Ovitrap surveys within the University premises of Sri Jayewardenepura for

Aedes aegypti and Aedes albopictus and for seasonal shift in relative abundance of these

mosquito species.

K.V.K.Gunathilake, B.G.D.N.K.De Silva, M.B.Wickramasinghe.

Introduction

Dengue fever (DF) and Dengue Haemorrhagic Fever (DHF) are endemic diseases.

Transmitted by female mosquitoes of the Genus Aedes.

Ae. aegypti is the most important primary vector while Ae. albopictus, Ae. polynesiensis and Ae. niveus have been incriminated as secondary vectors.

Dengue vector mosquitoes

Aedes albopictus Aedes aegypti

The role of vectors

Ae.aegypti-Primary vector Prefer artificial containers Mainly indoor breeder

Ae.albopictus –Secondary vector Prefers vegetation-leaf axils, tree holes Man made containers-artificial containers,

polythenes Mainly outdoor breeding mosquito.

The role of Aedes albopictus is prominent in semi urban areas,

Vegetation Host-Human

This has the ability to spread the virus in the presence of the infected host

Vector control is identified as the most important control measure .

• Absence of a vaccine.• Absence of treatment.• Prevention is better than cure

Vector surveillance

Gives information which can be used to control vectors. Determine the major breeding sources – can

carryout the source reduction Pinpoint high risk area. Determine seasonal population fluctuations Forecasting outbreaks. Recognizing significant changes-vector

density, distribution. etc

Indices used in vector surveillance

Container Index (CI) = Number of Containers positive for Ae. Aegypti larvae x 100 Number of Containers Inspected

House Index (HI) = Number of Houses positive for Ae. Aegypti larvae x 100 Number of Houses Inspected

Breteau Index (BI) = Number of Container positive for Ae. Aegypti larvae x 100 Number of Houses Inspected

Pupal Index (PI) = Number of pupae x 100 Number of Houses Inspected

In Sri Lanka, two peaks of dengue fever occur in association with the rainfall.

June – July -During South west Monsoon

October- December- During the North-East Monsoons.

It is highly important to study the relationship of vector mosquito with the rainfall.

Understand the role of Aedes albopictus as a vector mosquito.

Objectives

To identifying the possible Aedes breeding places in the university.

To study the seasonal shift in relative abundance of these mosquitoes.

Materials and methodsStudy site: The University of Sri Jayewardenepura

Study period : May 2009 to November2009

Study was carried out in 2 steps.1.Initial study –to identify mosquito breeding places( May 2009)2.To find out the abundance of Aedes vector

mosquitoes, their fluctuation with the rain fall, and to find out the relationship with the number of DF cases in neighboring areas.

Ovitrap survey

Ovitraps as described by Lee (1992) was used in this surveillance.

It is a 300 ml plastic container with straight, slightly tapered sides. The opening measures 7.8 cm in diameter, the base diameter is 6.5 cm and the container is 9.0 cm in height. The outer wall of the container is coated with a layer of black oil paint.

An oviposition paddle made from hardboard (10 cm x 2.5 cm x 0.3 cm) was placed diagonally into each ovitrap. Each ovitrap was filled with tap water to a level of 5.5 cm.

Ovitraps were randomly distributed, 30 ovitraps per one collection, both Indoor and outdoor :- Twice a month

Each trap along with the stripe were collected together after 5 days.

Each wooden stripe was put in to a basin with water and kept for another 5-7 days, giving enough time for the eggs to hatch.

Calculations

(i)Container Index (CI), the percent number of positive containers to the total number of recovered containers was determined.

(ii) Mean Number of Ae. albopictus larvae per total number of recovered ovitraps were also determined.

The fourth instar larvae were used for the identification.

The rainfall data-Rain gauge-Confirmed by the Dept of Meteorology, Colombo.

The number of DF cases in Maharagama, Piliyandala and Boralesgamuwa MOHs - from the Dept of Epidemiology.

Results

Out of 216 of sites observed, Twenty nine positive breeding sites for

mosquitoes were found.

The highest contribution of Aedes mosquito breeding was given by blocked drainage channels

Results

Relative abundance of the mosquito species found in the university premises

Aedes aegypti

Aedes albopictus

Armigeres sp

Culex sp

Plastic buckets

Concrete plates

Leaf bases

Bifurcations of trees

Curd potsFish Tanks

Drainage canals

Garbage heaps

Concrete pits

Dry period

Breeding of Ae.albopictus was detected in both indoor, (Mean Monthly CI=13.3)

and out door. (Mean Monthly CI=35.3)

But breeding of Aedes albopictus is significantly high in outdoor containers. (P<0.05)

No larval breeding of Ae. aegypti (primary vector) was recorded, during the study period from June to November, may be due to the clearing process.

For the study period, MLD (Mean Larval Density) 42% For Ae.albopictus MPD (Mean Pupal Density) 22% For Ae.albopictus

During the dry season (Rainfall changes from 45.6mm-152.6mm),

The Ae. albopictus population remained low

(CI Ranges from 0-12.5).

During the rainy season (rainfall changes from 294.9-414.4mm),

(CI increased From 13.3 to 80). The abundance of the mosquitoes increases with the

rainfall. (p<0.05)

Discussion

Ae. albopictus is found commonly outdoors and breeds in all types of natural containers including most of the water collecting plant axils.

Our studies shows that it can breed in indoor man-made containers as well.

There is a significance difference of these mosquitoes to breed in outdoor containers than indoor containers. (P<0.05)

Ae. albopictus -the percentage contribution for breeding is highest in blocked drainage channels.

Other containers –

plastic buckets, yoghurt cups, PVC pipes which are located for landscaping, fish tanks without fish, polythene, containers for birdbathing, regiform boxes for plant cultivation and bifurcations of trees are the major breeding sites of Ae. albopictus

Especially disposed polythene and broken plastics have provided good breeding sites for Ae. albopictus.

The abundance of Aedes mosquito density within the University premises increases with the rainfall, as it increases the number of breeding places. (P<0.05)

Similarly, the DF cases in the neighboring areas has a tendency of increasing with the rainy season. (P<0.05)

The number of DF cases increases with the rainfall in neighboring areas increasing the risk of spreading the virus among the population of the University.

At present there are large number of possible breeding places of this mosquitoes within the university premises.

Aedes mosquito has the ability of fly over 500m, hence has the ability to breed and transport the virus from the neighboring areas.

Similarly, the university is located in a semi-urbanized area .

The neighboring areas also have the similar climatic, ecological conditions.

The high value of the CI values could be utilized to minimize the dengue cases in neighboring areas.

Because similar ecology & vegetation provide similar breeding opportunities.

Recommendations and suggestions

Proper removal of garbage sites.

Continuous maintenance of drainage channels.

Removal of plants where water get accumulated by rain or by artificial watering practices .

Tree holes were observed in many locations, providing suitable breeding places for Aedes albopictus. – Cover with cement, Sand or mud

Month of June is the peak of dengue epidemic in Sri Lanka. Therefore taking control measures from May to September is mostly suitable to control epidemics.

Control measures we could recommend are,

Fumigation and Fogging Proper disposal of garbage Continuous clearing process Remove all possible water accumulating plants Maintain all drainage system

References

Lee,H.L,(1992) Aedes ovitrap and larval survey in several suburban community in Selangor, Malaysia. Mosquito Borne diseases Bulletin, 9(1): 9-15.

Dhang C.C., Benjamin,S., Saranum,M.M., Fook,C.Y., Lim,L.H., Ahmad,N.W. and Azirun,M.S. (2005) Dengue Vector Surveillance in urban residential and settlement areas in Selangor, Malaysia, Tropical Biomedicine, 22(1): 39-43.

Jayasooriya,D., Gunawardene,Y.I.N.S., Hapugoda,M., Manamperi .,A., Silva,H.A.De., Abeywicreme,W. (2007) Co-existence of double serotypes of Dengue in patients of Gampaha District. Proceedings of the 63rd Annual sessions of Sri Lanka Association for the Advancement of Science Part 1.

Abewickreme W., Hapangama H.A.C., Gunawardene Y.I.N.S., Hapugoda M.D., Gunawardene N.K. and Wickramasinghe A. R. (2008) Knowledge attitude and practices (KAP) on dengue control I Gampaha district . Proceedings of the 64th Annual sessions of Sri Lanka Association for the Advancement of Science Part 1.