Mosquito-borne diseases rely on their vector for transmission among humans. Environmental temperature is one of the key factors that influence fecundity, development, biting rate on hosts, and mortality, determining whether vectors are present in sufficient numbers for transmission. A new study models the effects of temperature on a suite of widespread mosquito vectors and viruses that currently lack complete temperature-dependent models. The new model suggests that West Nile virus spreads most efficiently in the United States at a very moderate 24–25°C (75.2–77°F) and that climate change may increase the areas in the United States with optimal temperatures for West Nile virus transmission.

The work is published in eLife in a paper titled, “Transmission of West Nile and five other temperate mosquito-borne viruses peaks at temperatures between 23°C and 26°C.”

“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, PhD, who was a postdoctoral fellow at Stanford University at the time the study was carried out, and is now a postdoctoral researcher at the University of California, Los Angeles.

To do this, Shocket and her colleagues developed models to assess the impact of temperature on six mosquito-borne viruses, four of which occur in the United States. These viruses—West Nile, St. Louis Encephalitis, Eastern and Western Equine Encephalitis, Sindbis, and Rift Valley fever viruses—were grouped together for this study as they share some of the same species of mosquito carriers.

The models used laboratory experiments that measured how different temperatures affect the mosquitoes’ survival, biting rate, reproduction, development, and ability to transmit the virus.

Compared to more tropical pathogens like dengue and malaria, transmission of temperate viruses has slightly cooler optimal temperatures and often substantially cooler cold thermal limits.

Shocket tweeted that this was, in part driven by differences in thermal response of lifespan. For the three mosquitos studied in this work, (Culex pipiens, Culex quinquefasciatus, and Culex tarsalis), lifespan continued to get longer at very cool temperatures (14–16°C) compared to other mosquito species (peaks at 22–25°C). The team validated their West Nile model using data on human virus transmission in the United States. They found that West Nile virus is transmitted most readily at moderate temperatures, while extreme temperatures limit where its mosquito carriers could live and successfully transmit the virus.

“Most of the viruses covered in this work are from more temperate areas than more commonly studied tropical diseases,” Shocket explained. “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.”

The researchers noted that the overall pattern of transmission matches general predictions for tropical versus temperate species: temperate diseases have wider thermal breadths. But trait responses for different mosquito vectors didn’t always match predictions based on their geographic ranges.

The results suggest that climate change could lead to the increased spread of West Nile virus in some places, while potentially causing a decrease in others, and provide insight on where and when these changes might occur.

The results also suggest that mosquito-borne diseases could take a greater toll in the United States as temperatures rise, especially as most of the population (70%) lives in places that are currently below the optimal temperature and will likely see increased transmission with climate warming. This is compared to 30% of the population who live in places where summer temperatures are above the optimal temperature, meaning transmission will likely decrease with climate warming. Temperature increases could also extend virus transmission seasons earlier into the Spring and later into the Fall.

“Climate change is poised to increase the transmission of West Nile and other mosquito-borne viruses in much of the United States,” concluded senior author Erin Mordecai, PhD, 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.”

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