WNV is a mosquito-borne virus that was first discovered in the United States in New York City in 1999.
It quickly spread throughout the continental United States and was first discovered on a horse in California in the summer of 2003.
It is preserved and amplified in a transmission cycle with mosquitoes and various species of birds. Humans and horses can be infected by the bite of an infected female mosquito.
California has reported about 15% of all human WNV infections since the virus was first reported to the state in 2003 – more than any other state.
The California Department of Health is working with partners across the state to provide reliable monitoring of WNV infection in mosquitoes, dead birds, and sentinel chickens.
It uses the collected data to manage vector control activity and reduce the risk of WNV transmission to individuals.
Robert Snyder and his colleagues at the California Department of Health, along with researchers from the University of California, Davis, summed up 16 years of monitoring people, mosquitoes, chickens, horses and dead wild birds for WNV in California.
Her research, published in the journal PLOS Neglected Tropical Diseases, represents the most comprehensive description of WNV monitoring data to date.
From 2003 to 2018, 6,909 human cases of WNV disease, including 326 deaths, were reported to the California Department of Health and 730 symptom-free WNV infections found in screening blood and organ donors.
A total of 4073 (59% of cases) have been reported as neuroinvasive West Nile disease.
In addition, 1299 WNV cases were identified in horses, along with evidence of WNV in 23,322 dead birds, 31,695 mosquito pools and 7,340 sentinel chickens.
Known as the vector for the West Nile virus, the Culex quinquefasciatus mosquito landed on a human finger. Eliminating puddles and small containers of water can significantly reduce the population of this mosquito. Photo: CDC / Jim Gathany
The highest WNV activity occurs in the Central Valley and Southern California from July through October. Less than five percent of WNV activities took place in other regions of the state or outside of this time.
The study team found that nearly 95% of all WNV activity in California between April and November occurs in the Central Valley and Southern California.
“WNV continues to be a major threat to public and wild bird health in California, particularly southern California and the Central Valley during the summer and early fall months,” wrote Snyder and colleagues.
“Local and government public health partners must continue nationwide monitoring of people and mosquitoes and enable effective mosquito control and bite prevention measures.”
Regarding horse surveillance, the authors said that only 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, resulting in 429 deaths (43.1%).
However, from 2006 to 2018, only 303 cases of horses and 105 deaths were reported to the California Department of Health.
From 2003 to 2018, Riverside County reported the most horse infections (170 or 13.2% of horse cases), followed by Sacramento County (136 cases, 10.5%).
The majority, 64.2%, were in the Central Valley, 23.8% in Southern California, and 11.9% in other parts of California.
Four counties have never reported cases of horses: Del Norte, Inyo, San Francisco, and San Mateo counties.
The study team says California consistently has more WNV activity than any other state in the Americas.
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.
“Mosquito, dead bird, and sentinel chicken surveillance is being used across the state to guide efforts to prevent WNV disease.”
The authors said that while WNV is endemic across California, its distribution and seasonal activity pattern vary regionally within the state.
“Annual changes in the geographic distribution of WNV outbreaks make disease risk interventions and public news more difficult.
“Even so, WNV has been discovered in every California county, with human disease reported in most residents. It remains important that all Californians stay informed and vigilant to protect themselves from mosquito bites and, consequently, from WNV infections. “
The researchers say that despite the state’s robust surveillance and control, it remains crucial that the emphasis on preventing mosquito bites through repellants and protective clothing continues.
“In addition, local and state public health partners must continue nationwide human surveillance for WNV and other emerging arboviruses (arthropod-borne viruses) while investing in and conducting robust, inexpensive enzootic surveillance to target activities To direct the prevention and control of mosquito-borne diseases. ”
The entire study team consisted of Snyder, Tina Feiszli, Leslie Foss, Sharon Messenger, Duc Vugia, Kerry Padgett, and Vicki Kramer, all from the California Department of Health; and Ying Fang, Christopher Barker, and William Reisen from 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: An Enduring Threat. PLoS Negl Trop Dis 14 (11): e0008841. https://doi.org/10.1371/journal.pntd.0008841
The study, published under a Creative Commons Licensecan be read Here.