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Vectors and Vector-borne diseases

Vectors and Vector-borne diseases

2014-10-17

Vectors are small organisms such as mosquitoes, mites and ticks that can carry disease from one infected person (or animal) to another person and place to place. The diseases caused by these vectors are called vector-borne diseases.

In Hong Kong, common vector-borne diseases in human include dengue fever, Japanese encephalitis, malaria, scrub typhus and spotted fever. Dengue fever and malaria are mostly imported from endemic countries via international travel, while most cases of scrub typhus and spotted fever got the infection locally and had history of going to vegetated areas in Hong Kong, e.g. hiking areas, vegetated areas near home, outdoor recreational areas, or outdoor workplaces.

Mosquitoes are responsible for transmitting dengue fever (Aedes mosquitoes), Japanese encephalitis (Culex tritaeniorhynchus mosquitoes) and malaria (Anopheles mosquitoes) while scrub typhus and spotted fever are transmitted by mites and ticks respectively.

Mosquito-borne Diseases

Dengue Fever

  • Vector
    • Aedes mosquitoes
    • Day-time biter
  • Local Epidemiology
    • Most cases were imported and local cases were reported in 2002, 2003 & 2010
    • A total of 505 cases were recorded from 2004 to 2013
      • The annual number of cases ranged from 30 to 103
      • No fatal cases were reported
      • Imported cases were mainly from Southeast Asian countries such as Indonesia, Thailand and the Philippines

Japanese Encephalitis

  • Vector
    • Culex tritaeniorhynchus mosquitoes
    • Dusk & night biter
  • Local Epidemiology
    • A total of 19 cases were recorded from 2004 to 2013
      • The annual number of cases ranged from 0 to 6
      • About 20% of the cases were fatal
      • Both local and imported cases were recorded
      • Most local cases lived in areas with pig farms within 2 km of their homes

Malaria

  • Vector
    • Anopheles mosquitoes
    • Night-time biter
  • Local Epidemiology
    • The last local indigenous case was reported in 1998
    • A total of 311 cases were recorded from 2004 to 2013
      • The annual number of cases ranged from 20 to 41
      • About 2% of the cases were fatal
      • All cases were imported except a case of reactivation of old infection due to immunosuppression
      • Imported cases were mainly from India, Pakistan, Nigeria, Ghana, Indonesia, etc

Mite or Tick-borne Diseases

Most cases acquired the infection locally with history of going to vegetated areas in Hong Kong, e.g. hiking areas, vegetated areas near home, outdoor recreational areas, or outdoor workplaces.

Scrub Typhus

  • Vector
    • Larval trombiculid mites
    • Congregate at areas where clothing is tight against the skin
  • Local Epidemiology
    • A total of 191 cases were recorded from 2004 to 2013
      • The annual number of cases ranged from 7 to 30
      • Less than 1% of cases was fatal
      • Most cases were local
      • About 13% of cases developed severe complications or required intensive care

Spotted Fever

  • Vector
    • Ticks of the Ixodidae family
    • They pierce their mouthpart deep into the skins of human or animals while transmitting the disease
  • Local Epidemiology
    • A total of 159 cases* were recorded from 2004 to 2013
      • The annual number of cases ranged from 8 to 22
      • No fatal cases were reported
      • Most cases were local
      • About 10% of cases developed severe complications or required intensive care

* Spotted fever has been statutory notifiable since 14 July 2008 and records before that were based on voluntary reporting

Prevention

To prevent vector-borne diseases, members of the public need to protect themselves from stings / bites of mosquitoes, mites and ticks, and help prevent their proliferation.

Protect yourselves against stings/bites

  • Wear loose, light-coloured long-sleeved tops and trousers
  • Use DEET-containing insect repellent on exposed parts of the body and clothing
  • Take additional preventive measures when hiking or going to scrubby areas

When members of the public hike or go to scrubby areas, here are some additional preventive measures:

  • Prepare for the visit
    • Wear shoes that cover the entire foot; avoid wearing sandals or open shoes
    • Tuck trousers into socks or boots to prevent arthropods from reaching the skin
    • Avoid using fragrant cosmetics or skin care products
  • During the visit
    • Stay on footpaths and avoid walking through vegetation. Do not brush along the vegetation at the sides of footpaths
    • Avoid resting on vegetation, or at humid and dark places
    • Do not hang clothing on vegetation
    • Do not feed wild or stray animals
    • Re-apply insect repellents according to instructions
  • After the visit
    • Inspect body parts and clothing.  Clear any attached arthropods carefully
    • Take a soapy shower and wash the clothes
    • Inspect and clean the bodies of accompanying pets
  • If an attached tick is found
    • Remove the tick by grasping with tweezers or fine-tipped forceps the head of the tick close to the skin and gently pulling with constant pressure
    • Do not crush or twist the tick during removal
    • After removing the tick, disinfect the bite area and wash hands with soap and water

 

Special notes when travelling abroad
Travellers going to Southeast Asian, African or South American countries should take extra precautions to protect themselves from stings and bites.

  • Before the trip
    • Arrange a travel health consultation with your doctor at least six weeks before the journey for risk assessment. During the consultation, the need for any vaccinations, vector preventive measures and anti-malarial drugs will be determined
    • Vaccines for Japanese encephalitis are recommended for travellers who plan to stay one month or longer in Japanese encephalitis-endemic countries, particularly in rural areas, and for short-term (less than one month) travellers if they plan to have significant extensive outdoor or night-time exposure in rural areas during the transmission season of the disease
    • There is no vaccine against malaria. Travellers who plan to stay in malaria-endemic areas should consult their doctors beforehand to determine if anti-malarial prophylaxis is required. This drug should be taken before and throughout the journey, then continued for some time after leaving the area
  • During the trip
    • If travelling in endemic rural areas, carry a portable bed net and apply permethrin (an insecticide) on it. Permethrin should NOT be applied to skin
    • Seek medical attention as early as possible if feeling unwell
  • Upon returning to Hong Kong
    • Travellers who return from affected areas and feel unwell e.g. run a fever, should seek medical advice as soon as possible, and provide travel details to their doctor

Help prevent vector proliferation

  • Prevent accumulation of stagnant water
    • Change the water in vases once a week
    • Clear the water in the saucers under potted plants every week
    • Cover water containers tightly
    • Ensure air-conditioner drip trays are free of stagnant water
    • Put all used cans and bottles into covered dustbins
  • Control vectors and reservoir of the diseases
    • Inspect and disinfest pets and pet beddings regularly
    • Store food and dispose of garbage properly to prevent rat infestation

Clinical Management

There is currently no specific treatment for dengue fever and Japanese encephalitis.  Management is generally supportive. Effective drugs against malaria are available but early diagnosis and prompt treatment is crucial. Patients suffering from scrub typhus or spotted fever should be treated with appropriate antibiotics and supportive care.

For details about the use of insect repellents, please refer to 'Tips for using insect repellents'.