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Updated on August 8, 2005

Summer is season for outbreaks!
Guidelines formulated to respond to the possible spread of West Nile fever to Japan

July 14, 2005
Bureau of Social Welfare and Public Health
Bureau of Industrial and Labor Affairs

West Nile virus activity is prevalent in North America, with this season's first patient confirmed in the United States as early as June 20.
The West Nile virus could conceivably spread to Japan by infected mosquitoes carried here by aircraft or the migration of infected birds. The Tokyo Metropolitan Government has formulated the Guidelines for West Nile Fever that outlines the swift and precise action to be taken by government agencies and the residents of Tokyo if and when the West Nile virus spreads to Japan.
Revisions have been made accordingly to the "West Nile Fever (encephalitis) Q&A".

Guideline Points

Regulation of appropriate countermeasures in the epidemic stage and introduction stage

1) In the event of an overseas epidemic

  • Establish a system for gathering data and liaison coordination
  • Raise awareness and caution among overseas travelers
  • Monitor trends of vector mosquitoes and dead birds (surveillance and fixed-point observation of mosquitoes at 8 locations, crows at 6 locations)
  • Develop a system to identify and eliminate mosquito habitats

2) In the event of virus detection or human illness in areas other than the Greater Tokyo Region

  • Secure a 24-hour communications system
  • Provide appropriate information to the public to prevent adverse rumors and panic
  • Expand surveillance, and publicly announce the results
  • Ensure early detection of suspected patients and undertake epidemiological studies

3) In the event of virus detection or human illness within the Greater Tokyo Region

  • Convene a meeting for West Nile fever countermeasures
  • Set up a hotline for consultations by the public
  • Conduct intensive epidemiological studies and take necessary measures accordingly
    Collect and undertake virus testing of mosquitoes within a 2km radius of the source of the outbreak (the range of a mosquito's activity) --> Elimination of larval and adult mosquitoes
    Identify and undertake virus testing of dead birds such as crows within a 10km radius of the source of the outbreak (the range of a crow's activity) --> Countermeasures at the source of the outbreak (elimination of larvae through environmental management or chemical agents)

These Guidelines and Q&A have also been posted on our website: (Japanese only)


Overview of West Nile Fever

1. Background

West Nile fever is a vector-borne infectious disease. The virus (West Nile virus) was first isolated in the West Nile District of Uganda in 1937.
The West Nile virus is currently distributed over a wide area encompassing Africa, Europe, the Middle East, Central Asia, West Asia and North America.

Distribution of the West Nile Virus


Source: "Information on Infectious Diseases Overseas", Quarantine Station, Ministry of Health, Labour and Welfare(as of July 11, 2005).

2. Routes of transmission

The West Nile virus is primarily carried by birds, and is maintained in a transmission cycle between birds and mosquitoes. Human infection is through the bite of an infected mosquito.
Human-to-human transmission (direct oral contact, contact, droplet spread, or airborne transmission), human-to-mosquito-to-human transmission and bird-to-human transmission are not possible; however, there have been reports suspecting transmission via blood transfusion, organ transplants, and breast milk.

3. Timing of outbreaks

In temperate regions, outbreaks of the virus normally occur from the latter half of summer to early autumn.

4. Clinical symptoms

The incubation period for the virus ranges from 2 to 14 days (usually 2-6 days), and approximately 80% of people infected develop no symptoms.
Symptoms include sudden fever (at least 39°C), headaches, muscular aches and pains, and rash (chest, back, and arms).
Infected patients can usually recover in about one week, however approximately 1% of those infected, mainly elderly patients, develop severe symptoms such as meningitis or encephalitis (West Nile encephalitis).
The mortality rate for patients severely ill is approximately 3 - 15%.

5. Treatment

Only supportive therapy is available.
Patients with severe symptoms will be hospitalized and treated with IV fluids, respiratory support, etc.
Vaccinations are still in the developmental stage.

6. Outbreaks (November 2002 - June 2005)

West Nile fever has been designated as a class IV infectious disease since November 1, 2002. There have been no reported cases in the Greater Tokyo Region or Japan.

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