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HK warns against seasonal rise of mosquitoes

16 June 2020

Aedes albopictus. Source: European Centre for Disease Prevention and Control

After grappling with Covid-19, Hong Kong is raising an alarm against mosquito-borne diseases such as dengue and Zika as the hot and rainy weather of summer becomes a favorable for massive mosquito breeding.

A report from the Food and Environmental Hygiene Department (FEHD) today (June 16) noted an increase to 10 per cent in the monthly gravidtrap index (MGI) for Aedes albopictus in May, from 4.7 per cent in April.

Among the 57 survey areas, the area gravidtrap index (AGI) in four areas exceeded the alert level of 20 per cent. They were Ma Wan (27.1 per cent), Sai Kung Town (21.3 per cent), Lai King (21 per cent) and Sha Tin East (20.2 per cent), the report said.

The monthly density index (MDI) for Aedes albopictus, on the other hand, rose to 1.4 in May, which indicated that 1.4 Aedes albopictus adults were found in the Aedes-positive gravidtraps, from 1.2 in April. This number of adult mosquitoes was not high, FEHD said.

Nevertheless, FEHD has called for community measures to prevent mosquito breeding to prevent the insect from spreading diseases. The FEHD has collaborated with other government departments to strengthen mosquito prevention and control work. It  reminded the public to carry out effective prevention and control measures. Fogging operations have been done at high-risk areas since April to eradicate adult mosquitoes.

“Aedes albopictus is a kind of mosquito that can transmit dengue fever (DF) as well as Zika virus infection. DF is commonly found in tropical and subtropical regions of the world, and has become endemic in many countries in Southeast Asia,” a FEHD pokesman said.

“The World Health Organization also issued warnings that the number of DF cases recorded in Asia last year was higher than before. As Hong Kong has recorded this year's first local DF case in April and the dengue activity in neighboring areas has remained high, and Hong Kong's hot and rainy summer is conducive to the proliferation of mosquitoes, the community must stay vigilant and work with the Government to carry out effective mosquito control measures,” the spokesman added.

Effective mosquito control requires the sustained effort of both the community and the Government.

As Aedes albopictus breeds in small water bodies, the public should carry out effective mosquito prevention and control measures including inspecting their homes and surroundings to remove potential breeding grounds, changing the water in vases and scrubbing the inner surfaces, removing the water in saucers under potted plants at least once a week, properly disposing of containers such as soft drink cans and lunch boxes, and drilling large holes in unused tires.


The spokesman also advised public and estate management bodies to keep drains free of blockage and level all defective ground surfaces to prevent accumulation of water. They should also scrub all drains and surface sewers with an alkaline detergent at least once a week to remove any mosquito eggs.

Since rural areas and the vicinity of shrubby areas are natural habitats for mosquitoes, other insects and animals, those living in these areas may install mosquito screens on windows and doors if necessary. 

They should take appropriate personal protective measures against mosquitoes, such as avoiding staying in the vicinity of shrubby areas for a long time, wearing light-colored long-sleeved clothes and trousers and applying DEET-containing insect repellent.

Members of the public are reminded to make reports to relevant government departments via 1823 if mosquito problems are detected.

The FEHD also said n inter-departmental anti-mosquito response mechanism has been activated in the four districts where the AGI reached the alert level.

“Relevant departments have also individually notified the groups under the survey areas concerned that had voluntarily subscribed to the gravidtrap rapid alert system when the AGI reached the alert level of 20 per cent, “ the report said. “Subscribers have been invited to post specially designed alert notices in the common parts of their premises to remind occupants and staff to carry out anti-mosquito measures promptly.”

A spokesman for the FEHD said, “The major anti-mosquito measures of the All-out Anti-mosquito Operations that commenced in April this year include carrying out fogging in the scrubby areas within a 100-metre radius around residences weekly to kill adult mosquitoes; carrying out inspections, removing stagnant water, applying insecticide and disposing of abandoned water containers weekly to prevent mosquito breeding; and trimming of grass to discourage resting of adult mosquitoes on the site.

“The FEHD and relevant government departments will continue the above mosquito prevention and control work in areas under their purview, and will strengthen publicity and education campaigns in the coming months.

“In addition, the FEHD collaborates with relevant government departments every year to conduct the three-phase Anti-mosquito Campaign. The second phase of the Anti-mosquito Campaign started on April 20 and will last until June 19. During the period, the district offices of the FEHD have targeted areas which have drawn particular concern, such as locations in close proximity to human residences, schools, construction sites, public housing estates, hospitals, illegal cultivation sites, waterfront public and private cargo handling areas, cross-boundary checkpoints, typhoon shelters and cross-boundary ferry terminals, to remove accumulated water and carry out mosquito prevention and control work. The FEHD will, after the second phase of the campaign, conduct territory-wide thematic mosquito prevention and control special operations so as to enhance the effectiveness of the campaign.”

Since April this year, the FEHD has put in place newly designed gravidtraps as a replacement for the ovitraps previously used to directly count the number of adult mosquitoes to enumerate the gravidtrap index and the new density index. The function of the new gravidtrap index is similar to that of the ovitrap index previously used in reflecting the extensiveness of distribution of Aedes albopictus in the survey area. The new density index indicates the average number of adult Aedes albopictus collected in each Aedes-positive gravidtrap in the survey area in order to better quantify the activity level of Aedes albopictus.

The AGI and the area density index (ADI) indicate the extensiveness of distribution and the density of Aedine mosquitoes respectively in that particular survey area, while the MGI and the MDI are enumerated by pooling together all AGIs and ADIs of the same month, which reflects the general situation of Aedes albopictus in all survey areas.

The gravidtrap index for Aedes albopictus is divided into four levels, reflecting the infestation level of 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 area surveyed is infested with the mosquito. Specific preventive and control measures will be initiated accordingly.

The FEHD will collect the data of the density index this year to evaluate the effectiveness of mosquito control work. After sufficient data has been collected, the FEHD will establish a reference level for the corresponding prevention and control measures for the density index.

Moreover, in order to enhance dengue vector surveillance, the FEHD will increase the number of survey areas from 57 to 62 from June this year. The five additional survey areas are Wong Tai Sin West, Tseung Kwan O East, Ngau Liu and Muk Min Shan, Ki Lun Shan and Cheung Chau North.

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