A record-breaking number of mosquitoes are carrying West Nile virus around Las Vegas

A record-breaking number of mosquitoes in and around Las Vegas are carrying West Nile virus, sparking warnings from local health officials who say the public should take precautions to avoid getting bit. 

West Nile virus can cause fever, headaches, vomiting and diarrhea and is fatal in about 1 of 150 cases. There are no vaccines or medications to treat or prevent the mosquito-borne illness.

In recent weeks, 169 of over 24,000 pools of mosquitoes tested for West Nile virus returned positive — meaning at least one insect in the pool carried the disease — across 25 southern Nevada ZIP codes. The number of mosquitoes recorded and the tally of positive pools this early in the season break the area’s records for both metrics, set in 2019. 

“These are huge numbers of mosquitoes, and we’ve already identified a concerning number of them carrying the West Nile virus,”  said Vivek Raman, an environmental health supervisor for the Southern Nevada Health District.

Health officials have also identified six pools in the Las Vegas area that tested positive for St. Louis encephalitis virus, a mosquito-borne disease that can cause fatal inflammation of the brain.

For decades, climate scientists and public health officials have warned that climate change  could expand the reach of various infectious diseases, especially those spread by mosquitoes. Las Vegas’ exploding mosquito population and the local uptick in West Nile prevalence offers an important case study on how climate could affect human health.

Climate change increases average global temperatures and precipitation levels, fostering conditions that are ideal for mosquitoes, which breed in still, warm water. It also extends the length of warm periods, prolonging the active season for mosquitoes. These changes increase the risk of human exposure to diseases like West Nile virus, even in places that have never recorded cases before.

The first case of West Nile virus in Las Vegas was recorded in 2004 — five years after the United States’ first case was documented in 1999 in New York City. Las Vegas’ most recent West Nile outbreak occurred five years ago, resulting in 43 human cases. District health officials are concerned that this summer could be far worse.

In Nevada and much of the Southwest, springtime weather has become warmer and summertime heat waves have grown increasingly extreme over the last few decades. Las Vegas has seen average springtime temperatures rise by 6.2 F since 1970; this month, the city has already experienced a weeklong, record-breaking heat wave. 

Southern Nevada’s rising temperatures are creating favorable conditions for mosquitoes, said Nischay Mishra, an assistant professor of epidemiology at Columbia University. What’s more, ongoing drought conditions in the state, which have led to low water table levels throughout the Colorado River Basin, including in Lake Mead, may also be counterintuitively beneficial for the insects. 

“Mosquitoes typically thrive in wet and hot places,” Mishra said. “But in Nevada, as smaller bodies of water dry up, they create shallow waters that are ideal for mosquito breeding.”

Las Vegas’ mosquito surge has been giant: Last year, district health officials measured 6,000 mosquitos in traps across Clark County from April to June. This year, counts have already exceeded 24,000. 

The vast majority have been Culex mosquitoes, a primary vector for West Nile virus.  But another mosquito species that does not carry the virus, Aedes aegypti, has also become more common in Las Vegas.  Aedes was first spotted in the area in 2017, and Raman attributes its spread there to the impacts of climate change, as well.

Along with climate, human behavior plays an important role in the spread of vector-borne diseases. Aedes and Culex mosquitoes both thrive in the backyards of many Las Vegas homes — the former breed in small pools of water such as those left from sprinklers, while the latter often breed along the surface of unmaintained swimming pools.

Raman said the best way to avoid infection is to empty any open containers filled with water outside, maintain swimming pools, wear protective clothing and use bug spray to avoid getting bit.

Louise Ivers, a professor of global health and social medicine at Harvard Medical School and the director of its Global Health Institute, said situations like the one in Las Vegas will become more common as climate change continues to boost infectious disease globally. 

“We should expect to see new infectious diseases, old infectious diseases back again and a change in the patterns of exposure of existing infectious diseases like West Nile virus,” Ivers said. “Things that we used to do freely without worrying as much about protection from vectors like mosquitoes or ticks, we might not be able to do anymore.”