Climate change will expose more people to mosquito-borne diseases

EPI researcher and medical geographer Sadie Ryan led a study that uses innovative maps to show how a warming world will open up new land regions, and the half billion people or more who inhabit them, to mosquito-borne diseases such as yellow fever, Zika, dengue and chikungunya over the next 30 years.

Nasty little dangerous insects mosquitoes flies against the bright red of the sunset sky

Nearly half a billion more people could be at risk for contracting mosquito-borne diseases such as yellow fever, Zika, dengue and chikungunya in the next 30 years as a result of climate change, new research shows.

Locales where tropical disease are currently unknown – Canada and parts of Northern Europe, for instance – will become prime real estate for the yellow fever mosquito (Aedes aegypti) and the tiger mosquito (Aedes albopictus).

That means government planners and public health officials in those countries who are unaccustomed to dealing with such crises need to start preparing now in order to protect their populations from future massive disease outbreaks, said Sadie Ryan, a UF Emerging Pathogens Institute researcher and lead author of the study published yesterday in PLOS Neglected Tropical Diseases.

“Newly exposed populations tend to see erupting epidemics,” said Ryan, who is also an associate professor of medical geography in UF’s College of Liberal Arts and Sciences. “And for the diseases we have seen recently, like Zika, first exposures tend to have worse outcomes, in terms of symptoms, and public health response, so we should be on the lookout for those new areas, under any future scenario.”

Ryan conducted the study with colleagues from Georgetown University, Stanford University and Virginia Tech. The team used mathematical models of climate-driven mosquito disease transmission, coupled with predictions of future climate change under different future emissions scenarios, to map when, where and how many people will be at potential risk for these diseases.

“These diseases, which we think of as strictly tropical, have been showing up already in areas with suitable climates, such as Florida, because humans are very good at moving both bugs and their pathogens around the globe,” Ryan said.

The purpose of this study, she said, is to show where different parts of the world will become newly suitable for this sort of introduction and establishment and to create a tool for global health planning in a changing climate.

One of the more nuanced findings has to do with how future temperature shifts will change the ranges of the yellow fever and tiger mosquitos in different ways. Worst-case warming predictions would contract the tiger mosquito range in southeast  Asia and west Africa, because these areas would become too hot for them, while at the same time increasing its range in Europe — upward of a billion more people will be exposed to yellow fever mosquito risk in this scenario. But intermediate warming changes would actually increase the tiger mosquito range as more places become warm, but not so hot that the insect reaches its thermal maximum for transmission. 

The authors write: “However, while year-round transmission potential from Aeaegypti is likely to expand (particularly in south Asia and sub-Saharan Africa), Aealbopictus transmission potential is likely to decline substantially in the tropics, marking a global shift towards seasonal risk as the tropics eventually become too hot for transmission by Aealbopictus.” (See figure, below, from the study.)

Two heat maps track Ae. aegypti and Ae. albopictus over 12 months.

By examining the number months people are exposed to risk throughout the year, the study estimates that there are a little more than 6 billion people currently exposed to transmission suitable climates for a month or more each year by the two mosquitoes. But as climate starts to push milder weather toward the poles, new areas, such as Canada and parts of Northern Europe, will start to be hospitable to these mosquitoes and their pathogens – within the next few decades.

By 2050, a net increase of around half a billion people will be exposed to one or more months of suitable transmission climate a year. However, the story is not a straightforward one of a warming world becoming a sicker world.

While there is a dengue vaccine, there are no widely available vaccines for chikungunya or Zika, diseases that both the yellow fever and tiger mosquito can transmit. These, as well as several other emerging diseases that could become global health threats, have different temperature ranges for ideal disease transmission. Put simply, it can become too hot for optimal transmission by the tiger mosquito, while becoming better for the yellow fever mosquito.

Climate change mitigation may not be the answer. A counterintuitive result emerges under emissions mitigation scenarios. The predictions are in fact worse, in sheer numbers of people at risk in the middle of the road scenarios, creating an interesting conundrum in the climate-health planning realm.

“Understanding the geographic shifts of risks really puts this in perspective,” Ryan said. “While we may see changing numbers and think we have the answer, imagine a world too hot for these mosquitoes.”

Ryan points out that the real impact will be most keenly felt where new population exposures will occur, and that is why we should keep an eye on the changing geography of disease.

That, she said, illuminates another issue — new diseases that can be transmitted by these mosquitoes can be picked up in new areas, making mapped results like these important to public health planning.

By: DeLene Beeland