Persistent threat of West Nile Virus in California highlighted in study

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A micrograph of the West Nile Virus, appearing in yellow.
A micrograph of the West Nile Virus, appearing in yellow. Image: Cynthia Goldsmith, P.E. Rollin, USCDCP, public domain

The persistent threat posed by West Nile Virus (WNV) to both humans and horses in California has been highlighted in a just-published study.

WNV is a mosquito-borne virus first detected in the United States in New York City in 1999.

It rapidly spread throughout the continental US, and was first detected in California during the summer of 2003, in a horse.

It is maintained and amplified in a transmission cycle involving mosquitoes and various bird species. Humans and horses may become infected through the bite of an infected female mosquito.

California has reported about 15% of all human WNV infections since the virus was first reported in the state in 2003 — more than any other state.

The California Department of Public Health works with partners throughout the state to conduct robust surveillance of WNV infections in mosquitoes, dead birds, and sentinel chickens.

It uses the data gathered to manage vector control activity and reduce the risk of WNV transmission to people.

Robert Snyder and his colleagues at the California Department of Public Health, together with researchers from the University of California, Davis, summarised 16 years of human, mosquito, chicken, equine, and dead wild bird surveillance for WNV in California.

Their research published in the journal PLOS Neglected Tropical Diseases, represents the most comprehensive description of WNV surveillance data to date.

From 2003 through to 2018, 6909 human cases of WNV disease, including 326 deaths, were reported to the California Department of Public Health, as well as 730 symptom-free WNV infections identified during screening of blood and organ donors.

In all, 4073 (59% of cases) were reported as West Nile neuroinvasive disease.

Additionally, 1299 equine WNV cases were identified, along with detections of WNV in 23,322 dead birds, 31,695 mosquito pools, and 7340 sentinel chickens.

Known as a vector for the West Nile virus, this Culex quinquefasciatus mosquito has landed on a human finger. Eliminating puddles and small containers of water can greatly reduce this mosquito's population. Photo: CDC/Jim Gathany
Known as a vector for the West Nile virus, this Culex quinquefasciatus mosquito has landed on a human finger. Eliminating puddles and small containers of water can greatly reduce this mosquito’s population. Photo: CDC/Jim Gathany

Peak WNV activity occurs from July to 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.

The study team found that almost 95% of all WNV activity in California occurs in the Central Valley and southern California between April and November.

“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,” Snyder and his colleagues wrote.

“Local and state public health partners must continue statewide human and mosquito surveillance and facilitate effective mosquito control and bite prevention measures.”

Turning specifically to equine surveillance, the authors said just one case was reported in 2003, in a 20-year-old unvaccinated horse in San Diego County. In 2004 and 2005, 996 cases were reported from 43 counties, leading to 429 fatalities (43.1%).

However, from 2006 to 2018, only 303 equine cases and 105 fatalities were reported to the California Department of Public Health.

From 2003 to 2018, Riverside County reported the most equine infections (170, or 13.2% of equine cases), followed by Sacramento County (136 cases, 10.5%).

The majority, 64.2%, were in the Central Valley, 23.8% were in southern California, and 11.9% were in other parts of California.

Four counties have never reported any equine cases: Del Norte, Inyo, San Francisco, and San Mateo counties.

The study team says California has consistently more WNV activity than any other state in the continental United States.

They said patterns of WNV activity are influenced by climate, the distribution and abundance of mosquito vectors and reservoir birds, as well as mosquito control and public health efforts.

“Surveillance using mosquitoes, dead birds, and sentinel chickens is employed throughout the state to guide WNV disease prevention efforts.”

The authors said that while WNV is endemic throughout California, its distribution and seasonal activity patterns vary regionally within the state.

“Annual changes in the geographic distribution of WNV outbreaks complicates intervention and public messaging regarding disease risk.

“Despite this, WNV has been detected in every California county, with human disease having been reported among residents of most. It remains critical that all Californians remain informed and vigilant to protect themselves from mosquito bites and therefore WNV infection.”

The researchers say that, even with the state’s robust surveillance and control, it remains critical that mosquito bite prevention via repellents and protective clothing continue to be emphasized.

“In addition, local and state public health partners must continue statewide human surveillance of WNV and other emergent arboviruses (arthropod-borne viruses), while investing in and conducting robust, cost-effective enzootic surveillance to guide mosquito-borne disease prevention and control activities.”

The full study team comprised Snyder, Tina Feiszli, Leslie Foss, Sharon Messenger, Duc Vugia, Kerry Padgett and Vicki Kramer, all with the California Department of Public Health; and Ying Fang, Christopher Barker and William Reisen, with the University of California, Davis.

Snyder RE, Feiszli T, Foss L, Messenger S, Fang Y, Barker CM, et al. (2020) West Nile virus in California, 2003–2018: A persistent threat. PLoS Negl Trop Dis 14(11): e0008841. https://doi.org/10.1371/journal.pntd.0008841

The study, published under a Creative Commons License, can be read here

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