ABOVE: Culex tarsalis, a main vector of West Nile virus

This year is shaping up to be a bad one for West Nile virus, a pathogen that has killed at least 2,300 people in the United States since it first arrived in the country in 1999. While the toll—21 deaths as of September 21, according to statistics published by the Centers for Disease Control and Prevention (CDC)—is much lower than the 51 deaths in 2020, case numbers tend to peak in late summer, and some states have been hit unusually hard. Arizona, for example, has reported some 97 cases and 4 deaths so far. 

New York has reported 14 cases, which is enough for some politicians to raise the alarm. A press release from US Senator Charles Schumer’s office blamed this year’s unusually high mosquito activity on a “particularly wet summer and changing climate”—both of which are relevant factors, experts say, although the relationship between climate and West Nile Virus is a little more complicated than that. 

“The climate link is actually much clearer here in Europe than in the United States,” says Jan Semenza, a public health expert at the University of Heidelberg in Germany. Because the virus is still a relative newcomer to North America, there is little natural immunity to hold back its spread. “It’s similar to COVID. If you have a pathogen that emerges in a pristine, naïve environment, then it expands very rapidly, regardless of climate change.” 

Specific events and conditions can also cause short-term spikes in disease numbers. Shortly after the financial crash of 2008, Semenza tells The Scientist, experts traced a West Nile outbreak in California to abandoned swimming pools. “There were so many foreclosures due to the economic meltdown that pools weren’t taken care of, and they became a breeding ground for mosquitoes,” he says. 

David Ewing, a mathematical biologist at Biomathematics and Statistics Scotland, has studied the situation in the UK and says that warmer temperatures will increase the risk of West Nile infections there. The disease “has been spreading northwards in recent years. They have had cases in Germany and in the Netherlands in recent years,” he notes. But the shift is not a simple consequence of the range of mosquitoes expanding with warmer weather. “The question is whether it is warm enough to sustain the transmission cycles of West Nile virus if it were to appear,” he says. 

West Nile is predominantly a disease of birds, which act as the main reservoir for the virus. Mosquitos pick up the virus when they bite an infected bird, and then spread it when they bite a second, uninfected, animal—usually another bird, but occasionally a person. Temperature can affect this transmission cycle because warmer conditions speed up the replication of the virus inside the cold-blooded mosquito. “It’s probably not really warm enough in the UK yet for the virus to replicate enough in the mosquito to pass it on before it dies,” Ewing says. “That risk will increase with climate change, and we estimate that outbreaks start to look quite plausible, mainly in the south and southeast of England, by the middle of the century.” 

We now know there’s what we call a Goldilocks temperature where mosquito-borne diseases spread the best.

—Marta Shocket, University of Florida

Recent research has shown another side to the equation: Temperatures can also get too hot for the disease to spread readily. “In the past ten years or so we’ve been making more realistic models for mosquito physiology, and we now know there’s what we call a Goldilocks temperature where mosquito-borne diseases spread the best,” says Marta Shocket, an ecologist at the University of Florida. “When it’s really hot, mosquitoes just don’t live that long.” 

The Goldilocks temperature is different for each disease, mainly because they are spread by different mosquitoes. Dengue fever flourishes at 29°C, for instance. For West Nile, the optimum is closer to 24°C or 25°C. “So really what we expect with climate change is that there will be shifts in mosquito-borne disease. Some places that are already in the Goldilocks zone are going to get too warm. And so they might actually have less disease,” Shocket says. 

That could be the case for many places in the US. In a study published last year, Shocket, then a postdoc at the University of California, Los Angeles, and her colleagues showed that about 70 percent of both the counties and the national population currently experience average summer temperatures cooler than West Nile’s optimal transmission temperature. But 30 percent are already above it. “In general, in the US we would expect more West Nile transmission [with climate change]. But there’s a mix of some places increasing [in hospitality to the virus] and some places decreasing,” she says. 

To help local officials plan their responses, researchers are working on models that could forecast outbreaks, she adds—aiming to, for example, “predict by July if this year is going to be a bad year.”

Correction (September 30): This article has been edited to correct Marta Shocket’s current affiliation and affiliation at the time she conducted the study described in the penultimate paragraph. The Scientist regrets the error.