Southern Nevada health officials tracking significant increase in mosquito activity • Nevada Current

Unlike common mosquitoes, which are stealthy, female yellow fever mosquitoes prefer to feed on humans and wait until nightfall to bite. Aedes aegypti, an aggressive urban mosquito commonly known as the yellow fever mosquito, was first identified in four zip codes in North Las Vegas in 2017. By 2022, the yellow fever mosquito had been found breeding in eight additional zip codes in Southern Nevada. Without coordinated management or intervention to stop the spread, urban mosquitoes have spread to 43 neighborhoods, one-third of all ZIP codes in Clark County. "This aggressive urban mosquito has spread rapidly," said Vivek Raman, environmental health supervisor for the Southern Nevada Health Department. This month, SNHD reported the highest levels of mosquito activity in the agency's 20-year monitoring history, driven in large part by an explosion of Aedes aegypti mosquitoes. As the outbreak spread, public complaints about mosquitoes skyrocketed from 99 in 2022 to more than 700 in 2023, according to SNHD. Vivek Raman, environmental health supervisor for the Southern Nevada Health Department, takes samples from a stagnant pond. (Photo courtesy of the Southern Nevada Health Department) Health officials expect mosquito infestations to be even worse this year, as the insects arrive earlier than normal: The health department has already received more than 300 mosquito complaints, up from 60 during the same period last year. It's not entirely clear why mosquito populations are exploding early in Southern Nevada, but there are some clues. High probability of flooding in recent years This is encouraging a rise in mosquito populations. About 90 traps set up by SNHD have captured more than 7,000 mosquitoes for testing in June 2023. As of June, the health department has already captured more than 26,000, about 19,000 more than last year. But even in drought conditions, yellow fever mosquitoes, which require standing water to breed, can still thrive. This is due to the peculiar reproductive habits of yellow fever mosquitoes. "Mosquitoes normally feed on birds, but Aedes aegypti prefers to feed on humans and has adapted to breed very efficiently and effectively in our yards," Raman said. Female yellow fever mosquitoes require much less water to breed than non-yellow fever mosquitoes. Aedes aegypti can lay eggs in the water equivalent of a bottle cap. Spare tires, plastic toys, discarded food containers, and saucers under flower pots are all potential sites for Aedes aegypti to lay the next generation of mosquitoes. Eggs can remain dormant for up to nine months in dry conditions. "Unlike other mosquito eggs commonly found here, they are able to remain dry and dormant in the small container until the next rain or until the container is filled with water, at which point they rehydrate and hatch," Raman said. Possible virus Yellow fever mosquitoes are not just aggressive biters, they are also capable of transmitting a number of pathogens, including the dengue and Zika viruses. Researchers at the Southern Nevada Public Health Laboratory test mosquitoes for viruses (Photo courtesy of the Southern Nevada Health Department) Locally acquired dengue and Zika infections (i.e., people do not get sick overseas) are still rare in the continental U.S. Although dengue and Zika are infectious diseases, they do not spread from person to person, only through vectors such as mosquitoes. For a mosquito to transmit the virus to another person, it must first bite an infected person. Still, several U.S. states, including neighboring Arizona and California, have reported locally transmitted cases of dengue in recent years. Last October, California health officials reported the state's first locally transmitted case of dengue in Pasadena. The first cases of community transmission could be a warning of things to come, said Louisa Messenger, an entomologist and assistant professor who studies mosquitoes and vector-borne diseases at the University of Nevada, Las Vegas. Of bigger concern in Nevada is the spread of diseases carried by common mosquitoes that can transmit West Nile virus and St. Louis encephalitis. Southern Nevada has reported two local cases of West Nile virus, another vector-borne virus, in 2023. As of June 17, SNHD has identified mosquitoes that tested positive for West Nile virus in 25 ZIP codes. Mosquitoes in another two ZIP codes have tested positive for the virus that causes St. Louis encephalitis. In their mildest form, West Nile and St. Louis encephalitis symptoms are similar to those of the common flu: fever, headache, nausea, vomiting and fatigue. But in the most severe cases, people infected with the virus develop a neuroinvasive illness that can lead to paralysis and coma. "This year is unprecedented," Messenger said, "We're seeing the highest numbers of positive mosquito swarms ever, at the very beginning of the season, which runs through around October." "We are very concerned that there may now be a significant number of mosquitoes in the environment carrying this virus, which could lead to a surge in disease transmission over the coming weeks," she continued. Climate Change The relationship between climate change and disease is complicated, and while much is still unknown about how virus-carrying mosquitoes respond to climate change, Messenger said research shows a clear link between heat and mosquitoes. Rising temperatures allow mosquitoes to grow faster and live longer. Cold winters reduce the survival rate of mosquito larvae, but warmer temperatures in the spring and fall due to climate change increase the chances of mosquito larvae surviving, lengthening the breeding season and allowing populations to grow. "Mosquitoes and other insects are extremely sensitive to small changes in temperature and humidity because their life cycle is intrinsically tied to temperature," she continued. Heavy rains from extreme weather also cause pools of water and stagnation, creating conditions for mosquitoes to breed. Last year, a series of severe storms, including Hurricane Hillary, flooded the valley, leaving behind large pools of warm, stagnant water that are perfect for mosquitoes to breed. "Then over the next few days, more rain, standing water, nice, hot weather, it creates perfect conditions for mosquitoes to breed," Messenger said. Nevada has no mosquito control program Each breeding season, SNHD sets up mosquito traps near areas where mosquitoes are expected to breed to monitor for the presence of West Nile virus and other viruses, but mosquito prevention is largely out of their control, Raman said. Despite an increased presence of mosquitoes in Southern Nevada, there is no established mosquito control program to oversee the treatment and elimination of mosquito breeding. Mosquito control programs in the United States have proven effective in reducing mosquito populations. The Centers for Disease Control and Prevention states that the only way to reduce mosquitoes and the danger they pose is to: A comprehensive, practical approachThese include monitoring, targeted use of pesticides and systematic intervention by local authorities. But establishing a mosquito control program will likely require dedicated funding and support from local governments, Raman said. "We need sustainable solutions. This isn't just a temporary fix. We do a program for a year and then we remove it. Long-term improvement requires sustainable solutions." 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