2020 West Nile virus summary report

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 2020 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 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.

For 2020, there were 731 human cases reported to CDC. Of those, 559 (76%) were reported as West Nile meningitis or encephalitis (neuroinvasive disease) and 172 (24%) were reported as West Nile fever (milder disease). In comparison, 971 human cases were reported to CDC in 2019, with 633 (65%) reported as West Nile meningitis or encephalitis (neuroinvasive disease), and 338 (35%) reported as West Nile fever (milder disease). There were a total of 66 fatalities reported in 2020, compared with 60 fatalities reported in 2019.

As shown in Figure 3, activity during the 2020 WNV season in Utah was below the five-year average for human and animal infection. Mosquito activity occurred in the highest numbers in Salt Lake, Weber, Utah, Davis, Box Elder, and Tooele counties. Six counties reported WNV activity during the 2020 season. Only PCR positive mosquito pools were included in surveillance data. Individual 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.

WNV activity was first detected in Utah in 2003. Similar to initial seasons in many other states, activity 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 significant activity in northern Utah along the Wasatch Front. During 2005, activity expanded into more northern regions of the state as Utah and Uintah counties served as focal points for detected activity. Thus far, the 2006 season was the most active season Utah has experienced. During that season, WNV activity was focused along the Wasatch Front, particularly in Salt Lake and Utah counties. With an increase in activity 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 activity 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 activity. 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, WNV activity 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.

Figure 2. WNV positive cases, Utah, 2010-2020

In 2020, WNV activity appeared in Utah in the middle of June. Mosquito activity was first detected in mosquito pools during the week of June 14, 2020, in Uintah and Utah counties. Activity was detected throughout the summer and into October, with WNV activity detected in horses, humans, and mosquitoes by mid-July. Utah’s first human case was reported on August 25, 2020, as shown in Figure 4. Active surveillance for the 2020 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 2015 and 2019.

Figure 3. WNV human epidemiologic curve by week, Utah 2020

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 2020 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. No blood donation screening tests were positive for WNV in 2020.

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.

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

Due to budget constraints, routine wild bird surveillance was not conducted in 2020.