As climate change drives mosquito populations to new heights, scientists urge us to take preventative measures against West Nile Virus in California.
Mosquito-borne disease is not a terror of the past.
We tend to think of mosquitoes as pesky bugs: they buzz behind our ears during cookouts and leave itchy bites on our arms. But just last year, two cases of dengue fever, a disease transmitted to humans through mosquito bites, were reported in Southern California. Dengue fever is typically contracted when someone visits a tropical region like Southeast Asia, is bitten by an infected mosquito, and returns home with the disease. But neither of these people had traveled outside of the United States, posing important questions about our risk for mosquito-borne disease in the coming years.
A common mosquito-borne disease in California is West Nile Virus. West Nile originates in birds, and mosquitoes become vectors for the disease when they feed on the blood of an infected bird or human. Though 80% of humans infected are asymptomatic, the virus can cause symptoms like fever, headache, nausea, and, in rarer cases, fatal neurologic illness. Across the state, there have been over 300 reported deaths from West Nile Virus since its introduction in 2003. Yet as climate change exacerbates heat waves and alters precipitation patterns, climate scientists expect these numbers to increase.
Mosquitoes thrive in warm, wet environments. Temperature is one of the most important factors for their survival, and a warmer climate means mosquitoes go through their lifecycle more quickly, speeding through the larval stage to become adults with the ability to contract and transmit disease. Higher temperatures may also make mosquitoes become infectious more quickly after biting a host.
The risk of West Nile has been especially alarming in California’s semi-arid Central Valley, as areas like Sacramento have hotspots of transmission each year. Since these insects lay their eggs in pools of standing water, flooding during California’s 2022/2023 winter created new breeding grounds and caused populations in the Central Valley to skyrocket.
However, this is not a one-way street. Mosquitoes are quite sensitive to temperature and actually have a narrow window at which they can survive. As global temperatures rise due to human- induced climate change, making warmer regions hotter and drier, we may see mosquitoes buzz northward to find more suitable habitat.
Desire Nalukwago, a Ph.D. student at Stanford University who is part of group working to map mosquito abundance in California, believes that “we're going to see disease shifting to places it's never been before.”
The mosquito species Aedes aegypti, which is known to spread dengue fever, has historically been found in tropical regions but was detected as an invasive species in California in 2011. Scientists fear this species, among others, is creeping northward. Previously low-risk areas, like the San Francisco Bay with its cooler climate, may be the next site of concern as global warming continues.
Climate change is also increasing the risk of West Nile infection in another way: drought. Although mosquitoes need water to reproduce, disease ecologist Andrew MacDonald argues that a scarcity of water is a larger concern for virus transmission.
“In periods of drought,” he explains, “you can actually have higher risk of West Nile Virus because there's more interaction between birds and mosquitoes.” When there is drought in an area, then animals, including humans, will seek out the few available water sources, making the exchange of disease more likely.
MacDonald’s research in the Central Valley involves using satellite technology to identify and monitor potential breeding areas for mosquitoes. His team studied the flooding event in 2023 and noted that despite an uptick in the number of mosquitoes in areas like Kern County, the same uptick was not seen in West Nile cases.
With the habitat range of mosquitoes growing, a host of communities will be newly exposed to mosquito-borne diseases. While the areas of California at highest risk of West Nile can’t be pinpointed with certainty, scientists do know that the species that transmit West Nile are found in both urban and rural areas. This means that communities like unhoused people in cities and the Central Valley’s agricultural workers, who are outdoors for most of the day, may be particularly vulnerable.
Tracking this risk is difficult, however. Barriers to healthcare access mean that many cases go unreported, so we may be grossly underestimating the number of people who have already been impacted by West Nile.
Mosquitoes may not be on your radar, but if you’ve lived through the COVID-19 pandemic, then you understand how vital it is to prepare for and protect ourselves from illness. There isn’t yet a treatment or vaccine for the mosquito-borne diseases transmitted in California, so preventative measures year-round remain essential to reducing risk.
Scientists urge us to wear protective clothing outdoors–think long sleeves, thick pants, and hats with mosquito nets. The mosquito species that transmit West Nile are most active between dusk and dawn, in the summer through early fall, so applying insect repellent with ingredients like DEET is imperative during these times.
We can also intervene in the mosquito lifecycle. Mosquitoes are able to breed in pools of water as tiny as flower pots and rain gutters, so scientists recommend emptying containers near your home every 4 days. California’s Vector Control Districts offer free inspection and treatment for standing water like ponds and will test dead birds for West Nile. These districts are also working to strengthen surveillance of mosquitoes to predict potential disease outbreaks.
Scientific communities are still working to understand who is at greatest risk for West Nile in the near future, but as Nalukwago notes, “At the end of the day, this is going to impact all of us.”
---
Aiyana is a storyteller who studies Earth Systems and Environmental Communication at Stanford. As an aspiring science writer, she hopes to tell stories that further climate justice and uplift communities most impacted by environmental threats. In the rare case she’s not walking around campus, you can find her in a theater or leading tours at Jasper Ridge.
Comments