At present, there are a total of 57 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 / Monthly Port Gravidtrap Index for Aedes albopictus (PGI/MPGI), and
  2. Port / Monthly Port Density Index for Aedes albopictus (PDI/MPDI).

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 Monthly Port Gravidtrap Index for Aedes albopictus (MPGI) 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 Monthly Port Density Index for Aedes albopictus (MPDI) 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 Gravidtrap Index for Aedes albopictus is divided into four levels, reflecting the infestation level of the Aedes albopictus. Level 1 (less than 5 per cent) indicates that infestation of the mosquito is not extensive in the area surveyed. Level 2 (5 per cent to less than 20 per cent) indicates that infestation of the mosquito is slightly more extensive in the area surveyed. Level 3 (20 per cent to less than 40 per cent) indicates that infestation of the mosquito exceeds one-fifth of the area surveyed. Level 4 (40 per cent or above) indicates that almost half of the surveyed area is infested with the mosquito. 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%
  • 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%
3 20% ≤ G.I. < 40%
  • 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%

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 phased Gravidtrap Indices and Density Indices for Aedes albopictus for each surveyed area in May 2020 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 2.1% 1.0
Sheung Wan & Sai Ying Pun 15.0% 1.4
Sai Wan 3.7% 1.5
Eastern Chai Wan West 3.6% 1.0 2.7% 1.0
Shau Kei Wan & Sai Wan Ho 22.2% 1.3 17.1% 1.3
North Point 5.8% 1.0 12.0% 1.3
Southern Aberdeen and Ap Lei Chau 18.5% 1.3 18.3% 1.4
Pokfulam 7.0% 1.0
Deep Water Bay & Repulse Bay 13.8% 1.0 11.5% 1.0
Wan Chai Tin Hau 9.1% 2.4 11.3% 2.4
Wan Chai North 3.8% 1.0 2.7% 1.0
Happy Valley 3.7% 1.0 1.9% 1.0
Islands
Islands Cheung Chau
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 21.3% 1.0
Tsim Sha Tsui East 0.0% N/A 3.9% 1.3
Mong Kok Mong Kok 6.8% 1.0
Sham Shui Po Cheung Sha Wan
Lai Chi Kok
Sham Shui Po East
Kowloon City Ho Man Tin 13.5% 1.0
Kowloon City North
Hung Hom 10.5% 1.2
Lok Fu West 6.7% 1.0
Kai Tak North 7.5% 2.0
Wong Tai Sin Wong Tai Sin Central 12.1% 1.0
Diamond Hill 1.7% 2.0 0.9% 2.0
Ngau Chi Wan 3.8% 1.0
Kwun Tong Kwun Tong Central 20.3% 1.2
Lam Tin 10.2% 1.2
Yau Tong 10.8% 1.0
Kowloon Bay 8.2 1.4

New Territories
District Surveyed area First Phase Gravidtrap Index First Phase Density Index Area Gravidtrap Index Area Density Index
Sai Kung Tseung Kwan O South 3.2% 1.0
Tseung Kwan O North
Sai Kung Town
Sha Tin Tai Wai 7.9% 1.0 10.4% 1.2
Sha Tin East 19.4% 2.2 20.2% 2.0
Ma On Shan 12.9% 2.0 12.1% 1.9
Sha Tin Town Centre and Fo Tan 3.7% 1.0 6.5% 1.1
Tai Po Tai Po 6.7% 1.0
North Fanling 3.4% 1.0
Sheung Shui 8.8% 1.2
Yuen Long Tin Shui Wai 6.5% 1.0
Yuen Kong 0.0% N/A
Yuen Long Town 7.7% 1.0
Tuen Mun Tuen Mun North 15.4% 2.5
Tuen Mun South 18.2% 1.3
Tuen Mun West 29.2% 2.2 15.6% 2.1
So Kwun Wat 10.3% 1.0 12.1 1.1
Tseun Wan Tsuen Wan Town 1.7% 1.0
Tseun Wan West 5.7% 1.0
Ma Wan 24.0% 1.2
Sheung Kwai Chung
Kwai Tsing Kwai Chung
Lai King
Tsing Yi South 12.1% 1.1
Tsing Yi North 1.9% 1.0