At present, there are a total of 64 survey areas selected for the vector surveillance throughout the territory and the following four different indices will be enumerated:

  1. Area Gravidtrap Index for Aedes albopictus (AGI),
  2. Area Density Index for Aedes albopictus (ADI),
  3. Monthly Gravidtrap Index for Aedes albopictus (MGI), and
  4. Monthly Density Index for Aedes albopictus (MDI).

The AGI and ADI indicate the extensiveness of the distribution and density of Aedine mosquitoes respectively in that particular survey area, while the MGI and MDI are enumerated by pooling together all AGIs and ADIs of the same month reflecting the territory-wide situation of Aedes albopictus.

In addition to the areas in the community, surveys of Aedes albopictus are also carried out to cover all major port areas. The following two different groups of indices will be enumerated:

  1. Port / Port Monthly Gravidtrap Index for Aedes albopictus (PGI/PMGI), and
  2. Port / Port Monthly Density Index for Aedes albopictus (PDI/PMDI).

The Port Gravidtrap Index for Aedes albopictus (PGI) indicates the extensiveness of the distribution of Aedine mosquitoes in a particular group of port areas. The Port Monthly Gravidtrap Index for Aedes albopictus (PMGI) is enumerated by pooling together all PGIs of the same month, which reflects the overall degree of occurrence of Aedes albopictus in port areas. Another index, the Port Density Index for Aedes albopictus (PDI) indicates the density of Aedes albopictus in positive traps in a particular group of port areas. The Port Monthly Density Index for Aedes albopictus (PMDI) is enumerated by pooling together all PDIs of the same month, which denotes the overall density of Aedes albopictus in locales detected with Aedine mosquito activities in port areas.

The period for setting gravidtraps in the survey areas is two weeks. During the survey period, the gravidtraps are collected on a weekly basis. At the end of the first week of surveillance, all the gravidtraps are retrieved and another batch of gravidtraps is set at the same location for surveillance for the second week. The glue boards inside the retrieved gravidtraps are immediately examined for the presence of adult Aedine mosquitoes (“Aedes-positive”) for the purpose of compiling the Gravidtrap Index (First Phase) and Density Index (First Phase). At the end of the second week of surveillance, all the gravidtraps are retrieved and instantly checked for the presence of adult Aedine mosquitoes (“Aedes-positive”) on the glue boards. Data of the two weeks’ surveillance will be combined to obtain the Area Gravidtrap Index (AGI) and Area Density Index (ADI).

FEHD will notify its District Environmental Hygiene Offices and other relevant government departments of the Gravidtrap Indices and Density Indices of various phases once the figures are available, so that they can promptly carry out targeted mosquito prevention and control work. To facilitate public access to the latest survey data, FEHD will announce the phased indices for Aedes albopictus for each of the surveyed areas on the FEHD webpage. Corresponding mosquito prevention and control measures can be taken by the public in a more expeditious manner so as to prevent the spread of dengue fever.

The Gravidtrap Index and Density Index for Aedes albopictus of each surveyed area are released in two phases. Figures released in the first phases are phased indices and the second phase indices represent the AGI and ADI of the surveyed area in that month.

three_phases

The Gravidtrap Index for Aedes albopictus is divided into four levels, reflecting the distribution level of the Aedes albopictus. Level 1 (less than 5 per cent) indicates that distribution of Aedes albopictus is not extensive in the area surveyed. Level 2 (5 per cent to less than 20 per cent) indicates that distribution of Aedes albopictus is fairly extensive in the area surveyed. Level 3 (20 per cent to less than 40 per cent) indicates that distribution of Aedes albopictus is extensive in the area surveyed. Level 4 (40 per cent or above) indicates that distribution of Aedes albopictus is very extensive in the area surveyed. Specific preventive and control measures will be initiated accordingly. Management of housing estates, hospitals, schools, construction sites and other venues or properties are also advised to take specific measures to contain mosquito problems in their own properties. The four levels of Gravidtrap Index for Aedes albopictus and actions to be taken by the management are listed in the following table.

Level Gravidtrap Index for Aedes albopictus Actions to be taken
1 G.I. < 5%
(Distribution of Aedes albopictus in the survey area is not extensive)
  • Closely monitor the hygienic condition to prevent breeding of mosquitoes;
  • Conduct weekly inspection to identify breeding / potential breeding places and eliminate such places as far as possible.
  • Public are advised to check and eliminate any possible breeding places within their premises at a frequency not less than once a week (Please go to Advice to the Public for details)
2 5% ≤ G.I. < 20%
(Distribution of Aedes albopictus in the survey area is fairly extensive)
3 20% ≤ G.I. < 40%
(Distribution of Aedes albopictus in the survey area is extensive)
  • To conduct special operations in addition to the regular weekly program to eliminate all breeding / potential breeding places;
  • Private pest control contractor might be employed to control the mosquito problem. Other control measures by using larvicides or adulticides might be feasible.
4 G.I. ≥ 40%
(Distribution of Aedes albopictus in the survey area is very extensive)

The phased Gravidtrap Indices and Density Indices for Aedes albopictus for each surveyed area in October 2021 are provided below:

Hong Kong Island

District

Surveyed area

First Phase Gravidtrap Index First Phase Density Index Area Gravidtrap Index Area Density Index
Central/Western Central and Admiralty 1.9% 1.0
Sheung Wan & Sai Ying Pun
Sai Wan
Eastern Chai Wan West 1.8% 1.0 3.5% 1.0
Shau Kei Wan & Sai Wan Ho 14.3% 1.1 11.5% 1.1
North Point 0.0% N/A 0.0% N/A
Southern Aberdeen and Ap Lei Chau 1.9% 1.0 1.9% 1.0
Pokfulam 1.9% 1.0
Deep Water Bay & Repulse Bay 0.0% N/A 1.9% 1.0
Wan Chai Tin Hau 9.3% 1.0 6.5% 1.0
Wan Chai North 0.0% N/A
Happy Valley 1.8% 1.0 2.7% 1.0
Islands
Islands Cheung Chau South
(Formerly: Cheung Chau)
Cheung Chau North
Tung Chung

Kowloon
District Surveyed area First Phase Gravidtrap Index First Phase Density Index Area Gravidtrap Index Area Density Index
Yau Tsim Tsim Sha Tsui
Tsim Sha Tsui East 3.8% 1.0
Mong Kok Mong Kok 0.0% N/A
Sham Shui Po Cheung Sha Wan
Lai Chi Kok
Sham Shui Po East
Kowloon City Ho Man Tin 0.0% N/A
Kowloon City North
Hung Hom 5.4% 1.0
Lok Fu West
Kai Tak North 2.9% 2.0
Wong Tai Sin Wong Tai Sin Central 9.6% 1.4
Wong Tai Sin West 1.9% 1.0
Diamond Hill 1.9% 1.0
Ngau Chi Wan 5.6% 1.0
Kwun Tong Kwun Tong Central
Lam Tin
Yau Tong 0.0% N/A
Kowloon Bay

New Territories
District Surveyed area First Phase Gravidtrap Index First Phase Density Index Area Gravidtrap Index Area Density Index
Sai Kung Tseung Kwan O West
(Formerly: Tseung Kwan O South)
Tseung Kwan O East 4.0% 1.0
Tseung Kwan O North
Sai Kung Town
Ngau Liu & Muk Min Shan
Sha Tin Tai Wai 4.8% 2.7 4.8% 2.0
Sha Tin East 1.6% 1.0 1.6% 1.0
Ma On Shan 7.9% 1.0 5.6% 1.0
Sha Tin Town Centre and Fo Tan 0.0% N/A 1.9% 1.0
Tai Po Tai Po West 1.7% 1.0
Tai Po East 7.8% 1.3 4.9% 1.2
North Fanling
Sheung Shui 3.5% 1.5
Yuen Long Tin Shui Wai 0.0% N/A
Yuen Kong 0.0% N/A
Yuen Long Town 3.7% 1.0
Ki Lun Shan 0.0% N/A
Sung Shan 0.0% N/A
Tuen Mun Tuen Mun North 6.2% 1.0
Tuen Mun South 0.0% N/A
Tuen Mun West 12.8% 1.0
So Kwun Wat 5.3% 1.0
Tseun Wan Tsuen Wan Town
Tsuen Wan West 1.9% 1.0
Ma Wan 0.0% N/A
Sheung Kwai Chung
Kwai Tsing Kwai Chung
Lai King
Tsing Yi South
Tsing Yi North

Remarks:
Gravidtrap Index figures in green:                Level I : Gravidtrap Index <5.0% (indicates that the distribution of Aedes albopictus in the survey area is not extensive)
Gravidtrap Index figures in yellow:               Level II : 5.0% ≤ Gravidtrap Index <20.0% (indicates that the distribution of Aedes albopictus in the survey area is fairly extensive)
Gravidtrap Index figures in red:                     Level III : 20.0%≤  Gravidtap Index <40.0% ( indicates that the distribution of Aedes albopictus in the survey area is extensive)
Gravidtrap Index figures in black (bold):    Level IV : Gravidtrap Index ≥40.0% (indicates that the distribution of Aedes albopictus in the survey area is very extensive)

Density Index currently released are only for reference. The FEHD will collect the data of the density index this year for analysis and will establish a reference level for the corresponding prevention and control measures for the density index after sufficient data has been collected.