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Clinical Features
Diagnostic Testing Forms
West Nile Virus Reporting and Notification
- West Nile Virus is a reportable disease under Title 17, Section 2500, California Code of Regulations. Please use the following form to report WNV infection of any type.
- CONFIDENTIAL MORBIDITY REPORT (
PDF 916 kb)
Clinical Features
Mild Infection
Most WNV infections are mild and often clinically unapparent.
Approximately 20% of those infected develop a mild illness (West Nile fever).
The incubation period is thought to range from 3 to 14 days.
Symptoms generally last 3 to 6 days.
Reports from earlier outbreaks describe the mild form of WNV infection as a febrile illness of sudden onset often accompanied by
- Malaise
- Headache
- anorexia
- Myalgia
- Nausea
- Rash
- vomiting
- lymphadenopathy
- eye pain
The full clinical spectrum of West Nile fever has not been determined in the United States.
Severe Infection
Approximately 1 in 150 infections will result in severe neurological disease
The most significant risk factor for developing severe neurological disease is advanced age.
Encephalitis is more commonly reported than meningitis.
In recent outbreaks, symptoms occurring among patients hospitalized with severe disease include
- Fever
- gastrointestinal symptoms
- weakness
- change in mental status
A minority of patients with severe disease developed a maculopapular or morbilliform rash involving the neck, trunk, arms, or legs.
Several patients experienced severe muscle weakness and flaccid paralysis.
Neurological presentations included
- taxia and extrapyramidal signs
- optic neuritis
- cranial nerve abnormalities
- polyradiculitis
- Myelitis
- seizures
Although not observed in recent outbreaks, myocarditis, pancreatitis, and fulminant hepatitis have been described.
Clinical Suspicion
Diagnosis of WNV infection is based on a high index of clinical suspicion and obtaining specific laboratory tests.
WNV, or other arboviral diseases such as St. Louis encephalitis, should be strongly considered in adults >50 years who develop unexplained encephalitis or meningitis in summer or early fall.
The local presence of WNV enzootic activity or other human cases should further raise suspicion.
Obtaining a recent travel history is also important.
Note: Severe neurological disease due to WNV infection has occurred in patients of all ages. Year-round transmission is possible in some areas. Therefore, WNV should be considered in all persons with unexplained encephalitis and meningitis.
Diagnostic Testing
The San Luis Obispo Public Health Laboratory (PHL) and the California Department of Health Services (CDHS) are providing testing services to assist in your evaluation of cases of Encephalitis*, Aseptic Meningitis (³ 17 years), and Acute Flaccid Paralysis/Atypical Guillain-BarrESyndrome, that may be caused by WNV. All specimens must be accompanied by a West Nile Case History Form ( PDF 40 kb) and West Nile Diagnostic Testing Guidelines ( PDF 124 kb). The San Luis Obispo PHL will perform a West Nile Indirect Fluorescent Antibody (IFA) test. This serologic screening test detects Arboviral antibodies from blood. Specimens that are IFA positive or borderline-positive will be sent to the Virus and Rickettsial Diseases Laboratory (VRDL) for confirmatory testing. This IFA kit cannot be used to test cerebral spinal fluid (CSF). The San Luis Obispo PHL will directly forward CSF specimens to the VRDL.
*For encephalitis cases- The California Encephalitis Project (CEP) at the VRDL will perform testing for both WNV and encephalitis. These specimens must be accompanied by a CEP Encephalitis Case History Form ( PDF 180 kb) and a California Encephalitis Project Test Request Form ( PDF 240 kb).
VRDL Testing
The most efficient diagnostic method is detection of IgM antibody to WNV in serum collected within 8-14 days of illness onset or cerebral spinal fluid (CSF) collected within 8 days of illness onset using the IgM antibody capture enzyme-linked immunosorbent assay (MAC-ELISA).
Since IgM antibody does not cross the blood-brain barrier, IgM antibody in CSF strongly suggests central nervous system infection.
Patients who have been recently vaccinated against or recently infected with related flaviviruses (e.g., yellow fever, Japanese encephalitis, dengue) may have positive WNV MAC-ELISA results.
Specimens that are MAC-ELISA positive will have a plaque reduction neutralization test (PRNT) performed.
Reporting Suspected WNV Infection
Refer to state health department reporting requirements at:
http://www.westnile.ca.gov/
WNV encephalitis is on the list of designated nationally notifiable arboviral encephalitides.
Aseptic meningitis is a reportable disease in California.
The timely identification of persons with acute WNV or other arboviral infection may have significant public health implications and will likely augment the public health response to reduce the risk of additional human infections.
Laboratory Findings
Among patients in recent outbreaks
Total leukocyte counts in peripheral blood were mostly normal or elevated, with lymphocytopenia and anemia also occurring.
Hyponatremia was sometimes present, particularly among patients with encephalitis.
Examination of the cerebrospinal fluid (CSF) showed pleocytosis, usually with a predominance of lymphocytes.
Protein was universally elevated.
Glucose was normal.
Computed tomographic scans of the brain mostly did not show evidence of acute disease, but in about one-third of patients, magnetic resonance imaging showed enhancement of the leptomeninges, the periventricular areas, or both.
Treatment
Treatment is supportive, often involving hospitalization, intravenous fluids, respiratory support, and prevention of secondary infections for patients with severe disease.
Ribavirin in high doses and interferon alpha-2b were found to have some activity against WNV in vitro, but no controlled studies have been completed on the use of these or other medications, including steroids, antiseizure drugs, or osmotic agents, in the management of WNV encephalitis.
For additional information about West Nile virus, please visit the:
California Department of Health Services West Nile website at:
http://www.westnile.ca.gov/
CDC’s Division of Vector-Borne Infectious Diseases website at: http://www.cdc.gov/ncidod/dvbid/westnile/index.htm
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