St. Louis Encephalitis & West Nile Virus
If you suspect St. Louis encephalitis virus (SLEV) or West Nile virus (WNV) infection, please call and report to Maricopa County Department of Public Health (MCDPH) at 602-506-3747 Monday-Friday, 8 a.m. - 5 p.m. within 5 working days. SLEV is clinically indistinguishable from WNV. Consider testing for both viruses as both viruses are endemic in Maricopa County.
WNV is the leading cause of domestically acquired arboviral disease. In 2021, Maricopa County saw more WNV cases than any year since the virus first arrived in Arizona. This was the largest recorded WNV outbreak in a U.S. County and included more than four times the number of cases reported (355) in the previous largest outbreak in the county during 2004.
Most years, there are only about 20 SLEV cases in the U.S., but occasional large urban outbreaks happen every 10-20 years. The last known locally-acquired outbreak in Maricopa County was in 2015 when there was simultaneous mosquito-borne outbreaks of WNV and SLEV. The total number of cases in Maricopa County for 2015 was 62 WNV cases and 22 SLEV cases. Prior to 2015, the highest year of reported SLEV cases was 2003, with five cases confirmed statewide, all from Maricopa County.
- Onset during mosquito season (May through October) PLUS
- Meningitis, encephalitis, spastic or flaccid paralysis, or profound muscle weakness with no other likely clinical explanation (neuroinvasive disease)
When collecting specimens for testing, consider both WNV and SLEV diagnostic tests due to the similarity in clinical presentation, especially if neuroinvasive disease is present. Most commercial laboratories do not have the capacity to test for SLEV but some hospital laboratories have arboviral panels that include SLEV. If they do have an arboviral panel, WNV is often not included in the panel and is ordered as a stand-alone test. Please check with your laboratory provider for test availability and specimen shipping recommendations. SLEV testing at a public health laboratory can be arranged through MCDPH by calling 602-506-6767 Monday-Friday, 8 a.m. to 5 p.m.
- Collect additional samples (blood in a serum separator tube or cerebrospinal fluid) to be sent to ASPHL as needed;
- Arrange for commercial laboratory testing concurrently with ASPHL testing.
- For WNV, order IgM serology through your usual commercial laboratory
- For SLEV, order IgM serology through your usual commercial laboratory
- If the laboratory does not have SLEV testing capacity, contact Maricopa County Department of Public Health to arrange for IgM serology at ASPHL
- Note: in cases of CNS disease, IgM antibodies should be detectable at presentation of CNS symptoms. In non-CNS disease, IgM antibody may not be detectable for the first 7 days of illness.
No specific antiviral therapy is available for either SLEV or WNV disease. Treatment is supportive and may include rest, fluids and analgesics and antipyretics.