Tokyo targets dengue fever mosquitos

The Tokyo metropolitan government is taking various measures to reduce the number of Asian tiger mosquitoes inhabitating major local parks in an effort to prevent a possible outbreak of dengue fever.

Measures include mowing grass and spraying pesticides into ditches around Yoyogi Park in Shibuya Ward, according to officials.

Last summer's outbreak of the infectious disease in the park led to a nearly two-month closure of most of the park.

In early July, personnel from Pest Control Tokyo - a public service corporation commissioned by the metropolitan government to eradicate the mosquitoes - counted how many adult mosquitoes had been caught in nets they had placed at 10 different locations in Yoyogi Park. "Not as many as we expected," said one "mosquito hunter."

Since late April, the association has conducted mosquito control operations once every two weeks. However, no mosquitoes were found in half the nets on July 9. Only three Asian tiger mosquitoes were captured in a five-day operation in June.

Akio Tamada, chairman of the Chiyoda Ward-based association, also participated in last autumn's effort to exterminate the insects there. Tamada is feeling that their anti-mosquito operations were proving effective.

Various countermeasures

In last year's outbreak, about 80 per cent of domestically infected people had visited Yoyogi Park or surrounding areas.

In an early September survey, mosquitoes carrying the dengue fever virus were captured, and much of the park was closed for almost two months.

The metropolitan government later decided to spray pesticide to exterminate the mosquitoes.

"I suppose a few eggs of the species could survive the winter at the park," a Tokyo government official in charge said.

It started mosquito control operations in April this year, categorizing projects into two types - one for "adult insects" and another targeting "larvae."

One of the countermeasures for adult-stage mosquitoes is very low-key: bush pruning and grass cutting to keep adult insects at bay.

Eggs of the species incubate into larvae several days after they are laid around waterside edges, and larvae grow to adulthood in about 12 days.

The Tokyo government removes trash in the park, including containers where rainwater can pool, while spraying growth inhibitor agents in roadside ditches to kill larvae.

Asian tiger mosquitoes are most active in the hot season, especially after the rainy season ends in late July through late August. Now is the critical period to curtail the number of mosquitoes in summer.

Similar countermeasures have been taken at other metropolitan government-run parks, too.

The Tokyo government is encouraging households - through websites and other media - to ensure there is no place at home that can collect rainwater, including waste cans and plant pot saucers in yards or balconies.

Concerned about a possible fresh epidemic, an official of Tokyo Metropolitan Park Association said, "By any means, we want to prevent a situation similar to what we faced last year."

Overseas cases

It is important to contain the virus brought from overseas to prevent a fresh domestic outbreak of the disease.

According to this year's data compiled by the National Institute of Infectious Diseases, 135 people were infected with the virus overseas and developed symptoms after returning to Japan as of July 26.

From this time of the year, the risk of dengue fever virus infection is much higher as the rainy season ended and mosquitoes have become more active, while a large number of people travel overseas for their summer vacations.

Last summer's outbreak was believed to have occurred as people carrying the virus went to Yoyogi Park without realizing they were infected. They were then bitten by mosquitoes and the virus was transmitted to others.

Those who develop suspicious symptoms should consult a doctor as soon as possible.

Dengue fever

An infectious disease caused by the dengue virus transmitted to people through virus-carrying mosquitoes. Tropical and subtropical zones are endemic centres of the disease. Symptoms usually develop after the incubation period for three to seven days.

Symptoms - fever, rash and pain in joints, for example - become severe in rare cases. Asian tiger mosquitoes in Japan can transmit the virus.