Zika Outbreak Could Be an Omen of the Global Warming Threat
The global public health emergency involving deformed babies emerged in 2015, the hottest year in the historical record, with an outbreak in Brazil of a disease transmitted by heat-loving mosquitoes. Can that be a coincidence?
Scientists say it will take them years to figure that out, and pointed to other factors that may have played a larger role in starting the crisis. But these same experts added that the Zika epidemic, as well as the related spread of a disease called dengue that is sickening as many as 100 million people a year and killing thousands, should be interpreted as warnings.
Over the coming decades, global warming is likely to increase the range and speed the life cycle of the particular mosquitoes carrying these viruses, encouraging their spread deeper into temperate countries like the United States.
Recent research suggests that under a worst-case scenario, involving continued high global emissions coupled with fast population growth, the number of people exposed to the principal mosquito could more than double, to as many as 8 billion or 9 billion by late this century from roughly 4 billion today.
“As we get continued warming, it’s going to become more difficult to control mosquitoes,” said Andrew Monaghan, who is studying the interaction of climate and health at the National Center for Atmospheric Research in Boulder, Colo. “The warmer it is, the faster they can develop from egg to adult, and the faster they can incubate viruses.”
“随着全球变暖，控制蚊子将越来越困难。”安德鲁·莫纳甘(Andrew Monaghan)说。安德鲁在科罗拉多州博尔德的美国国家大气研究中心(National Center for Atmospheric Research)研究气候与健康的互动关系。“天气越热，蚊子从孵化到成年的速度就越快，病毒繁殖的速度也就越快。”
Already, climate change is suspected — though not proven — to have been a factor in a string of disease outbreaks afflicting both people and animals. These include the spread of malaria into the highlands of eastern Africa, the rising incidence of Lyme disease in North America, and the spread of a serious livestock ailment called bluetongue into parts of Europe that were once too cold for it to thrive.
In interviews, experts noted that no epidemic was ever the result of a single variable.
Instead, epidemics always involve interactions among genes, ecology, climate and human behavior, presenting profound difficulties for scientists trying to tease apart the contributing factors. “The complexity is enormous,” said Walter J. Tabachnick, a professor with the Florida Medical Entomology Laboratory, a unit of the University of Florida in Vero Beach.
相反，传染病常常涉及基因、生态、气候、人类行为等多种因素的互动，所以科学家很难把单个因素分离出来。“原因极为复杂。”佛罗里达大学(University of Florida)位于维罗比奇(Vero Beach)的佛罗里达医学昆虫学实验室的教授瓦尔特·塔巴奇尼克(Walter J. Tabachnick)说。
The epidemics of Zika and dengue are cases in point. The viruses are being transmitted largely by the yellow fever mosquito, Aedes aegypti. That creature adapted long ago to live in human settlements, and developed a concomitant taste for human blood.
Cities in the tropics, the climate zone most favorable to the mosquito, have undergone explosive growth: Humanity passed a milestone a few years ago when more than half the population had moved to urban areas. But spending on health care and on basic public health infrastructure, like water pipes and sewers, has not kept pace. Mosquito control has also faltered in recent decades.
The mosquito lays its eggs in containers of water, of a sort that are especially common in the huge slums of Latin American cities. With unreliable access to piped water, people there store water in rooftop cisterns, buckets and the like. Old tires and other debris can also become mosquito habitat.
Water storage near homes is commonplace in areas where Zika has spread rapidly, like the cities of Recife and Salvador in northeastern Brazil, and where dengue experienced a surge in 2015, like São Paulo, Brazil’s largest state.
Altogether, dengue killed at least 839 people in Brazil in 2015, a 40 percent increase from the previous year. Worldwide, dengue is killing more than 20,000 people a year.
Several experts said in interviews that a main reason for the disease outbreaks was most likely the expansion of the number of people at risk, through urbanization, population growth and international travel. They see the changing climate as just another stress on top of a situation that was already rife with peril.
While they do not understand to what degree rising temperatures and other weather shifts may have contributed to the outbreaks, they do understand some of the potential mechanisms.
The mosquitoes mostly live on flower nectar, but the female of the species needs a meal of human blood to have enough protein to lay her eggs. If she bites a person infected with dengue, Zika or any of several other diseases, she picks up the virus.
The virus has to reproduce in the mosquito for a certain period before it can be transmitted to another person in a subsequent bite. The higher the air temperature, the shorter that incubation period. Moreover, up to a point, higher temperatures cause the mosquitoes to mature faster.
With rising temperatures, “You’re actually speeding up the whole reproductive cycle of the mosquitoes,” said Charles B. Beard, who heads a unit in Fort Collins, Colo., studying insect-borne diseases for the Centers for Disease Control and Prevention in Atlanta. “You get larger populations, with more generations of mosquitoes, in a warmer, wetter climate. You have this kind of amplification of the risk.”
温度上升“实际上会加速蚊子的整个繁殖周期，”查尔斯·B·彼尔德(Charles B. Beard)说，“人口增加了，在更潮湿、更炎热的气候里，存活的蚊子世代也增多了，这就放大了风险。”彼尔德领导着一个位于科罗拉多州科林斯堡的团队，为亚特兰大的疾病控制与预防中心(Centers for Disease Control and Prevention in Atlanta)进行昆虫传播疾病研究。
In principle, the risk from continued global warming applies not just to temperate countries, but to cities at high altitude in tropical countries. Researchers are keeping a close eye on Mexico City, for instance.
With 21 million people in the city and its suburbs, Mexico City is the largest metropolis of the Western Hemisphere. While the lowlands of Mexico are plagued by yellow fever mosquitoes and the viruses they transmit, the country’s capital sits on a mountain plain that has — up to now — been too cold for the mosquitoes.
But temperatures are rising, and the mosquitoes have recently been detected in low numbers near Mexico City.
“The mosquito is just down the hill, literally,” Dr. Monaghan said. “I think all the potential is there to have virus transmission if climatic conditions become a bit more suitable.”