West Nile Virus: Can Climate Change Cause Mosquito Migration?

Given the range of West Nile virus and other mosquito-borne diseases is environmentally determined, changes in the environment mean changes in where these diseases are most likely to spread.

West Nile virus spreads most efficiently in the United States at temperatures between 75.2—77 degrees Fahrenheit, according to a new study published in the journal eLife.

The results imply that climate change may lead to the increased spread of West Nile in some places, while causing a decrease in spread elsewhere.

“As the climate warms, it is critical to understand how temperature changes will affect the transmission of mosquito-borne diseases,” said lead author Marta Shocket, who is a Postdoctoral Researcher at the University of California, Los Angeles.

Shocket and the rest of the study team used modeling to estimate the impact of temperature on 6 different mosquito-borne viruses, 4 of which presently can be found in the US.

The 6 viruses were:

• West Nile,
• St. Louis Encephalitis,
• Eastern and Western Equine Encephalitis,
• Sindbis,
• and Rift Valley fever.

The 6 viruses share some of the same species of mosquito carriers.

The models were based on laboratory experiments which measured how temperatures affect mosquito survival, biting frequency, mating, developmental factors, and transmissibility of the virus.

Investigators validated the West Nile model using data on viral transmission in the US. West Nile virus is transmitted most readily at certain moderate temperatures, while extreme temperatures limit where relevant mosquito carriers can thrive.

“We primarily focus on Culex pipiens, Cx. quinquefasciatus, and Cx. tarsalis, well-studied species that are important vectors for many viruses and for which appropriate temperature-dependent data exist for nearly all traits relevant to transmission,” study authors wrote.

“Most of the viruses covered in this work are from more temperate areas than more commonly studied tropical diseases,” Shocket explained in a press release. “We compared these results to those of tropical diseases like malaria and dengue and found that the optimal temperatures and cold thermal limits for virus spread are cooler. This means the viruses spread more efficiently at cooler temperatures compared to more tropical diseases, as you would expect.”

While the claims around moderate temperature might not immediately bring global warming to mind, the results suggest that mosquito-borne diseases could present a greater burden in the US if global temperatures rise, because most of the population (70%) actually lives in places that are presently below the optimal transmission temperature.

Conversely, 30% of the population lives in places where summer temperatures are above the optimal temperature, so transmission would likely decrease with climate warming there.

“Temperature increases could also extend virus transmission seasons earlier into the Spring and later into the Fall,” study authors added.

“Climate change is poised to increase the transmission of West Nile and other mosquito-borne viruses in much of the US,” said senior author Erin Mordecai, an Assistant Professor of Biology at Stanford University. “But these diseases also depend on human contact with mosquitoes that also contact wildlife, so factors like human land use, mosquito control, mosquito and virus adaptations, and the emergence of new viruses make predicting the future of mosquito-borne disease a challenge.”