Affirming that Climate Change is Real, and its Impact on Mosquito-Borne Disease is Really Complex

Posted 7 November 2017

Affirming that Climate Change is Real, and its  Impact on Mosquito-Borne Disease is Really Complex
 
It would seem reasonable to assume that as increased carbon emissions from human activities cause our planet to warm, malaria—a disease associated with the sultry tropics—will inexorably expand its range.
 
But let scientists like Stanford biologist Erin Mordecai start digging into the details, and a different picture emerges. Her presentation Tuesday afternoon at TropMed17 depicted a situation in which rising temperatures could produce more of a relocation than an expansion of malaria.
 
Dr. Mordecai’s talk was part of the session, “Science is Real: Climate Change Impacts on Vector Borne-Diseases,” which offered solid proof that not only is climate science real, the science of predicting the impact of climate change on diseases like malaria, dengue, chikungunya and Zika is a complex undertaking
 
Dr. Mordecai said that for malaria, it’s critical to understand that as temperatures rise, the increased heat may have a different effect, positive or negative, on each of the many factors that determine the risk of malaria transmission. Those factors include mosquito development, density and survival, along with the development and incubation of the malaria parasites they carry. And when she and her colleagues took all of these issues into account and crunched the data, what emerged was a temperature tipping point of 25C—that’s about 77F—beyond which heat starts discouraging malaria transmission.
 
As a result, Dr. Mordecai’s research points “not to a widespread increase in malaria transmission but to a very specific shift to currently cooler areas that are warming into the optimal range,” she said. Her prediction: “As the climate warms, coastal West Africa will become less suitable for malaria,” while currently cooler areas like the higher elevations of East Africa will become more suitable.
 
Dr. Mordecai also noted that the effect of rising temperatures on mosquito-borne diseases varies depending on the disease involved and the mosquito that’s carrying it.
 
For example, she said when assessing the future risk for the spread of dengue, the optimal temperature for transmission peaks at 25C when the carrier is the A. albopictus mosquito, but goes all the way up to 29C, or about 84F, when A. aegypti is involved. And unlike malaria, that means climate change is likely to allow dengue to hold on to its current hot spots while increasing its range northward into new territory.
 
But the climate-disease picture is sometimes cloudy, even when a good amount of data are available. NASA scientist Radina Soebiyanto said it has been difficult to create a climate risk model for chikungunya because there were different patterns present in various parts of the Americas in 2013 when the disease suddenly emerged in the region. It first appeared in the Caribbean, she said, when conditions were wetter than normal, yet also managed to spread easily in southern Mexico, when conditions there were dryer than normal.
 
“This suggests the adaptation of vectors to hot and dry conditions,” she said.
 
Kerri Miazgowicz of the University of Georgia presented evidence that climate-related risk assessments also need to consider the differences in microclimates found in urban, suburban and rural areas. She worked with a team of researchers that examined mosquito development at sites in urban, suburban and rural areas in and around Athens, GA. They found temperature variability was sufficiently large in these different locales to alter the risk of disease transmission from one location to another.
 
But she cautioned that currently available climate data “often does not capture differences in microclimates, and those differences effect risk of disease transmission.”