Report purpose

This document provides Utah partners with a concise summary of this season’s major West Nile virus (WNV) results. Information displayed in this report has been compiled by the Utah Department of Health and Human Services (DHHS), but reflects information obtained from joint efforts. All activities related to WNV during the 2021 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 agencies were also involved in systematic planning and preparation for the 2021 season. The intent of this report is to document the results of the efforts put forth by these entities during the 2021 WNV season.

NOTE: This report describes the general trends that occurred during the 2021 season. Specific surveillance counts may be subject to change as data are reconciled. Data collection from this year may have been affected due to the COVID-19 pandemic.

Introduction

During the summer of 2021, WNV reemerged in Utah at above average levels when compared with the previous five years. WNV is transmitted by mosquitoes. Birds are the natural host of the disease with humans and horses serving as accidental hosts. The majority of people infected with WNV never develop symptoms. A small percentage of infected individuals will display West Nile fever symptoms (i.e., 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.

2021 season national highlights

West Nile virus neuroinvasive disease incidence 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 rate of human neuroinvasive disease (e.g., meningitis, encephalitis, or acute flaccid paralysis) by state for 2021 with shading, ranging from 0.01‐0.24, 0.25‐0.49, 0.50‐0.99, and greater than 1.00 per 100,000 population.

Figure 1:

The following states reported neuroinvasive disease cases to ArboNET in 2021: Alabama, Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, District of Columbia, Florida, Georgia, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, Virginia, Washington, and Wisconsin.

For 2021, there were 2,911 human cases reported to CDC. Of those, 2,008 (69%) were reported as West Nile meningitis or encephalitis (neuroinvasive disease) and 903 (31%) were reported as West Nile fever (milder disease). In comparison, 731 human cases were reported to CDC in 2020, with 559 (76%) reported as West Nile meningitis or encephalitis (neuroinvasive disease), and 172 (24%) reported as West Nile fever (milder disease). There were a total of 227 fatalities reported in 2021, compared with 66 fatalities reported in 2020.

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 2021 season was limited. Wild bird testing, sentinel chicken testing, and official coordinated equine testing efforts at UDAF were eliminated from routine surveillance in order to ensure the most critical surveillance 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. Health-care 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.

2021 season highlights—Utah

As shown in Figure 3, activity during the 2021 WNV season in Utah was above the five-year average for human and animal infection. Mosquito activity occurred in the highest numbers in Salt Lake, Davis, Bear River, and Weber-Morgan jurisdictions. 11 counties reported WNV activity during the 2021 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) have started conducting 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.

Table 1. WNV activity, Utah 2021 (positive counts only)
Jurisdiction Human Horse Positive mosquito pools Mosquito pools tested Mosquito pools % positive
Bear River 0 2 17 520 3.27%
Central 0 0 1 154 0.65%
Davis 9 0 301 2783 10.82%
Salt Lake 11 1 304 2784 10.92%
Southwest 2 0 0 655 0%
Summit 0 0 2 143 1.4%
Tooele 0 0 5 264 1.89%
Utah County 0 1 1 652 0.15%
Weber Morgan 6 8 23 941 2.44%
San Juan 0 0 0 0 0%
Southeast 0 0 0 107 0%
TriCounty 0 0 0 245 0%
Wasatch 0 0 0 0 0%
State total 28 12 654 9248 7.07%
Table 2. Human cases of WNV, Utah 2021
Age group Total Death Neuroinvasive
<18 1 0 1
18-39 2 0 1
40-64 18 2 13
>65 7 1 6
State total 28 3 21

Past season 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 detected 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. 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. From 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.

Table 3. Ten-year WNV season comparison, Utah 2011-2021
2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
Human 3 5 7 2 8 13 62 11 21 2 28
Horse 1 3 7 4 4 7 35 6 8 3 12
Positive mosquito pools 25 21 69 167 281 244 443 180 272 44 654
Positivity rate for pools 0.38% 0.31% 0.89% 2.18% 4.15% 4.24% 8.25% 3.58% 4.19% 1.24% 7.07%
Bird* 0 0 40 2
3 4 1
Chicken* 0 1 2 1 5
Counties with detection 6 8 9 9 8 8 17 16 20 8 11
* 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. It was found the die-off was the result of WNV infection.

Figure 2. WNV positive cases, Utah, 2011-2021

2021 Utah activity timeline

In 2021, WNV activity appeared in Utah at the end of June. Mosquito activity was first detected in mosquito pools during the week of June 27, 2021, in Box Elder, Salt Lake, Summit, Tooele, and Uintah counties. Activity was detected throughout the summer and into October, with WNV activity detected in horses, humans, and mosquitoes by the end of July. Utah’s first human case was reported on July 1, 2021, as shown in Figure 4. Active surveillance for the 2021 season stopped in the middle of October. However, we continue to test suspect human and horse cases throughout the year. Average counts for comparison were calculated from human case data reported between the years 2016 and 2020.

Figure 3. WNV human epidemiologic curve by week, Utah 2021

Human surveillance

Human surveillance relies primarily on major laboratories who report results indicative of acute infection. LHDs are immediately notified of positive lab results and conduct case investigations. During the 2021 season, specimens were sent to CDC for confirmation on the first two human cases, fatal suspect cases, and abnormal test results. Additionally, Utah’s major blood banks screened donations for the presence of WNV and reported positive results to DHHS. Zero blood donation screening tests were positive for WNV in 2021.

Table 4. WNV clinical comparison of human cases, Utah vs. United States, 2021
Utah US
Number of cases 28 2911
Fatalities (% fatal) 2 (7%) 227(8%)
Neuroinvasive disease (% neuroinvasive) 21 (75%) 2008 (69%)
Table 5. Clinical and demographic characteristics of human cases, Utah 2011-2021
2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
Number of cases 3 5 7 2 8 13 62 11 21 2 28
Fatalities 0 1 0 0 0 1 5 1 2 0 2
Percent male 66% 60% 57% 100% 75% 54% 70% 64% 67% 100% 64%
Median age (years) NA* 70 61 NA* 55 60 61 49 57 NA* 58
Age range (years) NA* 22-87 20-85 NA* 18-90 18-90 14-86 32-88 <18-92 NA* <18-79
* Not available: data suppressed due to small number of reported cases in this year
Table 6. WNV human case clinical and demographic characteristics, western states, 2021 (CDC ArboNet)
State Neuroinvasive Nonneuroinvasive Total Deaths
Arizona 1140 (66%) 575 (34%) 1715 125 (7%)
Colorado 101 (58%) 74 (42%) 175 11 (6%)
Idaho 12 (75%) 4 (25%) 16 2 (13%)
Montana 2 (100%) 0 (0%) 2 0 (0%)
New Mexico 31 (91%) 3 (9%) 34 7 (21%)
Utah 21 (75%) 7 (25%) 28 3 (11%)
Wyoming 0 (0%) 1 (100%) 1 0 (0%)

Mosquito surveillance

Mosquito abatement districts (MADs) and tribal mosquito abatement districts 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, routine wild bird surveillance was not conducted in 2021.

Sentinel chicken surveillance

Due to budget constraints, routine sentinel chicken surveillance was not conducted in 2021.

For questions about this report, please contact Kacy Nowak at 385-377-4920 or email: