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Indian Journal of Community Medicine

Measuring Inter Epidemic Risk in a Dengue Endemic Rural Area Using Aedes larval Indices

Author(s): N. Isaacs

Vol. 31, No. 3 (2006-07 - 2006-09)

N. Isaacs1

Introduction

Dengue has been traditionally known to be a disease of urban areas but has been spreading to rural areas as well1. An outbreak of dengue occurred in Narayankere, a village on the outskirts of Bangalore City, from 12th May to 4th July 1997. Atleast 19 cases of serologically positive dengue were reported from that village during the outbreak2.

In tropical countries, anything that will retain water can be turned into a breeding site for Aedes mosquitoes. Such breeding sites have been well documented in urban areas. However, few studies have described Aedes breeding sites in rural areas in India. Vector surveillance is indicated in areas that have suffered atleast one outbreak of dengue. The objectives of this study were to identify the various breeding sites of the Aedes mosquito in this rural area and to examine the entomological indices in order to estimate the risk of further dengue outbreaks. The study also aimed at identifying the various species of the Aedes mosquito present in these areas.

Materials and Methods

Narayankere village, which is about 17 Km from Bangalore, is covered by the Anugondanahalli Primary Health Centre and belongs to the Hoskote Taluk of Bangalore (rural) district. The population of the village was 1778 with 285 households as per the records of the local panchayat.

The sample size for the study was estimated to be 176, assuming that 5% of the houses (premises) were positive for Aedes larvae. The houses to be surveyed were selected by Simple Random Sampling.

The investigator visited 176 houses in the village and examined all the artificial containers of water both inside and outside the house. Larvae were collected from small containers like pots by emptying the water, through a commonly used fine meshed strainer. However, from large containers (like cemented rectangular tanks) larvae were collected by passing the strainer through the water. The strainer with the larvae on it was then immersed into a white plastic vessel with water. This was done to make it easier for the larvae to be picked up using a larval pipette. Larvae were then transferred into labelled glass test tubes and transported to the Malaria Research Centre (MRC), Bangalore.

At the MRC, the larvae were transferred into netted bowls and fed with larval food (a combination of dog biscuit powder and yeast). Once they emerged into mosquitoes, they were anaesthetised and examined under a binocular microscope. Species differentiation was carried out as per the pictorial key to commonly encountered species of Aedes mosquitoes3.

Results

Three entomological (larval) indices were calculated viz: House or Premises Index (HI), Container Index (CI), and Breteau Index (BI) according to Chan Kai Lok4. The Container index for the village was 6.72% while that for outdoor and indoor containers separately was 17% and 2.4% respectively (P< 0.05). House (Premises) Index was 12% and the Breteau Index was 13.64% for the village. The various container types encountered and those that were positive for Aedes larvae are described in table I. The indoor containers that were positive for Aedes larvae were mud pots (12.5%) and cement tanks (50%). Although there were a large number of metal pots in the household, none of them were found to be positive for Aedes larvae.

Table I: Container Types Seen in Narayankere Village and their

Container Type Number of containers examined Number positive
Mud Pots 88 (24.65%) 14 (15.9%)
Plastic Pots 123 (34.45%) 2 (1%)
Cement Tanks 13 (3.64%) 6 (46%)
Grinding stones 4 (1.12%) 2 (50%)
Tyres 2 (0.56%) 0 (0%)
Coconut shells 3 (0.84%) 0 (0%)
Metal Pots 124 (34.73%) 0 (0%)

The species of Aedes larvae that were encountered in Narayankere were albopictus, aegypti and vittatus. Aedes albopictus was the most frequently encountered species. A discarded grinding stone was found to have larvae of the three species co-existing in it. The species composition of the various containers is shown in table II.

Table II: Species Composition of Breeding Sites 

Species of Aedes larvae Mud Pots Cement tanks* Grinding stones
Ae. aegypti 9% 91% 68%
Ae. albopictus 78% 6% 21%
Ae. vittatus 13% 3% 11%
Total 100% 100% 100%
*Note: It wasn’t possible to collect all larvae from cement tanks because of the large volume of water but attempts were made to collect as many of them as possible.

Discussion

A Premises index of 12% and Breteau index of 13.4% during the inter-epidemic period is adequate to cause further outbreaks in this area given suitable climatic conditions. It can be assumed that in seasons of adequate humidity, rainfall and ambient temperature, the vector density in these breeding sites could increase many times. However the risk of transmission of dengue can be assessed accurately only when entomological studies are combined with studies on the immunological status of the population. The later, however, was beyond the scope of this study.

Breeding sites like discarded mud pots, cement tanks and grinding stones retained water for long periods of time and that made them ideal breeding sites for the aedes mosquito. Moreover, fewer containers with high densities of larvae pose a greater risk for outbreaks than the presence of many containers with low larval densities. Although the study area did not have high Container and House Indices, there were a few cement tanks used to store water, with high vector densities.

The cement tanks used for storing water were usually built inside the houses. They had no outlet for the water or had an outlet that was a few inches above the floor thus permitting water to stagnate for long periods of time. This also made it difficult to completely drain out the water to dry the tank. Aedes larvae, known to be resistant to adverse conditions can easily survive in these tanks. The predominant breeding site for Aedes albopictus was the mud pot whereas Ae. aegypti preferred cement tanks and grinding stones. Aedes albopictus is commonly found in rural areas as was observed in this study. It is a very efficient host for the dengue virus and can thus play an important role as a maintenance vector for the endemic strains. It is more adaptable and has shown to be capable of transovarial transmission. The presence of Aedes aegypti indicates that this vector is no longer confined to urban areas alone. The co-existence of three species of Aedes mosquitoes viz. aegypti, albopictus and vittatus shows that this area has become the home of these mosquitoes and there is no reason why they cannot multiply in large numbers. This makes vector control difficult. The mosquito can easily return after source reduction measures have been long carried out because they have established themselves in the area. Although Aedes vittatus was not encountered in as much density as the other two species, its importance in disease transmission is still not clear.

Conclusion

Breeding sites for aedes mosquitoes in rural areas may be concentrated in and around dwellings. This study shows that dengue surveillance in rural areas can be done on similar lines as that for urban areas since the breeding sites for aedes mosquitoes remain in close vicinity of people and develop mostly due to preventable human factors. Interepidemic vector surveillance and containment could go a long way in preventing such outbreaks in future.

Acknowledgements

I wish to thank Dr. S.K. Ghosh and Dr. Tiwari from the Malaria Research Centre, Bangalore for their help with emergence and identification of the mosquitoes. Dr D.T. Maurya from National Institute of Virology, Pune and Dr. C. Nagaraj, Medical Officer, Regional Office of Ministry of Health and Family Welfare, Bangalore, for his contribution and encouragement.

References

  1. Pavri KM. Ecology of mosquito-borne viruses in India and Southeast Asia. In: MW Loutit and JAR. Miles, Eds. Microbial Ecology (Springer-Verlag, Berlin) 1978; pp175.
  2. Personal Communication from Dr. C. Nagaraj, Research officer, Regional Office for Health and Family welfare, Government of India, Kendriya Sadan, Bangalore, 560034.
  3. Bina Pani Das, Kaul S.M., Pictorial Key to the common Indian Species of Aedes (Stegomia) mosquitoes. J Commun. Dis. 1998; 30(2); 123-127.
  4. Chan Kai Lok. Methods and indices used in the surveillance of dengue vectors, Mosquito Borne Disease Bulletin. 1985(1); 79-81.

1 Deptt. Of Community Health, St. John’s Medical College,
Bangalore, 560034.
Email: [email protected], [email protected]

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