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. 

  1. Screening
  2. Diagnostics
  3. Prevention
  4. Treatment
  5. Resources
  1. Onset during mosquito season (May through October) PLUS
  2. Meningitis, encephalitis, spastic or flaccid paralysis, or profound muscle weakness with no other likely clinical explanation (neuroinvasive disease)
  1. Office of Epidemiology & Data Services

    Physical Address
    4041 N Central Avenue
    Phoenix, AZ 85012