Tokyo on guard against mosquito-borne disease

The average summer temperature in Japan has risen 1.06 C over the past 100 years, an even faster pace than the increase in the average world temperature caused by global warming. The government will soon release an "adaptation plan" to mitigate the negative effects of the greater heat. This series covers unusual heat-related events that have begun to affect people's lives across the country.

Amid the throbbing cries of cicadas on Aug. 18, priest Akimitsu Horie turned over flower stands, in which rain water had pooled, on graves at Taichoji temple in Shibuya Ward, Tokyo.

"If I don't do this, mosquito larvae start to multiply. I never had to do this sort of thing before last year," said Horie, 57.

Mosquitoes became a new threat to Japan on Aug. 26 last year - that day authorities confirmed the first domestic case of dengue fever in 69 years. Ultimately, 162 people showed symptoms.

Taichoji temple is located between Yoyogi Park and Shinjuku Central Park, two places where numerous people were infected by mosquitoes. Supporters of the temple began to worry last summer about being infected if they visit their family graves.

Aoyama Cemetery, run by the Tokyo metropolitan government, is about 1.5 kilometers from Yoyogi Park. Insecticide is being put into rainwater drainage pits and other possible breeding spots to exterminate mosquito larvae.

There have not yet been any cases of dengue fever (see below) in Japan this year, but a staff member at the cemetery said they would "advise visitors to wear long sleeves and long pants leading up to the equinoctial week" in September.

Tropical infectious diseases that threaten humans, such as dengue fever, are spreading northward around the globe. A glimpse of Tokyo's future may lie about 2,400 kilometers to the southwest, in Kaohsiung, southern Taiwan.

On the afternoon of July 8, two officials of the Kaohsiung city health department entered an old house in the suburbs. Spraying insecticide from devices hanging on their shoulders, they were conducting emergency measures after a suspected case of dengue fever was reported to the city government.

"There were larvae in a water jar in the garden," the two officials warned a man in the house.

In an ordinary year, Kaohsiung sees from 500 to 1,000 dengue patients, but last year the number of cases exploded to about 15,000. "Warming makes the disease more infectious, and our prevention efforts cannot keep up," said Chi-Kung Ho, 55, the director general of the health department.

In Japan last year, the Aedes albopictus mosquito spread the dengue fever virus after drawing blood from a patient who had returned from overseas.

The ratio of blood-drawing female mosquitoes is about 40 per cent when the temperature is 20 C, but the figure rises to about 90 per cent at 30 C, according to experimental data from Taiwan. Other research says about 10 times as many viruses multiply inside a mosquito's body at 30 C than do at 26 C.

The average temperature in Tokyo in early August last year, when the first dengue fever patient was bitten by a mosquito, was 29.5 C. This was higher than the average temperature of 29 C in Kaohsiung during the same period.

There are no tropical Aedes aegypti mosquitos in Japan, which are more infectious than Aedes albopictus mosquitos, but they can survive the winter if the coldest month of the year has an average temperature of 10 C or higher.

The average January temperature in Tokyo over the last 10 years has been about 6 C, but this is about 3 C higher than 100 years ago. As the number of foreign tourists to Japan sharply rises, the number of "imported cases" of people who show symptoms in Japan after being infected with dengue fever overseas reached 144 in this year alone. The possibility of another outbreak is significant.

Nineteen prefectures told The Yomiuri Shimbun they are conducting fixed-point observations of the habitats of virus-carrying mosquitoes this fiscal year. This is a large increase from the just six prefectures that conducted such observations last fiscal year, reflecting the mounting concern felt by local governments.

"It's essential to quickly enact measures, such as improving sewerage systems, to create an environment where mosquitoes cannot thrive. We must also take measures against global warming, or tropical infections could slip out of our control," said Mutsuo Kobayashi, a senior researcher at the National Institute of Infectious Diseases.

Dengue fever

Symptoms appear three to seven days after infection, including fever, headache and a rash, and on rare occasions infection leads to the severe illness known as dengue hemorrhagic fever. There is no vaccine against this disease. According to estimates by the World Health Organisation, about 400 million people are infected worldwide each year.