for Prevention

Prevention and Detection of West Nile Virus Disease in Los Angeles County

Umme-Aiman Halai, MD, MPH

Van Ngo, MPH

Emily Barnes, MPH

May-June 2018

Facebook LinkedIn  Tweeter mail  Print  Print


West Nile virus (WNV) activity is predicted to escalate in Los Angeles County by June. Last year’s WNV season was one of the county’s worst with the highest number of human fatalities on record. It is important that clinicians recognize and report cases and promote prevention with their patients. This article will summarize the recent epidemiology of WNV, the clinical presentation of the disease, when to test for and how to report it, as well as prevention messages for patients.



Key Messages

  • WNV cases continue to be elevated in LA County.
  • WNV cases typically occur between June and November, though in 2017, cases were reported as early as March and as late as December.
  • Clinicians are encouraged to educate patients, especially those over 50 years of age and those who are immunocompromised, about how to reduce their risk of WNV.
  • WNV testing is recommended for all patients with aseptic meningitis, encephalitis, acute flaccid paralysis, or nonspecific illness consistent with West Nile fever.
  • Consider St. Louis encephalitis virus (SLEV) in the differential diagnosis, particularly if WNV testing is negative.



Nearly all WNV cases are due to mosquito bites. Very few cases have been documented from vertical transmission (from mother to child in utero or breastfeeding) or from an infected donor to a blood product or organ transplant recipient. Birds are the primary amplifying hosts and the virus is maintained in the environment in a bird-mosquito-bird cycle. Humans and other mammals are “dead end hosts” and do not pass the virus on to other mosquitoes.

The mosquitoes that transmit WNV are usually of the Culex species that bite primarily at dawn and dusk.



WNV first emerged in LA County in 2002, and has since caused severe illness in humans that has resulted in hospitalizations and deaths. The past six years in the county have shown steep increases in WNV cases (see graph below). In earlier years, WNV occurred in a cyclical pattern with the total numbers of cases as low as four in a year. In 2017, there were a total of 268 cases and 27 fatalities, a record number of WNV deaths for the county.


Human WNV Cases and Fatalities, LA County, 2002-2017

Graph showing West Nile Virus Cases and Fatalities in LA County 2002-2017

Source: LA County Department of Public Health Acute Communicable Disease Program

The LA County Department of Public Health (DPH) Acute Communicable Disease Control (ACDC) Program collaborates with multiple agencies to prevent and track human WNV infection. ACDC maintains a WNV data page and publishes weekly reports on WNV and other arboviral diseases during the WNV season.

Transmission Patterns

WNV is endemic in North America. The mosquitoes that can carry the disease are found extensively throughout LA County. WNV cases most frequently occur in the San Fernando Valley, San Gabriel Valley, and the Southeastern region of LA County. In 2017, WNV infection was recorded in 52 cities, which is more than half of all the cities in the county (see map below).

West Nile Virus Case Reports by Residence*, LA County, 2017

Map showing West Nile Virus Case Reports by Residence*, LA County, 2017


*Excludes Long Beach and Pasadena Data
Data points have been modified to prevent identification of exact home address.
Source: LA County Department of Public Health Acute Communicable Disease Program


WNV is a seasonal infection that typically circulates during warmer months when mosquitoes are active. However, the changing weather patterns and drought in LA County have affected the seasonality. In recent years, the county’s WNV season has lengthened. Previously, the typical season began in June and lasted through November. Last year, the first WNV case was detected in March and the final case had symptom onset on December 24 (see graph below).


West Nile Virus Cases by Month of Onset, LA County, 2017

Graph showing West Nile Virus Cases by Month of Onset, LA County, 2017

Source: LA County Department of Public Health Acute Communicable Disease Program


Clinical Presentation

Most persons infected with WNV are asymptomatic; only 20-30% of infections become symptomatic. When symptoms do occur, their onset is typically 3 to 14 days after infection. West Nile fever (WNF) is the most common clinical syndrome, which is characterized by an acute systemic febrile illness (see table below). Gastrointestinal symptoms and a transient maculopapular rash also are common.

Less than 1% of infected persons develop neuro-invasive disease, which typically manifests as meningitis, encephalitis, or acute flaccid paralysis. Rarely, cardiac dysrhythmias, myocarditis, rhabdomyolysis, optic neuritis, uveitis, chorioretinitis, orchitis, pancreatitis, and hepatitis have been described in patients with WNV disease. More information is available on the Centers for Disease Control and Prevention (CDC) WNV Clinical Evaluation and Disease website.


West Nile Virus Clinical Syndromes

Clinical Syndrome

Signs and Symptoms

West Nile fever (WNF)

Abrupt onset of fever, headache, muscle pain, fatigue, anorexia, nausea, and/or rash lasting less than a week

Aseptic meningitis

Symptoms of WNF, stiff neck, photo- and phonophobia, CSF pleocytosis


Symptoms of WNF, altered mental status, lethargy, focal neurologic signs (weakness, nerve palsies), movement disorders

Acute flaccid paralysis

Acute onset of asymmetric flaccid limb weakness or paralysis, which may occur with absence of fever or other features suggestive of WNV infection


Risk Factors and Complications

While all county residents are at risk for WNV, people over 50 years of age and those who are immunocompromised are at higher risk of severe disease and death. Being outdoors without appropriate mosquito protection, especially at dawn and dusk, increases the risk of exposure to mosquitoes that are infected with WNV.

In LA County, neuro-invasive disease accounts for approximately three-quarters of the county’s reported cases each year. The disease severity of documented cases is high as cases with significant symptoms are more frequently diagnosed and reported. Approximately 6-11% of patients hospitalized for severe WNV disease die.



Indications for Testing

In LA County during WNV season, clinicians should order WNV diagnostic tests for patients presenting with:

  • Aseptic meningitis
  • Encephalitis
  • Acute flaccid paralysis
  • Any non-specific illness consistent with WNF.

Diagnostic Tests

Laboratory diagnosis is usually done by testing serum or cerebrospinal fluid (CSF) for WNV-specific IgM antibodies or viral detection using polymerase chain reaction (PCR). These tests are available at many commercial and public health laboratories.

In LA County from May to November, the DPH Public Health Laboratory (PHL) provides WNV-IgM serum antibody testing for suspect cases as well as for confirmation of ambiguous serum test results. WNV-IgM testing on CSF is available through the California Department of Public Health and can be arranged through PHL. PHL does not charge the submitter for these services. Testing at PHL from December to April requires consultation with the ACDC program (213-240-7941).

For more information on submitting WNV specimens to PHL, see the Testing Information Section of the ACDC West Nile Virus Information for Medical Professionals.

Note regarding negative WNV-IgM results:

  • WNV-IgM antibodies are usually detectable 3 to 8 days after onset of illness. If specimens were collected within 8 days of symptom onset, false-negatives are possible. In early presentations, PCR testing can be ordered with serology. Alternatively, in highly suspect cases with negative WNV-IgM, repeating serology on a later sample will confirm diagnosis.

  • Consider SLEV infection if WNV testing is negative and there is no other likely etiology (see SLEV in LA County below).

More information on WNV laboratory diagnosis is located on the CDC WNV Diagnostic Testing website.



Saint Louis Encephalitis Virus (SLEV) in LA County

  • SLEV has re-emerged in mosquitoes in LAC since 2016, the first detections in over a decade.
  • Although no human SLEV cases have been confirmed since 1997, cases have occurred elsewhere in California.
  • Like WNV, SLEV infection is usually mild or asymptomatic, but can cause severe neurologic illness especially in the elderly.
  • Serologic testing is offered at commercial laboratories and PHL.



Human Infection

Human WNV infection is a reportable disease in California and health care providers are mandated to report cases to the local health department within one working day. Cases may be reported to the LA County Department of Public Health by:

  • Phone - call the Communicable Disease Reporting System at 888-397-3993 or
  • Mail or fax - complete a Confidential Morbidity Report (CMR) form and follow fax and mail instructions.

Other Reporting

WNV-infected birds are indicators of WNV activity. To help DPH monitor WNV and implement control measures, the public are encouraged to report dead birds by calling:

  • Los Angeles County DPH Veterinary Public Health 213-989-7060 or
  • California Dead Bird Hotline 877-WNV-BIRD.

Mosquito breeding sites and areas with water accumulation can be reported by residents to the county’s five independent mosquito abatement districts. Additional information and contact information is available at


Treatment and Prevention

How to avoid mosquito bites to prevent West Nile virus

There is no specific treatment for WNV disease. Clinical management is generally supportive. See the CDC WNV Treatment and Prevention website for more information.

WNV prevention strategies include controlling mosquitoes in the community, screening blood and organ donors, and promoting personal protective measures to decrease exposure to infected mosquitoes.

Residents should be encouraged to check for and remove mosquito breeding sources in their homes and yards at least once a week. Providers should routinely discuss the risk of WNV and mosquito bite precautions with patients, particularly those older than 50 years of age and those with chronic medical conditions.

Visit the LA County DPH West Nile Virus Health Education Materials webpage to download or order free posters and brochures on a variety of topics including mosquito abatement, FAQs, tips for using mosquito repellant, and information for seniors.





While WNV disease continues to occur in high numbers in LA County, there are many things that can be done to prevent and control its spread. A key role for clinicians is to educate patients about the prevention of mosquito bites and to recognize and report WNV infections. Clinicians are encouraged to visit the LA County DPH West Nile virus home page for a variety of resources including links to patient education materials, information for health care providers, human and environmental WNV surveillance data, and publications.



Protect Your Family from Mosquito Threats: User-friendly information for patients in English and Spanish. Includes a vector control agency zip code locator  

Mosquito and/or Vector Control Agencies in Los Angeles County: One page map and listing of agencies.

LA County DPH West Nile Virus homepage:

LA County DPH Speakers' Bureau: The speakers' bureau provides presentations for the general community on WNV that includes risk factors, symptoms, treatment, and prevention.

California Mosquito-borne Virus Surveillance and Response

Centers for Disease Control and Prevention


Reference Documents


Subscribe to Rx for Prevention button

Contact Information:

Umme-Aiman Halai, MD, MPH
Medical Epidemiologist
Vector Borne and Neurological Infections

Van Ngo, MPH
Arboviral Surveillance Coordinator

Emily Barnes, MPH
Epidemiology Analyst

Department of Public Health Acute Communicable Disease Control
County of Los Angeles

Rx for Prevention, 2018

Published: May 9, 2018