2024 mosquito-borne diseases summary report
Utah Department of Health and Human Services
Report purpose
This document gives Utah partners a concise summary of this season’s major mosquito-borne disease results. Information displayed in this report was compiled by the Utah Department of Health and Human Services (DHHS), but reflects information obtained from joint efforts. All activities related to the mosquito-borne diseases seen in Utah during the 2024 season involved major contributions from many different agencies. These include: Utah blood banks, local health departments (LHDs), Utah Department of Agriculture and Food (UDAF), Utah Division of Wildlife Resources (UDWR), Utah Mosquito Abatement Association (UMAA), the Utah Public Health Laboratory (UPHL), and the Utah Veterinary Diagnostic Laboratory (UVDL). In addition to the direct contribution of surveillance data, these entities were also involved in systematic planning and preparation for the 2024 season. This report documents the results of the efforts made by these entities during the 2024 season. Visit our disease-specific webpages (https://epi.utah.gov/atozlist/) to learn more information about each disease described in this report.
NOTE: This report describes the general trends that occurred during the 2024 season. Specific surveillance counts may be subject to change as data are reconciled.
Introduction
This report historically focused on each season’s West Nile virus results, but was expanded in 2023 to incorporate other mosquito-borne diseases for which Utah conducts surveillance. Since West Nile is the most common mosquito-borne disease in Utah, the majority of this report will cover this virus. Utah Department of Health and Human Services (DHHS) conducts surveillance for endemic and travel-associated mosquito-borne diseases. Mosquito-borne diseases considered endemic in Utah include West Nile virus (WNV) and St. Louis Encephalitis (SLE). Travel-associated diseases Utah DHHS monitors include chikungunya, dengue, malaria, and Zika. Public health investigations of travel-associated mosquito-borne diseases are important to determine the locations of exposure and help us monitor any potential locally transmitted events associated with these diseases. Malaria is transmitted by the Anopheles mosquito, which has been detected in almost every county in Utah since the establishment of Utah’s mosquito abatement districts (MADs) beginning in 1924. Chikungunya, dengue, and Zika are all transmitted by the Aedes aegypti mosquito, which has been detected near St. George and Moab. Both of these mosquito species are detected in low numbers, and the risk of locally acquired transmission of these diseases is small. However, due to the recent reports of local transmission of dengue in Arizona (2022) and California (2023) and malaria in Florida, Texas, Maryland, and Arkansas (2023), Utah DHHS has enhanced surveillance and monitoring of all reported cases of these mosquito-borne diseases for proactive awareness and response. SLE is transmitted to humans from the bite of an infected Culex species mosquito. A SLE human case has not been reported since 2016. However, mosquito pools have been positive for SLE in 2022, 2023, and 2024. WNV is also transmitted by the bite of an infected Culex mosquito. Birds are the natural host of the disease with humans and horses serving as accidental hosts. WNV is the most common disease acquired in Utah with a 10-year average reported human case count of 16. The majority of people infected with WNV never develop symptoms. A small percentage of infected individuals will display West Nile fever symptoms (fever, headache, and body aches). A more serious form of the disease, West Nile neuroinvasive illness, may also occur when the virus infects the central nervous system. People with this form of the disease experience high fever, severe headache, neck stiffness, and mental confusion. Hospitalization is often required and death is possible. This type of infection can also cause long-lasting neurological symptoms. WNV is the most common mosquito-borne disease found in Utah. The remainder of this report will primarily focus on WNV.
2024 season national highlights
West Nile virus case maps present data reported to the Centers for Disease Control and Prevention’s (CDC) ArboNET surveillance system by state and local health departments. Figure 1 shows the WNV human reported cases by state of residence for 2024 with shading, ranging from 1–5, 6–8, 9–25, and >25 cases.
Figure 1:
For 2024, there were 1,791 human cases reported to CDC. Of those, 1,335 (74.5%) were reported as West Nile meningitis or encephalitis (neuroinvasive disease) and 456 (25.5%) were reported as West Nile fever (milder disease). In comparison, 2,406 human cases were reported to CDC in 2023, with 1,599 (67%) reported as West Nile meningitis or encephalitis (neuroinvasive disease), and 807 (33%) reported as West Nile fever (milder disease). There were a total of 161 fatalities reported in 2024, compared with 162 fatalities reported in 2023.
WNV surveillance in Utah
West Nile virus activity surveillance involves several different components. Since the disease is zoonotic in nature, we track activity in both humans and animals. In past years, WNV surveillance in Utah involved various populations including humans, mosquitoes, wild birds, horses, and sentinel chickens. Due to the involvement of these different populations, surveillance efforts this season enlisted the expertise and abilities of many different departments. Because of budget constraints, surveillance for the 2024 season was limited. Wild bird testing, sentinel chicken testing, and official coordinated equine testing efforts at UDAF were eliminated from routine surveillance to make sure the most critical systems were maintained. Necessary trapping and identification for mosquito surveillance was performed by local mosquito abatement districts (MADs) and tribal abatement districts in conjunction with the Utah Mosquito Abatement Association. Confirmatory testing for mosquito pools was conducted at UPHL. Healthcare providers across the state submitted human samples to both UPHL and private laboratories such as Associated Regional and University Pathologists (ARUP). The three major blood banks serving Utah (American Red Cross, ARUP, and Mountain Star) coordinated the screening of donated blood to identify any viremic donors. All LHDs in Utah were involved with disseminating, investigating, and responding to surveillance data indicative of local WNV activity.
2024 season highlights—Utah
Activity during the 2024 WNV season started later than the 5-year average, with the first positive mosquito pool reported at the end of July. Mosquito activity occurred in the highest numbers in Salt Lake, Davis County, and Bear River health jurisdictions. Nine counties reported WNV activity during the 2024 season. Only PCR-positive mosquito pools were included in surveillance data. Individual Rapid Analyte Measurement Platform (RAMP) tests without PCR confirmation were not included. Many mosquito abatement districts (MADs) now conduct their own PCR testing at their respective locations. Surveillance data from MADs whose testing was validated by UPHL are included in DHHS surveillance data throughout the mosquito season.
| Jurisdiction | Human | Horse | Positive mosquito pools | Mosquito pools tested | Mosquito pools % positive |
|---|---|---|---|---|---|
| Bear River | 5 | 2 | 36 | 534 | 6.74% |
| Central | 0 | 1 | 5 | 227 | 2.2% |
| Davis | 3 | 0 | 99 | 4199 | 2.36% |
| Salt Lake | 0 | 0 | 123 | 3459 | 3.56% |
| Tooele | 0 | 2 | 3 | 167 | 1.8% |
| TriCounty | 0 | 0 | 19 | 265 | 7.17% |
| Utah County | 3 | 6 | 5 | 787 | 0.64% |
| Weber Morgan | 3 | 5 | 29 | NA* | NA* |
| San Juan | 0 | 0 | 0 | 0 | 0% |
| Southeast | 0 | 0 | 0 | 138 | 0% |
| Southwest | 0 | 0 | 0 | 0 | 0% |
| Summit | 0 | 0 | 0 | 56 | 0% |
| Wasatch | 0 | 0 | 0 | 0 | 0% |
| State total | 14 | 16 | 319 | 9861 | 3.23% |
| * ‘Mosquito pools tested’ value for this jurisdiction is unavailable |
| Age group | Total | Death | Neuroinvasive |
|---|---|---|---|
| <18 | 1 | 0 | 1 |
| 18-39 | 4 | 0 | 4 |
| 40-64 | 5 | 0 | 5 |
| >65 | 4 | 1 | 4 |
| State total | 14 | 1 | 14 |
Past-seasons comparison
WNV activity was first detected in Utah in 2003. Similar to initial seasons in many other states, detection was muted. Only one human case was reported during the 2003 season in Utah, in addition to an asymptomatic viremic donor. The main indication of WNV illness in Utah in 2003 was in horses. That changed in 2004 as WNV became firmly established with multiple detections in northern Utah along the Wasatch Front. During 2005, detection expanded into more northern regions of the state as Utah and Uintah counties served as focal points for detected transmission. Thus far, the 2006 season was the most active season Utah has experienced. During that season, WNV detection was focused along the Wasatch Front, particularly in Salt Lake and Utah counties. Along with an increase in transmission came an increase in fatalities, with five deaths reported. In 2007, the number of cases (as well as the number of deaths) began to decline. During the 2007 season, the virus moved farther north with most cases reported in Cache and Box Elder counties. WNV detection continued to decrease in Utah between 2008 and 2011. Due to inconsistencies with RAMP testing, mosquito pools were counted after they were confirmed by PCR. This led to a decrease in the number of positive mosquito pools detected throughout the state. The southwestern portion of Utah saw the most animal (mosquito) activity for the 2010–2012 seasons. In 2013, Washington County, in southwest Utah, saw the majority of human and animal WNV cases. During 2014–2016, WNV transmission was centered mostly along the Wasatch Front, but was also seen in Box Elder, Grand, and Weber counties. In 2017, WNV detection dramatically increased in Utah, with the largest number of human and animal infections, along with the greatest number of positive mosquito pools, in more than 10 years.
| 2014 | 2015 | 2016 | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | 2023 | 2024 | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Human | 2 | 8 | 13 | 62 | 11 | 21 | 2 | 28 | 5 | 8 | 14 |
| Horse | 4 | 4 | 7 | 35 | 6 | 8 | 3 | 12 | 7 | 16 | 16 |
| Mosquito pools | 167 | 281 | 244 | 443 | 180 | 272 | 44 | 654 | 158 | 300 | 319 |
| Positivity rate for pools | 2.18% | 4.15% | 4.24% | 8.25% | 3.58% | 4.19% | 1.24% | 7.07% | 1.76% | 2.5% | 3.23% |
| Bird* | 2 |
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3 | 4 | 1 |
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1 | 0 |
| Chicken* | 1 | 5 |
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1 | 2 |
| Counties with detection | 9 | 8 | 8 | 17 | 16 | 20 | 8 | 11 | 9 | 15 | 9 |
| * Wild bird and sentinel chickens were not part of Utah’s surveillance in 2011–2013. However, a large eared grebe and bald eagle die-off in October 2013-January 2014 led to an increase in testing. The die-off was the result of WNV infection. |
Figure 2. WNV positive cases, Utah, 2014-2024
2024 Utah activity timeline
In 2024, WNV activity appeared in Utah at the end of July with our first WNV positive mosquito pool reported in Salt Lake County on July 27, 2024. The first human case was reported about a month later from Utah County Health Jurisdiction August 22,2024 (as shown in Figure 3). Activity was reported throughout the summer and into October, with WNV activity detected in horses, birds, humans, and mosquitoes by late August. Active surveillance for the 2024 season stopped in mid-October with the last human case reported on October 10,2024 and the last positive mosquito pool reported out of Davis County on October 12, 2024. However, we continued to test suspect human and horse cases throughout the year. Average counts for comparison were calculated from human case data reported between the years 2019 and 2023.
Figure 3. WNV human epidemiologic curve by week, Utah
2024
Human surveillance
Human surveillance relies primarily on major laboratories that report results that indicate acute infection. LHDs are immediately notified of positive lab results and conduct case investigations. During the 2024 season, specimens were sent to CDC for confirmation for the first human case, fatal suspect cases, and abnormal test results. Utah’s major blood banks also screened donations for the presence of WNV and reported positive results to DHHS. Seven blood donation screening tests were positive for WNV in 2024. In 2024, 14 WNV cases were reported, and all 14 cases were neuroinvasive (as shown in Table 4). Table 5 compares Utah human WNV cases reported in 2024 to previous years, and table 6 compares Utah 2024 human WNV case counts to human WNV cases from other mountain west states.
| Utah | US | |
|---|---|---|
| Number of cases | 14 | 1466 |
| Fatalities (% fatal) | 1 (7.14%) | 115 (7.84%) |
| Neuroinvasive disease (% neuroinvasive) | 14 (100%) | 1,063 (72.5%) |
| 2014 | 2015 | 2016 | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | 2023 | 2024 | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Number of cases | 2 | 8 | 13 | 62 | 11 | 21 | 2 | 28 | 5 | 8 | 14 |
| Fatalities | 0 | 0 | 1 | 5 | 1 | 2 | 0 | 2 | 0 | 1 | 1 |
| Percent male | 100% | 75% | 54% | 70% | 64% | 67% | 100% | 64% | 60% | 75% | 79% |
| Median age (years) | NA* | 55 | 60 | 61 | 49 | 57 | NA* | 58 | 69 | 68 | 51.5 |
| Age range (years) | NA* | 18-90 | 18-90 | 14-86 | 32-88 | <18-92 | NA* | <18-79 | 63-74 | 21-87 | 21-89 |
| * Not available: data suppressed due to small number of reported cases in this year |
| State | Neuroinvasive | Nonneuroinvasive | Total | Deaths |
|---|---|---|---|---|
| Arizona | 20 (66.7%) | 10 (33.3%) | 30 | 2 (6.7%) |
| Colorado | 38 (50.7%) | 37 (49.3%) | 75 | 4 (5.3%) |
| Idaho | 4 (66.7%) | 2 (33.3%) | 6 | 0 (0.0%) |
| Montana | 2 (66.7%)) | 1 (33.3%) | 3 | 0 (0.0%) |
| New Mexico | 23 (88.5%) | 3 (11.5%) | 26 | 3 (11.5%) |
| Utah | 14 (100.0%) | 0 (0.0%) | 14 | 1 (7.1%) |
| Wyoming | 1 (50.0%) | 1 (50.0%) | 2 | 1 (50.0%) |
Other mosquito-borne disease human case summary
In 2024, Utah saw elevated case counts for both dengue and malaria. There were 25 reported dengue cases in 2024 and 32 reported malaria cases. No chikungunya, SLE, or Zika cases were reported in 2024. Table 7 summarizes the case counts and 10-year average for chikungunya, dengue, malaria, SLE, and Zika between 2014 and 2024.
| 2014 | 2015 | 2016 | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | 2023 | 2024 | Average | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Chikungunya | 7 | 5 | 1 | 0 | 1 | 1 | 0 | 0 | 2 | 2 | 0 | 1.9 |
| Dengue | 3 | 1 | 8 | 6 | 6 | 10 | 3 | 2 | 4 | 11 | 25 | 5.4 |
| Malaria | 4 | 8 | 22 | 10 | 10 | 10 | 4 | 3 | 12 | 24 | 32 | 10.7 |
| St. Louis Encephalitis | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0.1 |
| Zika | 0 | 1 | 29 | 9 | 11 | 9 | 2 | 0 | 0 | 1 | 0 | 6.3 |
| 2024 case counts have not been finalized at the time of posting this report. |
Mosquito surveillance
Local and tribal mosquito abatement districts (MADs) across the state perform the primary function of trapping mosquitoes at various locations throughout Utah. Trapped mosquitoes are identified and sorted into “pools” of 50–100 mosquitoes based on species. Some MADs conduct their own PCR testing which is verified by a UPHL proficiency panel. Other MADs ship their mosquito pools to UPHL for PCR testing. All PCR tests are reported to DHHS and included in surveillance measures. RAMP tests without PCR confirmation are also reported to DHHS; however, they are not included in weekly surveillance measures.
Horse surveillance
Equine disease surveillance related to WNV infection in Utah is coordinated by the UDAF. Veterinarians across the state are encouraged to submit samples from suspect equine cases to the UVDL-Logan for testing. UDAF reports serum test results and appropriate notification for positive cases to DHHS. The majority of samples submitted for testing are from domestic, privately owned horses with symptoms indicative of infection and no history of vaccination. Pamphlets and periodic updates are sent through the Utah Veterinary Alert Listserv to increase disease awareness among veterinarians and horse owners. UDAF also maintains an interactive map which shows positive equine cases across the state. https://www.arcgis.com/home/webmap/viewer.html?webmap=a5a404efda6b43e4b265c2cebe5bdeee&extent=-119.0804,35.0043,-101.744,43.0138
Wild bird surveillance
Due to budget constraints, active wild bird surveillance has not been conducted since 2013 and was not conducted in 2024. However, UDWR reports any wild bird identified through passive surveillance that tests positive for WNV throughout the season.
Sentinel chicken surveillance
Due to budget constraints, routine sentinel chicken surveillance has not been conducted since 2013 and was not conducted in 2024. However, a few mosquito abatement districts continue to perform their own surveillance efforts and provide positive data to DHHS.
For questions about this report, email Kacy Nowak at onehealth@utah.gov.