2019 West Nile Virus Summary Report

Utah Department of Health


The purpose of this document is to provide Utah partners a concise summary of this season’s major West Nile Virus (WNV) results. This report has been compiled by the Utah Department of Health (UDOH), but reflects information obtained from joint efforts. All activities related to WNV during the 2019 season involved major contributions from many different agencies. These include: blood banks of Utah, local health departments (LHDs), Utah Department of Agriculture and Food (UDAF), Utah Division of Wildlife Resources (UDWR), Utah Mosquito Abatement Association (UMAA), Tracy Aviary, Hogle Zoo, 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 2019 season. The intent of this report is to document the results of the efforts put forth by these entities during the 2019 WNV season.

NOTE: The purpose of this report is to describe general trends that occurred during the 2019 WNV season. Specific surveillance counts may be subject to change as data continue to be reconciled.


During the summer of 2019, WNV reemerged in Utah at above average levels when compared with the previous five years. This was the 16th year that WNV activity was detected in Utah. 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.

WNV Surveillance in Utah

Surveillance for WNV involves several different components. Since the disease is zoonotic, both human and animal surveillance are conducted. In past years, WNV surveillance in Utah involved human, mosquito, wild bird, horse, and sentinel chicken populations. Budget constraints limited surveillance for the 2019 season. To ensure the most critical surveillance systems were maintained, wild bird testing, sentinel chicken testing, and official coordinated equine testing efforts at UDAF were again eliminated from routine surveillance. Local mosquito abatement districts (MADs) and tribal abatement districts, in conjunction with the Utah Mosquito Abatement Assocation, performed necessary trapping and identification for mosquito surveillance. Confirmatory testing of 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 screening of donated blood for identification of viremic donors. All LHDs in Utah were involved with disseminating, investigating, and responding to surveillance data indicative of local WNV activity.

2019 Season National Highlights

West Nile virus neuroinvasive disease incidence maps present data reported by state and local health departments to CDC’s ArboNET surveillance system. Figure 1 shows the states reporting WNV activity. Figure 2 shows the rate of human neuroinvasive disease (e.g., meningitis, encephalitis, or acute flaccid paralysis) by state for 2019.

Figure 1. West Nile Activity by State - United States, 2019 (as of November 19, 2019)
*WNV human disease cases or presumptive viremic blood donors have a positive screening test which has not necessarily been confirmed.
\(\dagger\)WNV veterinary disease cases, or infections in mosquitoes, birds, or sentinel animals.

Figure 2. West Nile Virus Neuroinvasive Disease Incidence by State - United States, 2019 (as of November 19, 2019)

Neuroinvasive disease cases were reported to ArboNET from the following states in 2019: Alabama, Arizona, Arkansas, California, Colorado, Connecticut, District of Columbia, Florida, Georgia, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, South Carolina, Tennessee, Texas, Utah, Virginia, Washington, Wisconsin and Wyoming.

Of the 971 human cases reported to CDC, 633 (65%) were reported as West Nile meningitis or encephalitis (neuroinvasive disease) and 338 (35%) were reported as West Nile fever (milder disease). In comparison, 2,647 human cases were reported to CDC in 2018, with 1,658 (63%) reported as West Nile meningitis or encephalitis (neuroinvasive disease), and 989 (37%) reported as West Nile fever (milder disease). A total of 60 fatalities were reported in 2019, compared with 167 fatalities reported in 2018.

2019 Season Highlights - Utah

Activity during the 2019 WNV season in Utah was above the five-year average for human and animal infection, as shown in Figure 3 (navigate to Past Season Comparisons for figure). The number of WNV positive pools was highest in Davis, Salt Lake, Uintah, and Washington counties. A total of 16 counties detected WNV activity during the 2019 season. Note that only PCR positive mosquito pools were included in surveillance data. Many mosquito abatement districts (MADs) have started conducting their own PCR testing at their respective locations. Surveillance data from MADs whose testing has been validated by UPHL are included in UDOH surveillance data throughout the mosquito season.

Table 1. All WNV Activity, Utah 2019 (positive counts only)
Jurisdiction Human Horse Mosquito
Bear River 1 0 13
Central 1 2 0
Davis 5 0 90
Salt Lake 2 0 87
San_Juan 0 1 0
Southeast 3 0 9
Southwest 5 2 24
TriCounty 2 3 33
Utah County 1 0 4
Weber Morgan 1 0 12
Summit 0 0 0
Wasatch 0 0 0
State Total 21 8 272
Table 2. Human Cases of WNV, Utah 2019
Age Group Total Death Neuroinvasive
<18 2 0 2
18-39 1 0 1
40-64 8 0 1
>65 10 2 8
State Total 21 2 12

Past Season Comparison

WNV activity was first detected in Utah in 2003. Similar to many initial seasons in other states, activity was muted. Only one human case was reported during the 2003 season in Utah, in addition to an asymptomatic viremic blood donor. WNV illness detected in horses was the main indication of WNV presence in the state in 2003. WNV became firmly established in Utah in 2004 with significant activity in northern Utah along the Wasatch Front. During 2005, activity expanded into more northern regions of the state and Utah and Uintah counties served as focal points for detected activity.

Thus far, the 2006 season was the most active season we have experienced in Utah. During this season, WNV activity was focused along the Wasatch Front, particularly in Salt Lake and Utah counties. With an increase in activity, there was also 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 this season, the virus moved farther north with the bulk of cases reported in Cache and Box Elder counties.

WNV activity continued to decrease from 2008-2011. During this time, due to inconsistencies with RAMP testing, it was decided that mosquito pools would only be counted if 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 the southwest portion of the state, saw the majority of activity, both human and animal. From 2014-2016, WNV activity was centered mostly along the Wasatch Front, but was also seen in Box Elder, Grand, and Weber counties.

In 2017, Utah saw a dramatic increase in WNV activity, with the largest number of human and animal infections, along with the greatest number of positive mosquito pools, in more than ten years. A decrease in WNV activity was reported in 2018 compared to 2017.

Table 3. Ten-year WNV Season Comparison,Utah 2009-2019
2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Human 2 2 3 5 7 2 8 13 62 11 21
Horse 6 3 1 3 7 4 4 7 35 6 8
Mosquito Pools 284 31 23 21 69 167 281 244 443 180 272
Bird* 0 0 0 0 40 2
3 4 1
Chicken* 1 1 0 1 2 1 5
Counties with Detection 5 5 6 8 9 9 8 8 17 16 20
* Wild bird and sentinel chickens were not part of Utah’s surveillance in 2011–2013. However,in October 2013-January 2014 the
large increase in WNV positive birds was due to an eared grebe and bald eagle die-off.

Figure 3. WNV Positive cases, Utah, 2009-2019

2019 Utah Activity Timeline

In 2019, WNV activity appeared in Utah during the beginning of July. Positive mosquito pools were first detected in mosquito pools during the week of July 8, 2019, in Grand County. WNV activity continued to be detected throughout the summer and into October, with WNV activity detected in horses, humans, and mosquitoes by August. Utah’s first human case was reported on July 27, 2019 as shown in Figure 4. Active surveillance for the 2019 season ceased in the middle of October. However, testing of suspect human and horse cases continued year-round. Average counts for comparison were calculated from human case data reported between the years 2014-2018.

Figure 4. WNV Human Epidemiologic Curve by Week, Utah 2019

Human Surveillance

Human surveillance relies on positive lab results and WNV positive test screening of blood donors.

Additionally, major blood banks serving Utah screen donations for the presence of WNV and report positive results to UDOH. However, no blood donation screening tests were positive for WNV in 2019.

Table 4. WNV Clinical Comparison of Human Cases, Utah vs. United States, 2019
Utah US
Number of cases 21 872
Fatalities (% fatal) 2 (9.5%) 46 (5.3%)
Neuroinvasive disease (% neuroinvasive) 12 (57%) 570 (65%)
Table 5. Clinical and Demographic Characteristics of Human Cases, Utah 2009-2019
2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Number of cases 2 2 3 5 7 2 8 13 62 11 21
Fatalities 0 0 0 1 0 0 0 1 5 1 2
Percent Male 50% 100% 66% 60% 57% 100% 75% 54% 70% 64% 67%
Median Age (years) NA* NA* NA* 70 61 NA* 55 60 61 49 57
Age Range (years NA* NA* NA* 22-87 20-85 NA* 18-90 18-90 14-86 32-88 <18-92
* 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, 2019 (CDC ArboNet)
State Neuroinvasive Nonneuroinvasive Total Deaths
Arizona 134 (78%) 38 (22%) 172 17 (10%)
Colorado 49 (41%) 70 (59%) 119 6 (5%)
Idaho 4 (36%) 7 (64%) 11 0 (0%)
Montana 3 (100%) 0 (0%) 3 0 (0%)
New Mexico 30 (75%) 10 (25%) 40 4 (10%)
Utah 12 (57%) 9 (43%) 21 2 (10%)
Wyoming 1 (33%) 2 (66%) 3 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 the state. 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 testing by PCR. All PCR tests are reported to UDOH and included in surveillance measures. RAMP tests without PCR confirmation are also reported to UDOH, however, they are not included in weekly surveillance measures.

Horse Surveillance

Surveillance of equine disease related to WNV infection in Utah is coordinated by the UDAF. Veterinarians across the state were encouraged to submit samples from suspect equine cases to the UVDL-Logan for testing. Results of these serum tests were reported by UDAF to UDOH with appropriate notification occurring for positive cases. The majority of samples submitted for testing were from domestic, privately owned horses with symptoms indicative of infection and no history of vaccination. Disease awareness among veterinarians and horse owners was increased through distribution of pamphlets and periodic updates using the Utah Veterinary Alert Listserv. UDAF also maintains an interactive map showing 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 2019. However, WNV positive reports of a crow from Davis County were received.

Sentinel Chicken Surveillance

Due to budget constraints, routine sentinel chicken surveillance was not conducted in 2019.

For questions about this report, please contact the Utah Department of Health Bureau of Epidemiology at (801) 538-6191 or email .