West Nile virus in California, 2003–2018: A persistent threat

Abstract

The California Arbovirus Surveillance Program was initiated over 50 years ago to track endemic encephalitides and was enhanced in 2000 to include West Nile virus (WNV) infections in humans, mosquitoes, sentinel chickens, dead birds and horses. This comprehensive statewide program is a function of strong partnerships among the California Department of Public Health (CDPH), the University of California, and local vector control and public health agencies. This manuscript summarizes WNV surveillance data in California since WNV was first detected in 2003 in southern California. From 2003 through 2018, 6,909 human cases of WNV disease, inclusive of 326 deaths, were reported to CDPH, as well as 730 asymptomatic WNV infections identified during screening of blood and organ donors. Of these, 4,073 (59.0%) were reported as West Nile neuroinvasive disease. California’s WNV disease burden comprised 15% of all cases that were reported to the U.S. Centers for Disease Control and Prevention during this time, more than any other state. Additionally, 1,299 equine WNV cases were identified, along with detections of WNV in 23,322 dead birds, 31,695 mosquito pools, and 7,340 sentinel chickens. Annual enzootic detection of WNV typically preceded detection in humans and prompted enhanced intervention to reduce the risk of WNV transmission. Peak WNV activity occurred from July through October in the Central Valley and southern California. Less than five percent of WNV activity occurred in other regions of the state or outside of this time. WNV continues to be a major threat to public and wild avian health in California, particularly in southern California and the Central Valley during summer and early fall months. Local and state public health partners must continue statewide human and mosquito surveillance and facilitate effective mosquito control and bite prevention measures.

Characterizing Areas with Increased Burden of West Nile Virus Disease in California, 2009–2018

“…However, we focused on the raw number of cases, which has previously been observed to be higher in metropolitan areas (Petersen 2019 ) where populations tend to be younger (Parker et al 2018 ). The WNV incidence in these areas is not the highest in California (Snyder et al 2020b ), but by sheer numbers and density of people, there were more cases.…”

“…As a result, 99% of our high-burden ZIP codes fell within the service area of a vector control agency. Those agencies collect environmental data, such as mosquito abundance and infection rates, to estimate risk of human WNV transmission (Snyder 2020b ). We hope to quantify how those environmental indicators were associated with the number and timing of human cases in a given area.…”

“…Reducing mosquito exposure through vector control, public education, and personal protective behaviors are the primary forms of prevention (Petersen et al 2013 ). States report disease cases to the CDC by county of residence (CDC 2020), and California has already identified that >95% of all cases occur in southern California or Central Valley counties (Snyder et al 2020b ). However, California's counties are large, with 47 of the 58 having areas greater than 2000 km 2 , including San Bernardino County, the largest in the continental United States, covering 51,947 km 2 .…”

A panel of real-time PCR assays for the detection of Bourbon virus, Heartland virus, West Nile virus, and Trypanosoma cruzi in major disease–transmitting vectors

“…1 This assay has been used in the detection or surveillance of WNV in people, mosquitoes, birds, or horses. 14,30,34 To meet the need for simultaneous testing of multiple pathogens, we modified the assay protocol and validated the assay, which showed excellent sensitivity and specificity. A study using the primers/probe set detected as few as 10 1.7–2.3 pfu of WNV/mL in infected mosquitoes, C. pipiens quinquefasciatus.…”

A Review: Aedes-Borne Arboviral Infections, Controls and Wolbachia-Based Strategies

“…Examples of Aedes -borne arboviruses having RNA genomes are dengue virus (DENV), Zika virus (ZIKV), chikungunya virus (CHIKV), yellow fever virus (YFV), and Ross River virus (RRV) [ 7 ] ( Figure 1 ). Other RNA arboviruses which are not Aedes -borne include West Nile virus (WNV) and Sindbis virus ( Culex -borne) [ 8 , 9 ], Tick-borne encephalitis virus and Crimean-Congo haemorrhagic fever virus (tick-borne) [ 10 , 11 ], and Toscana virus (fly-borne) [ 12 ].…”