The Mosquito: The Uninvited Bloodsucker of Summer

There’s an old Japanese saying that’s rather apt for this time of year: “Bunbō gyūyō o hashirasu,” which roughly translates to “mosquitoes and horseflies can make cattle run.” The implication is that even the weak can move the mighty or the trivial can cause disaster. Mosquitoes aren’t a rare sight — or bite — in Japan, and they’re as much a part of a Japanese summer as cicadas or cockroaches. However, these tiny little bloodsuckers are more than just annoyances: Zika virus, dengue fever, malaria – mosquitoes are capable of spreading an assortment of infectious diseases to humans, sometimes with horrific consequences. In fact, these tiny flies cause more human suffering than any other creature on the planet, helping to kill over a million people worldwide each year. While Japan has largely succeeded in containing major epidemics from diseases spread by mosquitoes through the use of vaccines and careful border inspections, the country is by no means immune. In 2014, Japan recorded its first dengue outbreak in 70 years, while a few cases of Japanese encephalitis are reported every year. And with climate change, scientists say the habitat of Aedes albopictus, the (Asian) tiger mosquito, a major carrier of viruses responsible for dengue fever, Zika, and other diseases, is creeping north through the archipelago as temperatures rise, a phenomenon felt especially acutely this summer as the mercury rises and some prefectures see their highest temperatures in decades. The centuries-old battle against the buzzy little nuisances has resulted in a robust market for bug killers and repellents, with some Japanese firms exporting their solutions to tropical regions where mosquitoes are active year round. Despite these efforts, though, Japanese researchers have recently discovered that a series of mutations has made some mosquitoes in Asia “super resistant” to popular pyrethroid-based insecticides like permethrin. “Mosquitoes are by far the deadliest insects for humans,” says Shinji Kasai, a researcher at the National Institute of Infectious Diseases who led the study. “Our research indicates that we need to think of other, effective strategies to combat them.” The Yoyogi Park outbreak of 2014 On Aug. 27 and 28, 2014, three locally acquired cases of dengue were confirmed in Tokyo. By October, a total of 160 cases had been reported. The tiger mosquito, active mostly during daylight hours in outdoor settings, was suspected to be the culprit, since the species is a vector competent in transmitting the dengue virus and is present in much of Japan, excluding Hokkaido. None of the people infected had a history of recent overseas travel, and a majority were associated with visits to the capital’s central Yoyogi Park. A portion of the park was shut down and mosquito traps were set up to collect the insects for examination while pest control specialists in hazmat suits walked around the vicinity, spraying insecticide. “We detect dengue fever in patients every year, but typically these are people who are infected while traveling overseas, and who show symptoms after returning to Japan,” says Aya Kaebeta, a manager at the Tokyo Metropolitan Government’s infectious disease control department. “But in 2014, we detected cases of domestic transmission, likely through mosquitoes sucking the blood of infected patients and passing it along to others. It was the first dengue epidemic since 1945,” she says. The outbreak was a warning that any temperate nation that welcomes travelers from endemic countries in tropical and subtropical areas faces the threat of local outbreaks. And that risk is growing in Japan as the government considers tourism one of the central pillars of its economy. The age of ‘super-resistant’ mosquitoes With the Yoyogi Park outbreak serving as a preview for what could happen if a tropical disease were to spread in Japan, the urgency to study mosquitoes as a common vector for transmission has become more pressing. In an article published in December in Science Advances, Kasai and a team of researchers found a new genetic mutation that makes Aedes aegypti — a species of mosquito commonly called the yellow fever mosquito that is found in mostly tropical regions — far more resistant to chemicals such as pyrethroid, which is used to kill them. Shinji Kasai, a researcher at the National Institute of Infectious Diseases, holds a model of an Asian tiger mosquito. | Alex K.T. Martin Kasai’s team examined mosquitoes from several Asian countries, as well as Ghana (the species originated in Africa), and discovered a succession of mutations had made some effectively impervious to permethrin. Between 78% and 99% of the mosquitoes in Hanoi, Ho Chi Minh City and Phnom Penh had the new genetic mutation identified as L982W. Some of them exhibited substantially higher levels of pyrethroid resistance — even up to a thousandfold higher — than any other field population ever reported. And while the L982W strain hasn’t yet been detected in any country except Vietnam and Cambodia, Kasai believes it could be spreading west to other areas of Asia, potentially posing an unprecedentedly serious threat to the control of dengue fever and other tropical diseases. “Our research is continuing, and I’m working with researchers and public safety experts in Laos, Thailand and Cambodia to investigate the situation,” Kasai says from his office in Tokyo’s Shinjuku Ward. And the threat isn’t limited to the Indochinese Peninsula. While the A. aegypti species doesn’t naturally distribute in Japan, it would be introduced here via international air transportation. In fact, both its larvae and adult forms have been discovered on six separate occasions at Japanese airports, beginning in 2012 at Narita International Airport in Chiba Prefecture. “The A. aegypti are considered unable to survive the winter unless the average temperature in January is higher than 10 degrees Celsius,” Kasai says. “However, if the median temperature in Japan were to rise by 3 degrees, for example, in theory, the mosquito would be able to take hold in a much wider area, even reaching the coastal Kanto region.” And that’s not an unrealistic future, Kasai adds, pointing out that Tokyo’s average temperature has risen by around 3 degrees over the past century. The buzz spreads north There are approximately 3,600 species of mosquitoes in the world, and 112 species known in Japan. Despite their vampiric reputation, most feed mainly on flower nectar, with only a limited number going after human blood and transmitting infectious diseases. The National Institute of Infectious Diseases was frequently in the news during the coronavirus pandemic for its research into COVID-19. It’s also home to a department dedicated to medical entomology, which Kasai heads, and is considered the top facility when it comes to the study of mosquitoes in Japan. Kasai’s fascination with the insect is apparent from the numerous bite marks on his arms. He has mosquitoes suck on him while capturing extremely close-up photographs of them, an activity he can trace back to his childhood in Gifu Prefecture. An Anopheles sinensis mosquito | Courtesy of Shinji Kasai / National Institute of Infectious Diseases “I was into butterfly photography in high school and in college began researching mosquitoes,” he says. “And while studying mosquitoes at the National Institute of Infectious Diseases, I became drawn to their beauty and started taking photographs of them.” Among his recent projects is the tracking of the distribution of A. albopictus, the species at the center of the Yoyogi Park outbreak. A known carrier of dengue, its habitat is spreading north as temperatures increase, and it has been confirmed in places as far as Aomori Prefecture at the tip of Japan’s main island of Honshu. “We expect them to eventually reach Hokkaido, perhaps starting with the city of Hakodate, which can be reached via airplanes, bullet trains, and ferries,” he says. “Last year we went to Hokkaido for fieldwork and examined local mosquito populations, visiting around 40 locations over the span of three days.” Fortunately the team didn’t find any tiger mosquitoes this time around, but Kasai is worried that “it just could be a matter of time.” The magic coil Mosquitoes and Japan go back a long way. After all, the nation’s ubiquitous rice paddy fields, the primary staple food of its population, are mass breeding sites for the…

FOLLOW US ON GOOGLE NEWS

Read original article here