Dengue Fever Outbreak Catches Tokyo by Surprise

A dengue fever outbreak in Tokyo, the first seen in Japan in almost 70 years, has to date sickened over 100 people and appears to be spreading beyond the borders of the capital. Officials are scurrying to uncover the route of the infection and have taken emergency measures to contain it. We take a look at the disease and consider what global warming could mean for other mosquito-borne illnesses.

Major Parks Closed to Curb Spread of Disease

Dengue fever is a mosquito-borne disease common to tropical and subtropical climates. Symptoms include high fever, severe headache, and joint pain. While most patients recover from the malady, it has the potential to be fatal if complications develop. No vaccine or medication exists for the illness and the only treatment is to alleviate symptoms. Dengue is widespread in Southeast Asia as well as Central and South America. According to Japan’s National Institute of Infectious Diseases, about 200 cases are seen domestically each year, all of which are the result of infection during overseas travel. 

On August 26, the NIID confirmed that several students from a high school dance group had been infected with dengue fever while practicing together at Yoyogi Park in Tokyo’s Shibuya Ward. As none of the students had recently traveled overseas, this marked the first recorded incident of transmission of the illness in Japan for nearly 70 years. According to a September 11 Ministry of Health announcement, the number of cases has continued to rise, topping 100 in and around Tokyo.

Dengue fever is not transmitted between humans. Authorities believe the outbreak is the result of tiger mosquitoes, a chief carrier of the condition, spreading the virus to victims after biting an individual who had been infected while traveling overseas. 

After announcing September 4 that several mosquitos collected from within the confines of Yoyogi Park, one of Tokyo’s largest public spaces, had been found to be carrying the dengue virus, the metropolitan government cordoned off the roughly 4.5-hectare north section of the park and began fumigating the area. Additional efforts to keep the spread of the disease under control have included the closing on September 5 of some pathways on the grounds of the Meiji Shrine, which sits adjacent to Yoyogi Park, and the shuttering on September 7 of Shinjuku Gyoen, an expansive public park to the northeast of Yoyogi.

The tiger mosquito remains active until mid-October. However, the virus is not thought to be transmitted from parent to egg, making it likely that the outbreak will subside as the weather cools.

Visitors are sprayed with insect repellent prior to an event held at Yoyogi Park on September 6. (Photo: © Jiji)

Mosquitoes Moving North

In Asia, countries like the Philippines, Vietnam, Laos, and Malaysia account for a large share of dengue fever infections. In 2013, more than 160,000 cases resulting in over 500 fatalities were reported in the Philippines alone. Japanese nationals contracting the disease often do so while traveling in tropical and subtropical regions, with a large number of cases being traced to the Indonesian island of Bali. 

The two most common purveyors of dengue fever are the tiger and yellow fever mosquitos. The latter had previously been found on the island of Amakusa in Kumamoto Prefecture and throughout Okinawa Prefecture, but has not been seen on Japan’s main islands since the 1970s. The tiger mosquito is common to all areas of Japan, excluding the northernmost island of Hokkaidō. According to Japan’s Ministry of the Environment, global warming is gradually expanding the habitat of the mosquito, which now reaches as far north as Akita and Iwate Prefectures. 

The last major outbreak of dengue fever in Japan occurred during World War II, lasting from 1942 to 1945, and saw around 200,000 fall ill in Kobe, Osaka, Hiroshima, Nagasaki, and other major cities.

Growing Disease Risk from Global Warming

As the Earth warms, health officials are becoming increasingly concerned about the spread of other mosquito-borne diseases. Japanese encephalitis, a potentially debilitating illness characterized by acute inflammation of the brain, spinal cord, and surrounding membranes, is prevalent in many countries in Asia and the Western Pacific, including all areas of Japan excluding Hokkaidō. Several different varieties of mosquito are known to spread the condition, which is transmitted to humans by insects that have bitten pigs or other animals carrying the virus.

In Japan, vaccination efforts have controlled the spread of the disease, but the country still sees around 10 cases annually, with some of these resulting in fatalities. Globally, 30,000–40,000 individuals contract the illness each year, which has prompted Western countries to recommend vaccination for those staying in Japan or Southeast Asia for an extended period of time. 

Chikungunya is a viral infection passed to humans by disease-carrying mosquitoes. It shares similar symptoms to dengue fever, including sudden fever and severe joint pain, and is often misdiagnosed. It is most prevalent in sub-Saharan Africa, India, and Southeast Asia. There have been no domestic incidents of Chikungunya in Japan to date, but there have been recent cases of people being infected while traveling overseas, and health officials worry that an outbreak similar to that seen with dengue fever could occur. Currently there is no vaccine or known treatment for the disease.

Low Risk for Malaria

Malaria, a parasitic infection transmitted by mosquitoes, is not unknown to Japan, but at present, the 100–150 cases seen annually are invariably tied to overseas travel, with no domestic occurrences being recorded. Experts on infectious diseases consider the risk of an outbreak in Japan to be low, pointing to the high level of public health and sanitation, as well as the structure of homes and other factors hindering the spread of the mosquitoes that are responsible for transmitting the condition.  

(Banner photo: An official at Yoyogi Park puts up a closure notification on September 4 after mosquitoes carrying the virus causing dengue fever were found there. © Jiji Press)