The expansion of the yellow fever mosquito (Aedes aegypti) and evidence of its establishment in the Las Vegas metropolitan area, Nevada, United States of America

Abstract

Aedes aegypti (L.) is the primary vector of several arboviruses, including chikungunya, dengue, Zika, and yellow fever viruses. While models have predicted a low likelihood for establishment of Ae. aegypti in the arid climate of the Las Vegas Metropolitan (Statistical) Area, United States, surveillance indicates it is established. Since its initial detection in 2017, its distribution has spread over the Las Vegas metro area, which has low precipitation and hot summers. We mapped the spread of Ae. aegypti over 5 yr using adult catches. A total of 407 census tracts in metro Las Vegas were surveyed at least once for a total of 8,895 trap nights. In 2017, 5 of 117 (4%) surveyed census tracts were positive. By 2023, the number of positive census tracts increased to 147 of 276 (53%). Our study highlights the need for effective vector control measures and public health strategies to mitigate the growing risk of vector-borne diseases in Las Vegas and other urban areas experiencing similar introductions of Ae. aegypti.

Introduction

Aedes (Ae.) aegypti (L.) is the primary vector of arboviruses that cause chikungunya, dengue, Zika, and yellow fever diseases in humans. Predicting where arboviruses vectored by Ae. aegypti will cause human disease, especially at the margins of the known distribution of Ae. aegypti, is complex (Olson et al. 2021). However, the establishment of Ae. aegypti increases the risk of local transmission of arboviruses, which can lead to morbidity among the local population. For example, Florida experienced 437 locally acquired dengue cases between 2010 and 2023 (Ortiz et al. 2025) and 94 local cases in 2024 (Morrison et al. 2025). In the Americas, dengue cases surged to 13 million in 2024 (Pan American Health Organization 2025), surpassing the previous 2023 record of 4.1 million cases and 3.1 million cases in 2019 (Pan American Health Organization 2024).

Aedes aegypti populations expanded their distribution within the Las Vegas–Henderson–Paradise Metropolitan Statistical Area, NV, United States (a.k.a. the Las Vegas metropolitan area) since its introduction in 2017 (Pless and Raman 2018). The most likely source of introduction was the Southwest United States, with Southern California serving as an intermediary stop (Pless and Raman 2018). In California, Ae. aegypti reached the northern cities of Anderson and Redding (Shasta County) (California Department of Public Health 2023). Further north, it was detected in Oregon in 2024 (Oregon Housing Authority 2024, Jackson County Vector Control District 2025). To the east of Nevada in Utah, Ae. aegypti was detected in Washington County in 2013 (Meiners 2021) and 2024 (Southwest Mosquito Abatement District 2024), and in neighboring Moab, in 2019 (Gloria-Soria et al. 2022) and 2022 (Amsberry 2022). Further east, in Colorado, introductions were observed in La Junta in 2017 (Ostrum and Mutebi 2019) and Grand Junction in 2024 (McKee 2024, Mesa County Public Health 2024). This range expansion highlights Ae. aegypti’s ability to establish populations not only at higher altitudes (Lozano-Fuentes et al. 2012) but also at higher latitudes, ie closer to the poles.

At a lower altitude—approximately 475 km southeast of Las Vegas—the city of Phoenix, Arizona, has been monitored through monthly vector surveillance since 1969. These records document consistent detectable reintroductions of Aedes aegypti as early as 1994 (Engelthaler et al. 1997). The species has been well established in Maricopa County (which includes Phoenix) since the early 2010s, with year-round detections reported from 2014 onward (Holeva-Eklund et al. 2022, Coalson et al. 2023). Because both altitude and latitude serve as proxies for temperature, this further underscores the crucial role of temperature in the mosquito’s biology (Eisen et al. 2014).

The Las Vegas metropolitan area is characterized as a hot desert (Köppen climate classification: BWh) (Beck et al. 2018), with long, harsh summers and short winters. The metropolitan area has an average of 310 sunny days annually. Summer temperatures frequently exceed 38°C (100°F), with nighttime lows often remaining above 27°C (80°F). Winters have daytime highs typically ranging from 13–18°C (55–65°F) and occasional cold spells that can bring temperatures near freezing, 4–7°C (39–44°F). Precipitation averages less than 127 mm (5 inches) per year, mostly occurring during the winter and during the North American Monsoon season in late summer, which can bring brief but intense thunderstorms (National Oceanic Atmospheric Administration 2022, Runkle et al. 2022). Given these conditions, it is possible for Ae. aegypti for a percentage of eggs to survive low temperatures (Kramer et al. 2020). This study aims to document the establishment and expansion of Ae. aegypti in Las Vegas and identify the factors that facilitated its persistence.

Materials and Methods

From 2017 to 2023, mosquitoes were trapped all through metropolitan Las Vegas based on nuisance reports from residents to the Southern Nevada Health District (SNHD). Additionally, systematic sampling was conducted from 2017 to 2022 in subdivisions surrounding the initial discovery site. Notably, a nuisance report was instrumental in the original detection of Ae. aegypti in 2017. The systematic collections were conducted in 2 subdivisions from April through September (each year from 2017 to 2022) to establish a baseline for future mosquito treatment evaluation; the subdivisions were in census tracts (CT) FIPS: 32003003609 and 32003003646. Local city policies and the COVID-19 pandemic prevented planned control activities, and Ae. aegypti control measures were not implemented.

Mosquitoes were collected using BG-Sentinel 2 traps (BGS) (Biogents AG, Regensburg, Germany) equipped with a BG-Lure (Biogents AG, Regensburg, Germany), and CO₂ was dispensed at a rate of 250 to 350 ml/min.

We estimated Ae. aegypti presence as the percentage of positive collections by dividing the number of collections that captured Ae. aegypti by the total number of traps deployed within a CT each year. Relative abundance was estimated as the mean number of female Ae. aegypti per trap per week (μ), derived from a negative binomial distribution. The negative binomial model was chosen to account for statistical overdispersion in the counts, where the variance exceeds the mean, a common characteristic in biological count data. The estimated parameter μ represents the expected number of mosquitoes per trap and provides a robust measure of the relative abundance. The 95% confidence intervals were drawn with a bootstrapping procedure. We used the fitdistrplus package (version 1.1.11) (Marie Laure and Christophe 2015) to determine the most likely estimate of μ. The uncertainty of μ was assessed with 10,000 bootstrap iterations with the “boot” library (version 1.36) (Davison and Hinkley 1997).

Temperature and precipitation records were taken from the North Las Vegas airport (GHCND: USW00053123), which was closest to the main sampling area. Long-term data were obtained from the National Oceanic and Atmospheric Administration (NOAA) (Menne et al. 2012a, 2012b).

Results

A total of 8,895 trap nights were conducted in 407 CT in Metro Las Vegas starting in 2017. From these, baseline surveillance consisted of 4,498 trap nights, corresponding to 51% of all collections. The last baseline surveillance collection was conducted in 2022; subsequently, the baseline areas were only visited as a response to mosquito nuisance calls.

Presence Estimation

Figure 1 illustrates the change in presence of Ae. aegypti from 2017 to 2023. In 2017, only 5 of 117 (4%) sampled CTs were positive. By 2023 the number of positive CTs surged to 147 of 276 (53%). The increase in presence was progressive in the intervening years: 2018 (2/141), 2019 (3/186), 2020 (4/175), 2021 (9/143), and 2022 (25/167). The number of collections and number of positive at each CT, by year, is presented in Supplementary Table S1.

Figure 1.

Spatial distribution of Aedes aegypti in the Las Vegas metropolitan area, 2017 to 2023. Only positive CTs are included.

Relative Abundance Estimation

In Fig. 2A, each data point represents a week in which relative abundance (μ) was successfully estimated using the Maximum Likelihood Estimate (MLE) method. However, in certain weeks, the estimation process failed, likely because of small sample size, irregular patterns, or the presence of outliers that prevented the convergence of the fitting algorithm on a reliable μ value. Consequently, these weeks are not included in the figure and create occasional gaps in the temporal sequence of the visualization.

Figure 2.

Abundance, precipitation, and temperature for Las Vegas, 2017 to 2023. A) Relative weekly abundance (μ), error bars denote bootstrapped 95% CI of μ. B) Daily accumulated precipitation, red dotted line is a 30-yr average. C) Monthly average maximum and minimum temperatures. Error bars are the 95% confidence intervals (mean ± 1.96 SD).

Despite these omissions, the visualized data revealed the relative abundance of Ae. aegypti varied significantly across years, with a notable increase observed in 2023 (Fig. 2A). In 2017, abundance remained consistently low throughout the sampling period (black triangles), with no clear seasonal trend. In contrast, both 2022 (blue squares) and 2023 (red circles) demonstrated a marked increase in mosquito abundance, particularly from late August to October. The peak abundance in 2023 was substantially higher than in previous years, with several weekly estimates exceeding a relative abundance of 15 mosquitoes per trap. The intervening years (2018 to 2021, gray points) exhibited lower levels of abundance with a small increase from year to year.

Weather

Precipitation

In Fig. 2B we present the daily accumulated precipitation for the 2017 to 2023 period (blue lines) and the 30-yr average (red line). NOAA does not provide confidence intervals for this metric, but 2 seasonal periods of precipitation are marked by sharp curve changes. The first period was during January to March and the second during August to September. The dry period starts around late March and ends with rains in August. The years 2017 to 2021 are presented as dashed lines, while 2022 to 2023 are presented as continuous lines.

The year 2022 was drier than expected, with no meaningful rain events early in the year and summer rains not large enough to reach the expected accumulation. In contrast, in 2023 remnants of Hurricane Hilary resulted in heavy rains and likely drove an increase in the number of mosquito-biting complaints. Of note, in 2017 Las Vegas received no precipitation beyond the early rains (flat line); 2022 was the driest start of the year; and 2019 was the wettest year with a total rainfall of 175 mm (7 in) during the 2017 to 2023 period.

Temperature

Figure 2C depicts the variation in average monthly temperatures from January 2017 through December 2023. The observed average monthly maximum temperature (red) and the average minimums (blue) are plotted with error bars that indicate the 95% confidence interval for the respective minimum and maximum temperatures. The CIs were estimated from 30-yr of observations and provided by NOAA.

Most of the observed temperatures fall within the expected normal range—with a 95% CI, it would be expected that 5% of the observations would fall outside the CI range. However, notable exceptions exist, particularly in the hottest (July and August) and coldest months (January and December). During July and August, maximum temperatures exceeded the normal range in half of the observed years, with July 2023 being exceptionally hot where the monthly maximum reached 43°C (109°F); the maximum temperature for August 2022 and 2023 remained within the expected range. Every recorded minimum temperature for July exceeded the expected normal range. In the winter months, while the maximum temperatures generally remained within the normal range, the minimum temperatures frequently exceeded it, indicating milder winter conditions; the minimum temperatures for January and December 2023 were above the expected range, reaching 4°C (39°F) and 7°C (45°F) respectively. The figure illustrates that while the temperature trends for North Las Vegas mostly align with historical averages, a tendency exists towards higher minimum temperatures during summer and winter months.

Discussion

Our observations demonstrate that Ae. aegypti established itself in the Las Vegas metropolitan area despite the harsh, hot, dry summer weather conditions. The results highlight a progressive increase in the number of census tracts where Ae. aegypti was present, with minimal detections in 2017 and widespread distribution by 2023. The establishment is of concern because Ae. aegypti is a highly competent vector of several arboviruses (Facchinelli et al. 2023), and its increased distribution raises the risk of arbovirus transmission if viruses, including chikungunya, dengue, yellow fever, and Zika, are introduced to the Las Vegas area.

The low detections of Ae. aegypti from 2017 through 2021 can likely be considered real absences because SNHD visited many sites (see Supplementary Table S1 for details). However, 2023 was different because of the heavy rains brought by Hurricane Hilary. Las Vegas residents were likely more aware of mosquitoes occurring in standing water because of news coverage of the weather event. It is difficult to determine whether reports of biting mosquitoes were caused by actual increases in mosquito populations associated with the rain or simply a result of heightened awareness from media coverage. However, in 2024, the presence of Ae. aegypti remained largely unchanged (data not shown), and SNHD received fewer calls, likely because the year was drier (personal comm.).

Since the initial detections of Ae. aegypti, SNHD personnel have placed ovitraps and searched for larval habitats (data not shown). In 2017, SNHD personnel found larvae (eclosed to adult) in an abandoned, covered spa holding rainwater. All ovitraps have been negative, and it was not until 2023 that another larval habitat was found in a stormwater pipe in an apartment complex. The likely reason for negative ovitraps is that small amounts of water get sufficiently hot to prevent the development of larva, and high temperatures deter oviposition (Day 2016). The optimal water temperature for larval survival is approximately between 24 and 34°C. Above this range, mortality increases rapidly, and around 38°C mortality is 100% (Eisen et al. 2014). It is important to recognize that within a temperature range, increases in temperature can lead to increased oviposition (do Nascimento et al. 2022), but how larval sites are heated is a complex interaction of many abiotic factors and not just air temperature (Steinhoff et al. 2016).

Although some modeling studies did not project Ae. aegypti establishment in the Las Vegas metropolitan area, giving Clark County a low probability of presence (Johnson et al. 2017, Monaghan et al. 2019), our findings indicate that this species has adapted to the Las Vegas conditions. Another example (Kraemer et al. 2019) projected a suitability of approximately 50%, but not until 2050. Our data indicate that Ae. aegypti has successfully overcome the assumptions made by all 3 models (Johnson et al. 2017, Kraemer et al. 2019, and Monaghan et al. 2019).

The abundance estimation should be interpreted with caution, as most sampling was opportunistic, relying on resident reports of mosquito nuisance. However, 2023 showed a similar seasonality observed in other areas where Ae. aegypti is established (Coalson et al. 2023, Morreale et al. 2024), with the populations peaking in the late summer.

Future studies incorporating systematic rather than opportunistic sampling would provide a more precise representation of seasonal dynamics and environmental drivers of Ae. aegypti populations in Las Vegas.