Sterile Mosquito Releases Show Promising Results in Curbing Dengue Fever in Southern California

Officials in Southern California are fighting an uphill battle against the invasive Aedes aegypti mosquito, which has caused a recent surge in the local spread of dengue fever. To tackle this issue, two vector control districts have launched programs to release thousands of sterile male mosquitoes in targeted neighborhoods. These sterile males mate with wild females, but the resulting eggs do not hatch, thus reducing the overall mosquito population. The West Valley Mosquito and Vector Control District, serving a significant portion of Los Angeles County, initiated its pilot program in 2023. The district saw nearly an 82% reduction in invasive Aedes aegypti mosquitoes in the Sunland-Tujunga area compared to a control area. Following the initial success, the district expanded its efforts in 2024, increasing the number of sites and the frequency of releases. By focusing on hot spots—areas with particularly high mosquito counts—the district was able to achieve substantial reductions with a limited budget of about $4 million. Similarly, the Greater Los Angeles County Vector Control District began its sterile mosquito release program in 2024, targeting two neighborhoods in Sunland-Tujunga. The district unleashed an average of 30,000 sterile males per week from May to October, aiming to outnumber wild males by a 10-to-1 ratio. Preliminary results showed an average decrease of 44% in mosquito counts across several heavily infested areas, marking the first decline in roughly eight years. These reductions have translated into fewer complaints from residents, indicating that the program is making a tangible difference. Brian Reisinger, community outreach coordinator for the West Valley district, noted a notable decrease in public disturbances and calls. The program also helps mitigate the risk of other mosquito-borne diseases such as yellow fever, Zika, and chikungunya, which Aedes aegypti can carry. However, scaling this intervention to cover larger areas poses significant challenges. The Greater Los Angeles County district, which has a budget of nearly $24 million and serves nearly 6 million residents, plans to expand its program but requires additional funding. Officials have proposed raising the property tax assessment by up to $20 annually per single-family home, in addition to the existing $18.97 fee. If approved by a sampling of property owners, the increase will be put to a vote in the fall, as mandated by Proposition 218. The effectiveness of the sterile mosquito program has led to increased interest from other districts. An Orange County district will begin releasing 100,000 to 200,000 sterile male mosquitoes weekly in Mission Viejo from late May to November. The Coachella Valley district is also developing its own program, expected to launch next spring. Despite the program’s successes, the threat remains significant. California experienced 18 locally acquired dengue cases in 2023, with 14 occurring in Los Angeles County. Of these, at least seven cases were linked to an outbreak in Baldwin Park, and others emerged in Panorama City, El Monte, and the Hollywood Hills. Dengue can cause severe symptoms, including high fever and body aches, and may even be fatal in rare cases. Over a third of the cases in L.A. County required hospitalization, according to Dr. Aiman Halai, director of the vector-borne disease unit at the L.A. County Department of Public Health. The situation is exacerbated by climate change, which allows Aedes mosquitoes to thrive in new areas and intensifies their activity in established regions. Shorter winters have extended the mosquito season, making it more challenging to control their populations. For instance, mosquitoes now emerge as early as March and can persist into December, compared to the traditional May to October cycle. New technologies, such as mosquitoes infected with Wolbachia bacteria, are being explored to further enhance control efforts. Wolbachia-infected males produce eggs that do not hatch, similar to the sterile male mosquitoes. However, global dengue cases reached unprecedented levels in 2023, with over 13 million reported in the Americas and the Caribbean. High case numbers in many countries mean that travelers are more likely to bring the virus back, posing an ongoing threat to local communities. Industry insiders laud the sterile mosquito program for its environmental benefits, noting that it avoids the need for chemical pesticides and can be adapted to target other disease-carrying mosquitoes like the native Culex species, which spreads the West Nile virus. While the approach shows promise, sustained efforts and additional funding will be crucial to achieve broader impact and prevent the establishment of a permanent dengue presence in Southern California. The success of these programs, however, demonstrates a significant step forward in managing mosquito populations and mitigating the spread of dangerous diseases.