes breeding vis a vis dengue transmission [19].

Our hypothesis proofed that pupal development takes place only in few key containers. Four key containers (PWSC, cement tanks, sintex tanks and desert coolers) harboured 86% of Aedes pupae in Delhi, except desert coolers, all were the perennial source of breeding. Coolers contributed only during summer and rainy season for pupae growth. Similarly, Seng et al.reported that two container types (jar and concrete tanks) harbour 89.3% of the Aedes pupae recovered during the survey and an intervention successfully targeting both containers would be of considerably effective in reducing dengue outbreak. Observations from several areas also suggested most pupae of Ae. aegypti and Ae. albopictus reproduce in only few types of containers [12,20]. Results also show that PWSC contributed maximum (48%) for the pupal production in Delhi. Edillo et al., 2012 [20] also reported that plastic containers as key site for Ae. aegypti pupae (50.7%) in Philippines. However, in Kerala, plastic sheets and buckets were found the key containers for pupae [26]. However, present study indicated that pupae developed in specific containers in different seasons, which is useful for planning of targeted control measures. Since the pupae of dengue vectors emerge to become adults, controlling the key breeding sites that produce the most pupae could have the greatest impact on the adult population [27]. Therefore, it needs to target these pupal habitats for prevention and control of Dengue.

It was recorded that number of pupae production was also found maximum during rainy season followed by summer, autumn, spring and winter. Average pupal index ranged from 0.18 to 4.91 in all five seasons of Delhi. Pupal index is important to know the intensity of transmission and were considered the better and alternate indicator for adult mosquito abundance, than traditional larval surveillance [28]. In Delhi, there is variation in pupal production in different containers in all five seasons and it was found varied in different containers according to the dry and wet seasons in Kerala [26].

The results indicate that seasonal variation of Aedes immatures as well as pupal breeding in Delhi. During rainy season, number of containers with pupae and total number of pupae collected, both were found higher in number as compared to other seasons, it might be due to most favourable condition like high humidity and temperature for development of pupae. Similarly other studies also showed that Aedes breeding was high in monsoon and post-monsoon and in semiarid areas of India, Ae. aegypti populations fluctuate with rainfall and other water storage practices [29].

During rainy season, maximum pupae productivity and relative importance of particular container was recorded in PWSC, cement tankand coolers, so best measures for prevention of breeding to empty and dry the containers (Table 4). We have promoted to cover tanks and plastic water containers with clean clothes, if lids are not available. NCDC also develop a mosquito free desert cooler.

However, Aedes breeding spread in small water collection in solid waste in outdoor and on top of the roof and good proportion of pupae production (14.3%). Source reduction and cleaning of garbage are recommended to eliminate such type of breeding habitats. During this season, control measures are required to be implemented in war foot manner. Awareness in the community through health education is required to be developed for cleaning and drying the containers, at least once in a week. Top up all depressions to prevent hatching out of eggs. Change water in vases completely and remove water in saucers underneath potted plants every week. Source reduction at least once a week is the best solution for prevention and control of Aedes breeding in and around the houses and on roof top by involving community.

However, Aedes breeding persist during autumn season so all measures of rainy season should be planned for autumn also. During Summer, breeding extents in other storage containers due to demand of more water in hot weather and irregular water supply and in desert coolers which needs to be targeted vector control measures in IN & OD areas only. In Delhi, during winter season and spring season, Aedes immatures infestation was limited in three types of breeding foci namely PWSC, cement tanks and sintex tanks, which ensured year round availability of water, acted as mother foci during winter. Larvae and pupae stay there for long period of time due to slow development due to extreme cold temperature and their eggs can withstand at edges and sides of the containers for long period of desiccation, and will hatch when the temperature becomes suitable. Aedes mosquitoes infected with dengue virus can pass the virus from generation to generation through their eggs (Kumari et al., 2011). Therefore, it is important to target these permanent containers for vector control during winter season, so as to prevent their proliferation. It needs to involve the community to empty and dry such containers/tanks with proper cleaning and scrubbing edges and sides with household detergent to remove possible deposited Aedes eggs of previous season to eliminate mosquito eggs so as to prevent their proliferation.

Result of pupal survey shows that during rainy season 7 different types of containers produced pupae, followed by summer when 6 types of containers produced pupae while in autumn, pupae production was recorded in 4 types of containers. However, during spring and winter season pupae production was limited in to 2 types of containers (PWSC & tanks), that acted as mother foci and showing most conducive environment for survival of pupae during extreme cold condition. However, as per earlier reports, cement tanks, clay jars and overhead tanks acted as mother foci of Aedes [30,6] during pre-monsoon season. Pupal productivity in key containers identified in all seasons in Delhi provided valuable information to target for implementation of control measures. The containers that produced the larger proportion of pupae, vector control intervention should be targeted to maximize their intact.

Thus, in Delhi population of Ae. aegypti and Ae. albopictus fluctuates with rainfall, water storage practices, temperature. During the post monsoon season and vector density increases and virus load also increases in vectors [9,10] that lead to transmission of dengue at the end of rainy season followed in autumn, and it declined on the onset of winter. Similarly, Pandya et al. [31] reported that dengue in Delhi encounter during or after rainfall as an outcome of rise in vector. Rajesh et al. [23] also reported that the population of Ae. aegypti fluctuates with temperature rainfall and humidity.

Prevalence of Ae. albopictus occurred during monsoon and post monsoon season in Delhi while Ae. aegypti reported throughout the year. However, its population increases during rainy season and maintain until autumn. It has been evidenced from studies that Ae. albopictus is now well adopted for artificial containers in urban areas of Delhi and harbouring dengue virus [10]. Therefore, I needs to study bionomic of Ae.albopictus, since the report is available, it is slowly displacing Ae. aegypti from its habitat in USA [32]. In SEAR, Ae. aegypti has been incriminated as the principal epidemic vector, while Ae. albopictus has been incriminated as the principal epidemic vector, while Ae. albopictus has been given the status of secondary vector, responsible for maintenance of the virus [33].

Four key perennial containers, identified as the most favourable sites for Aedes larvae as well as pupae. Among them, PWSCs contributed maximum (48%) for the pupal production in Delhi, which need public health attention. Pupal survey was also found the most productive breeding sites for Ae. aegypti and Ae.albopictus in order to reduce dengue mosquito populations in Philippines. This was relevant for both the National Dengue Prevention and Control Program and special program and the special program for Research and Training in Tropical Diseases of the WHO [34]. This might be used as a model for dengue prevention in the Philippines and in other countries as well.

Pupal surveys of Ae. aegypti are based on the assumption that pupal mosquito production is a better proxy for adult mosquito reproduction than traditional indices or larval counts [34] and is more appropriate for directing dengue control programs because traditional larval indices correspond poorly with the actual number of pupae per person [12]. Barrera et al., 2006 [35] emphasizes that pupal survey technique generates an estimate of pupal density of Ae. aegypti in containers as a proxy for the number of adults. The pupal survey may be feasible approach for vector control programs because the preliminary observations in several urban areas suggested that most pupae of Ae. aegypti were produced in a few types of containers [12]. Thus vector control efforts on be concentrated on eliminating or treating the most productive types of containers to reduce mosquito density below a target threshold. However, the threshold levels of vector infestation that constitute a trigger dengue transmission are influenced by many factors, including mosquito longevity and immunological status of the human population. There are instances (in Singapore), where dengue transmission occurred even when the HI was <2% [36].

Conclusion

It is an important finding that four key containers (PWSC, cement tanks, sintex tanks and desert coolers) harboured 86% of Aedes pupae in Delhi, except desert coolers, all were the perennial source of breeding. Therefore, an intervention successfully targeting these containers would be of considerably effective in reducing dengue outbreak. Among them, PWSCs are recorded as main source for production of pupae of Aedes aegypti and Ae. albopictus and identified as most abundant and perennialkey breeding habitat during all five season in Delhi. The containers that produced the larger proportion of pupae, vector control intervention should be targeted to maximize their intact. Therefore, public health attention is required to create awareness to cover them properly and dry these containers at least once in a week. The source reduction of Aedes breeding habitats in and around living and working areas (ID & PD) needs be taken into consideration, since the presence of water in storage containers is probably the most important factor in determining the breeding of dengue vectors.

During rainy season 7 different types of containers produced pupae, while first time it was recorded that during winter and spring seasons, Aedes breeding was limited to plastic storage containers and tanks that acted as mother foci and it needs focussed intervention. Identification of season wise key containers for production of pupae of Aedes mosquitoes, are important for season wise planning of effective control interventions.

Acknowledgement

The authors gratefully acknowledge all the co-operation extended by the Municipal Corporation of Delhi. Technical help of all staffs of Centre for Medical Entomology & Vector management, National Centre for disease Control is gratefully acknowledged.

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