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By
Tran Vu Phong and Vu Sinh
Nam
National Institute of Hygiene and Epidemiology
1, Yersin Street, Hanoi 10.000, Vietnam
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Abstract
From 1994 to 1997, a total of 6,357 water containers in four
districts of Hanoi were investigated for the
types, capacity, water volume, positive percentage with Aedes
aegypti larvae and number of larvae in
different instars. Six types of water containers were found as breeding
sites of DF/DHF vector, including drums (38.8%), concrete tanks (26.2%), clay jars (26.0%), discarded objects (5.0%), aquariums
(3.5%) and buckets (0.5%). Two species, Aedes
aegypti and Aedes
albopictus, were found from the 47,479 Aedes larvae collected, of which Aedes aegypti
were dominant (87.2%). 91.7% of Aedes aegypti larvae were collected from three types of
big water containers (drums, tanks and jars). Larval breeding significantly
increased during the rainy season from April to November.
Key words: Aedes aegypti,
Aedes albopictus,
Key containers, Vietnam
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Introduction
During
the recent past, DF/DHF has not yet become a serious health problem in the
urban area of Hanoi, Capital of Vietnam(1).
However, surveillance of dengue vector population and its breeding sites play
a very important role in predicting DF/DHF epidemics and effectively
establishing active vector control measures. This study aims at determining
the key containers which breed Aedes aegypti and the seasonal change of their population
in Hanoi.
Materials and methods
The
studies were conducted in four communes of Hanoi city which included: Van Ho 3
alley, Le Dai Hanh commune, Hai
Ba Trung district, Hang
Be street, Hang Bac commune, Hoan
Kiem district, Kham Thien
street, Trung Phung
commune, Dong Da district, Doi Can street, Doi
Can commune, Ba Dinh
district. The study lasted for four years, from 1994 to 1997.
Adult
mosquito surveys were conducted on indoor resting mosquito collection (2
well-trained technicians, 15’/house, 15 houses/study site, and twice a month
at 15-days interval). Water containers were classified by types, capacity,
and water volume, and percentage of water containers positive with Aedes aegypti
larvae. Aedeslarvae were collected
by entomological nets and identified in the medical entomology laboratory.
Data collected were analysed using EPI INFO 6.0 and
EXCEL 7.0 software.
Result and discussion
DF/DHF vector surveillance
From
1994 to 1997, 8 mosquito species comprising of 4 genus
were found in Hanoi with the presence of both Aedes aegypti and
Aedes albopictus.
Aedes aegypti
were dominant among the total of collected Aedes
mosquitoes. Almost all Aedes albopictus adult mosquitoes were found only at their
breeding sites(2,3,4).
Surveillance
results of adult mosquito, larval population and DF/DHF cases (Fig. 1 and
Fig. 2) show that there was no relationship between the mosquito density
index, the Breteau index and the number of DF/DHF
cases (r=0.288; 0.140). This means that these indices had very small
potential for predicating the DF/DHF epidemic in Hanoi. Data on vector surveillance
also revealed that Aedes aegypti population significantly increased during the
rainy season from April to November.
Breeding sites
The
results of the investigations on water containers (Table 1) showed that the
types of the most popular domestic water containers were: tanks (38.9%) (average capacity: 950 litres),
clay jars (30.2%) (average capacity: 120 litres), drums (26%) (average
capacity: 200 litres), buckets (1.7%)(average
capacity: 9.5 litres), discards (1.9%) (average capacity: 0.35 litres)
and aquariums (1.2%) (average capacity: 30.5 litres). Aedes aegypti larvae were mainly found in drums (40.35%), jars
(28.22%), concrete tanks (20.85%) and discarded objects (8.96%). Meanwhile, Aedes albopictus
larvae were mainly found in jars (40.70%), discarded objects (24.45%), drums
(16.63%), and aquariums (9.86%). This means that the percentage of each type
of water container positive with Aedes aegypti larvae differed from one type to another.
Types of containers producing almost all of Aedes
larval population were called "key breeding sites". These are
drums, jars and tanks in Hanoi city (Fig.3).
Key
breeding site distribution plays a very important role in finding out the
most suitable and effective method of vector control in specific areas. In Hanoi, up to 89% Aedes
aegypti larvae were collected in large-size
containers.
Key
breeding sites of Aedes aegypti larvae differed from one area to another. In Haiphong city (100 km east of Hanoi), Aedes
aegypti larvae were mainly found in concrete
tanks (33.5%), in jars (29.7%) and in discarded objects (17.5%) (Un-
published data). In Nghiadong commune, a rural area
of Namdinh province (120 km south of Hanoi), Aedes
aegypti larvae breeding sites were jars
(58.4%), discarded objects (23.6%) and concrete tanks (14.87%). Surprisingly,
larval breeding sites in Huongchu commune of Thua Thien Hue province
(central coast of Vietnam) were ant traps (38.45%) and in Thanhbinh commune of Danang
province these were toilet tanks (41.74%). In these areas, Aedes aegypti
larval control measure should be different from those in Hanoi.
Table 1. Surveillance results of Aedes
aegypti and Aedes albopictus larval breeding sites in Hanoi, 1994 - 1997
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Containers
|
Ae. aegypti
|
Ae. albopictus
|
|
No
|
Types
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No
|
%
|
Cont. (+)
|
%
|
No of larvae
|
%
|
Cont. (+)
|
%
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No of larvae
|
%
|
|
1
|
Tanks
|
2475
|
38.93
|
211
|
26.15
|
8635
|
20.85*
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6
|
8.69
|
507
|
8.35
|
|
2
|
Clay jars
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1920
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30.20
|
210
|
26.02
|
11684
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28.22
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26
|
37.68
|
2472
|
40.70
|
|
3
|
Drums
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1652
|
25.99
|
313
|
38.79
|
16707
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40.35
|
5
|
7.25
|
1010
|
16.63
|
|
4
|
Buckets
|
110
|
1.73
|
4
|
0.50
|
18
|
0.04
|
0
|
0.00
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0
|
0.00
|
|
5
|
Discards
|
123
|
1.93
|
41
|
5.08
|
3708
|
8.96
|
27
|
39.13
|
1485
|
24.45
|
|
6
|
Aquariums
|
77
|
1.21
|
28
|
3.47
|
654
|
1.58
|
5
|
7.25
|
599
|
9.86
|
|
Total
|
6357
|
|
807
|
|
41406
|
|
69
|
|
6073
|
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*underlined figures
relate to key breeding sites.
Aedes aegypti larvae were found in all types
of water containers. The number of larvae increased 2.2 times during the
rainy season from April to November (Fig.4). These result are different from
the theory propounded by Rakesh Katyal (1996)(5)
about "Primary and secondary" breeding sites of Aedes
aegypti*
Conclusion
 Both Aedes aegypti and Aedes albopictus mosquito species were found in Hanoi,
and Aedes aegypti was the
dominant species (larvae accounted for 87.2% and adult mosquitoes accounted
for 86.9%).
Key breeding sites of Aedes
aegypti in Hanoi
were drums (40.35%), clay jars (28.22%), tanks (20.85%) and discarded objects
(8.96%). Almost all Aedes albopictus
larvae concentrated in clay jars (40.7%), discarded objects (24.25%) and
drums (16.63%).
There was no significant change in the types
of Aedes aegypti breeding
sites by month in Hanoi, but the
number of larvae significantly increased during the rainy season from April
to November.
The relationship between Aedes
aegypti indices and the number of cases was very
low. Therefore, finding out other effective indices to predict DF/DHF
outbreaks is necessary.
References
1. 1989.
Distribution of endemic DF/DHF in Vietnam,
1980-1988. Annual Report of National Institute of Hygiene and Epidemiology,
Vietnam 215.
2. Vu
Sinh Nam,
1995. Bio-ecological characteristics of Aedes aegypti and its control measures in north Vietnam, Ph.D.
thesis, Hanoi University:
87-92.
3. Vu
Sinh Nam,
1996. New direction of DF/DHF vector surveillance and control in Vietnam.
4. Vò Sinh Nam,
1996. Breeding places of Aedes aegypti
in North Vietnam.
Plant Protection Journal, 5(149): 97-101. Preventive Medical Journal, No 12:
223-225.
5. Rakesh K, Gill KS,
Kumar K. Seasonal variations in Aedes aegypti population in Delhi.
Dengue Bull. 1996; 20:78-81.
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