Forecasting Mosquitoes' Global Spread
By BCH - 6. March 2019
Outbreaks of mosquito-borne illnesses like yellow fever, dengue, Zika and chikungunya are rising around the world. Climate change has created conditions favorable to mosquitoes' spread, but so have human travel and migration and accelerating urbanization, creating new mini-habitats for mosquitoes.
In today's Nature Microbiology, a large group of international collaborators combined these factors into prediction models that offer insight into the recent spread of two key disease-spreading mosquitoes -- Aedes aegypti and Aedes albopictus. The models forecast that by 2050, 49 percent of the world's population will live in places where these species are established if greenhouse gas emissions continue at current rates.
"We find evidence that if no action is taken to reduce the current rate at which the climate is warming, pockets of habitat will open up across many urban areas with vast amounts of individuals susceptible to infection," says Moritz Kraemer, PhD, of Boston Children's Hospital and the University of Oxford (UK). Kraemer was co-first author on the study with Robert Reiner of the University of Washington (Seattle), Oliver Brady of the London School of Hygiene and Tropical Medicine, Jane Messina of the University of Oxford and Marius Gilbert of the Universite Libre de Bruxelles (Belgium).
A spatial-temporal analysis
The team gathered historic data on the distributions of Aedes aegypti and Aedes albopictus over time in more than 3,000 locations in Europe and the U.S., going back as far as the 1970s and 1980s. They also mapped the locations based on their present-day suitability as mosquito habitats, then projected their suitability in 2020, 2050 and 2080 based on various climate models, projections of urban growth and other variables. They also included human migration and travel patterns, using data from census data and mobile phone records.
In recent times, they found, Aedes aegypti has tended to spread over long distances, while Aedes albopictus's spread has been more localized. Within the U.S., Aedes aegypti spread north at a relatively constant rate, about 150 miles per year. Aedes albopictus spread most quickly between 1990 and 1995; its advance has since slowed to about 37 miles per year. In Europe, Aedes albopictus has spread faster, advancing about 62 miles per year increasing to 93 miles per year in the past five years.
"We observed striking differences in the spread of both mosquito species, which has direct implications for surveillance and control strategies," says Kraemer. "We hope that these high-resolution maps will be used to target specific geographic areas for surveillance, control and elimination of these harmful mosquito populations."
What's next for mosquitos?
The models suggest that under current climate conditions and population densities, both mosquito species will continue to spread globally over the coming decades.
Aedes aegypti is predicted to spread mostly within its current tropical range, but also in new temperate areas in the U.S. and China, reaching as far north as Chicago and Shanghai, respectively, by 2050. In the U.S., this spread is expected to occur in large urban areas, through long-distance introductions of the insect. Aedes aegypti is expected to decline in the central southern United States and Eastern Europe, which climate models predict will become more arid. It's not expected to reach Europe except for parts of southern Italy and Turkey.
Aedes albopictus, however, is forecast to spread widely throughout Europe, ultimately reaching large areas of France and Germany over the next 30 years. It's also expected to establish a toe-hold in parts of the northern U.S. and the highland regions of South America and East Africa.
Climate change: the wild card
In the next 5 to 15 years, the models predict that spread of both species will be driven by human movement, rather than environmental changes. But thereafter, expansion will be driven by changes in climate, temperature and urbanization that create new mosquito habitats. And if climate change isn't curbed by 2050, the spread is predicted to be even greater.
"With this new work, we can start to anticipate how the transmission of diseases like dengue and Zika might be influenced by a variety of environmental changes," says Simon I. Hay, director of Geospatial Science at IHME and Professor of Health Metrics Sciences at the University of Washington. "Incorporating this information into future scenarios of risk can help policymakers predict health impacts and help guide strategies to limit the spread of these mosquito species, an essential step to reduce the disease burden."
(*) Hay and Nick Golding of the University of Melbourne were co-senior investigators on the study.
More information: Past and future spread of the arbovirus vectors Aedes aegypti and Aedes albopictus, Nature Microbiology (2019). DOI: 10.1038/s41564-019-0376-y , https://www.nature.com/articles/s41564-019-0376-y
Journal information: Nature Microbiology
- Moritz U. G. Kraemer, Robert C. Reiner, Oliver J. Brady, Jane P. Messina, Marius Gilbert, David M. Pigott, Dingdong Yi, Kimberly Johnson, Lucas Earl, Laurie B. Marczak, Shreya Shirude, Nicole Davis Weaver, Donal Bisanzio, T. Alex Perkins, Shengjie Lai, Xin Lu, Peter Jones, Giovanini E. Coelho, Roberta G. Carvalho, Wim Van Bortel, Cedric Marsboom, Guy Hendrickx, Francis Schaffner, Chester G. Moore, Heinrich H. Nax, Linus Bengtsson, Erik Wetter, Andrew J. Tatem, John S. Brownstein, David L. Smith, Louis Lambrechts, Simon Cauchemez, Catherine Linard, Nuno R. Faria, Oliver G. Pybus, Thomas W. Scott, Qiyong Liu, Hongjie Yu, G. R. William Wint, Simon I. Hay, Nick Golding. Past and future spread of the arbovirus vectors Aedes aegypti and Aedes albopictus. Nature Microbiology, 2019; DOI: 10.1038/s41564-019-0376-y
Source: Boston Children's Hospital
Rising Temperatures Will Help Mosquitos Infect a Billion More People
“Plain and simple, climate change is going to kill a lot of people.”
Nexus Media - Mar 28 2019
By Marlene Cimons
Mosquitoes are unrelenting killers. In fact, they are among the most lethal animals in the world. When they carry dangerous viruses or other organisms, a bite can be unforgiving. They cause millions of deaths every year from such infectious diseases as malaria, dengue, Zika, chikungunya, yellow fever and at least a dozen more.
But here’s the really bad news: climate change is expected to make them even deadlier. As the planet heats up, these insects will survive winter and proliferate, causing an estimated billion or more new infections by the end of the century, according to new research.
“Plain and simple, climate change is going to kill a lot of people,” said biologist Colin J. Carlson, a postdoctoral fellow in Georgetown University’s biology department, and co-author of the study, published in the journal PLOS Neglected Tropical Diseases. “Mosquito-borne diseases are going to be a big way that happens, especially as they spread from the tropics to temperate countries.”
The study predicted an amenable climate could prompt some of these new cases within regions not previously regarded as vulnerable, including the United States. These viruses can result in volatile outbreaks when conditions are right, as was the case with Zika. “We’ve known about Zika since 1947, and we watched it slowly spread around the world until 2015, when it arrived in Brazil and suddenly we had an explosive epidemic on our hands,” Carlson said.
“Chikungunya has done something not too different from that,” he added. “These viruses proliferate quickly in populations that don’t have any immunity — and we’re very scared about that. If you only have one month that’s warm enough for outbreaks, the question is: ‘how much damage you can do…?’ For viruses like these, it’s a lot.”
The study underscores the growing evidence that climate change is having — and will continue to have — a deleterious impact on global health, not only from the direct effects of extreme weather events like heat waves and flooding, but also because mosquitoes thrive in warm temperatures and carbon dioxide, encouraging them to flourish and spread disease. Rising temperatures also are causing many to migrate to new locations.
“This is a very important forward-looking report,” said Robert T. Schooley, an infectious diseases expert at the University of California, San Diego, and editor of the journal Clinical Infectious Diseases, who was not involved in this study. “Understanding in as much detail as possible the risks we face as we move toward a warmer planet is essential. Mosquitoes are able to spread blood-borne pathogens faster than epidemiologists can track an epidemic. There are many one-way doors through which we will go as the planet warms. The spread of mosquitoes and other vectors that can transmit multiple pathogens is an important one that those who don’t think climate change is a serious problem should ponder.”
The research team, also led by Sadie J. Ryan, of the University of Florida’s emerging pathogens institute and associate professor of medical geography, looked what would happen to the two most common disease-carrying mosquitoes, Aedes aegypti and Aedes albopictus, as temperatures increase during the coming decades. It found that global warming will expose almost all of the world’s population to mosquitoes at some point in the next 50 years. Also, there likely will be year-round transmissions in the tropics and seasonal risks almost everywhere else, along with a greater intensity of infections, according to the study. Moreover, shorter, warmer winters will mean more mosquitoes will survive.
“Where the number of temperature-suitable months of the year increase, so too will the winter months decrease, lowering the threshold for overwintering survival of the mosquitoes,” Ryan said. “We already have evidence, for example, in New England, that tire piles can provide sufficiently ambient overwintering habitat already for albopictus, by virtue of retaining pooled water that doesn’t get too cold. With fewer months at those very low temperatures, the pressure on survival is lessened, and more overwintering mosquitoes will make it to the next season.”
The scientists’ goal was to better understand what increasing temperatures would mean for the handful of viruses that Aedes mosquitoes spread. “We used a model of virus spread at different temperatures to mark out where in the world these viruses might be over time, and used climate models to map people at risk now and in the future, that is, 2050 and 2080,” Carlson explained. “This study is a bit of a numbers game: with 7 billion people on Earth, who’s most at risk now? Who’s at risk in a generation? We don’t know where mosquitoes will be in the future. What we can do is say where they might be able to transmit viruses if they show up.”
The burden on developing countries, already hard hit, likely will increase, especially in the East African portion of sub-Saharan Africa, “one of the top regions to experience increases in people at risk,” Ryan said. “These are regions in which we tend to focus on malaria and malaria control…However, we know that there is also dengue circulating in these regions. It is essential to think about surveillance of these diseases now. This is a part of the world that will be facing the intersection of multiple vulnerabilities under climate change, with underfunded infrastructure to manage multiple health impacts.”
Dengue, which causes high fever, headache, and joint pain, is the most common vector-borne viral disease in the world, with up to 100 million infections and 25,000 deaths annually, and, in recent years, has appeared in the United States. It caused an epidemic in Hawaii in 2001, as well as a cluster of cases in Florida about ten years ago, with additional sporadic infections since then. It also has shown up with increasing frequency along the Texas-Mexican border.
The 2009 Florida cases, in fact, were the first dengue cases acquired in the continental United States — outside of the Texas-Mexico border — since 1945, and the first locally transmitted cases in Florida since 1934. “We’ve seen dengue showing up in Hawaii and Florida, then we saw Zika arrive in Florida and really grab public attention,” Ryan said. “Because Aedes aegypti is such a globalized mosquito, the potential for it to facilitate new outbreaks of many diseases is high.”
Often, cases that arise in the United States typically result after Americans travel and are infected abroad. Once they return home, a local mosquito bites them and acquires the virus. It then can transmit it to others. “Travelers overseas can bring back pathogens that can establish in local mosquito populations,” Ryan said. “This potential is a very real issue.”
There are several thousand species of mosquitoes, but only a few transmit disease. Anopheles mosquitoes carry the parasites that causes malaria and filariasis — also called elephantiasis — and the virus that causes encephalitis. Culex mosquitoes carry encephalitis, filariasis, and the virus that causes West Nile, while the two Aedes species studied in this paper transmit the viruses that cause yellow fever, dengue, and encephalitis.
“We’ve only managed to capture the uncertain futures for two mosquitoes that spread a handful of diseases — and there’s at least a dozen vectors we need this information on,” Carlson said. “It’s very worrisome to think how much these diseases might increase, but it’s even more concerning that we don’t have a sense of that future. We have several decades of work to do in the next couple years if we want to be ready.”
He believes that climate mitigation could save millions of lives, “but I also don’t want us to fall into the trap of mitigating climate change just to keep dengue and Zika in the tropics, and out of the U.S. and Europe,” he said. “Facing something as massive as climate change gives us a chance to rethink the world’s health disparities, and work towards a future where fewer people die of preventable diseases like these. Facing climate change and tackling the burden of neglected tropical diseases go hand-in-hand.”
Marlene Cimons writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture.
May 2, 2019