Background West Nile virus (WNV) is a mosquito-borne disease most commonly spread by the bite of an infected Culex mosquito. WNV first emerged in the eastern United States in 1999 and is the leading cause of mosquito-borne disease in the contiguous U.S.
In Chicago, human WNV cases are typically seen during the summer and fall seasons, when mosquitoes are more prevalent. While most people infected with WNV do not have symptoms, about 1 in 5 develop mild illness and about 1 in 150 develop a serious illness that affects the central nervous system and can result in death. Serious illness can occur in people of any age, but those over the age of 60 and those with certain medical conditions are at greater risk.
Symptoms The incubation period for WNV disease can range from 2 to 14 days, though immunocompromised individuals may not present with symptoms for several weeks.
Most people infected with WNV do not develop any symptoms.
About 20% develop an acute febrile illness with other symptoms such as headache, body aches, joint pains, vomiting, diarrhea and maculopapular rash. Most recover completely, but fatigue and weakness can last for weeks or months.
Though rare, severe illness typically manifests as meningitis, encephalitis or acute flaccid paralysis. Symptoms include high fever, severe headache, neck stiffness, stupor, disorientation or confusion, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. Recovery might take several weeks or months and some effects to the central nervous system could be permanent.
Information for Health Care Providers
Testing
Laboratory diagnosis is accomplished through the testing of serum or cerebrospinal fluid (CSF) to detect WNV-specific IgM antibodies, which are usually detectable 3-8 days after onset of illness.
IgM detection may also reflect a past infection, as antibodies may persist for 30-90 days or even longer.
Note, if serum is collected within 8 days of illness onset and antibodies are not present, WNV should not be entirely ruled out yet and testing may need to be redone on a later sample.
The presence of WNV-specific IgM antibodies is good evidence of recent infection but may also result from cross-reactivity with other flaviviruses and should be taken in context with clinical signs and symptom and a detailed travel history. Where possible, CDPH recommends providers test for both WNV and St. Louis Encephalitis virus because both are endemic in Chicago.
All human specimens (serum and CSF) should be sent to private or reference laboratories for diagnostic testing.
WNV IgG antibodies persist for many years following an infection and cannot be used to differentiate between a new and past infection.
Treatment
There are no approved or recommended treatments for WNV disease and clinical management is supportive.
The best way to prevent WNV disease or any other mosquito-borne illness is to reduce the number of mosquitoes around your home and to take personal precautions to avoid mosquito bites. Precautions include:
Avoid being outdoors when mosquitoes are most active, especially between dusk and dawn.
When outdoors, wear shoes and socks, long pants and a long-sleeved shirt, apply EPA- registered insect repellent that includes DEET, picaridin, oil of lemon eucalyptus or IR 3535 according to label instructions. Consult a physician before using repellents on infants.
Make sure doors and windows have tight-fitting screens. Repair or replace screens that have tears or other openings. Try to keep doors and windows shut, especially at night.
Eliminate all sources of standing water that can support mosquito breeding, including water in bird baths, ponds, flowerpots, wading pools, old tires and any other receptacles. Use 311 to report areas of standing water along the roadside, flooded yards and similar locations in the public way.
Per the Control of the Communicable Disease Code of Illinois, WNV cases must be reported to CDPH within 3 days.
Most laboratories automatically report test results to public health electronically. To report additional required information, please use I-NEDSS (Illinois Notifiable Electronic Disease Surveillance System) or call 312-743-9000, select option number 7.
*For urgent questions after hours (4:30pm – 8:30am), weekends, and holidays, call 311 and ask for the communicable disease physician on-call (or 312-744-5000 if outside the City of Chicago)
