In recent years, some reports have claimed that as global temperatures rise, more areas will be affected by diseases that are spread by mosquitoes, such as malaria, dengue, and yellow fever. As temperatures rise, the logic goes, the mosquito habitat will increase and more people will be exposed to them, making them potential disease victims.
However, a study published this week in the scientific journal Proceedings of the National Academy of Sciences shows that the situation is much more complicated. For mosquitoes to thrive, they need water, and warmer temperatures may also mean lower precipitation, allowing fewer mosquitoes to survive.
University of Arizona researchers Cory Morin and Andrew Comrie developed a climate-driven mosquito population model to simulate the abundance across the southern United States of one type of mosquito known to carry and spread West Nile virus to humans. They found that, under the future climate conditions predicted by climate change models, many locations will see a lengthening of the mosquito season but the populations will be smaller due to hotter and dryer conditions that allow fewer mosquito larvae to live.
“It used to be an open question whether climate change is going to make disease-carrying mosquitoes more abundant, and the answer is it will depend on the time and the location,” said Morin, who did the study as part of his doctoral dissertation in the lab of Andrew Comrie, UA Provost and professor in the UA’s School of Geography and Development.
“One assumption was that with rising temperatures, mosquitoes would thrive across the board,” Morin said. “Our study shows this is unlikely. Rather, the effects of climate change are different depending on the region and because of that, the response of West Nile virus transmitting mosquito populations will be different as well.”
“The mosquito species we study is subtropical, and at warmer temperatures the larvae develop faster,” Morin explained. “However, there is a limit – if temperatures climb over that limit, mortality increases. Temperature, precipitation or both can limit the populations, depending on local conditions.”
Their findings suggest that disease transmission studies and programs designed to control populations of disease-carrying mosquitoes must be targeted locally to maximize their effectiveness, the authors argue. In the southwestern US for example, hotter and drier summers are expected to delay the onset of mosquito season; however, late summer and fall rains are expected to result in a longer season. Conversely, the south-central US will see fewer mosquito days due to less rain during summer and early fall. Higher temperatures projected for the shoulder seasons – spring and fall – will likely make for a longer mosquito season across much of the US, except in the Southwest during spring where severe drying inhibits population development.
“Which locations are likely to experience epidemics in the future? – those are the kinds of questions studies like ours may help prepare for,” said Morin. “We don’t model the actual virus, we only look at the vector, but our study informs at least one part of the ecology of the virus. It is unique in projecting the impacts of climate change on a West Nile vector.”
Morin said the study could help managers and decision makers better anticipate how mosquito populations will respond to changes in climate and prepare accordingly.
“For example, if projected precipitation and temperature changes for a given area are indicating a longer mosquito season, public health officials can plan to adapt to that possibility through abatement and awareness campaigns.”
Their study reinforces one done by Penn State researchers who found that warmer temperatures seem to slow the transmission of malaria-causing parasites. Both indicate that even if warmer temperatures do expand the range of disease-transmitting mosquitoes, dryer conditions will lead to fewer of the insects, and even fewer of them are likely to be infectious.
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