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Notes from the Field: Concurrent Outbreaks of St. Louis Encephalitis Virus and West Nile Virus Disease — Arizona, 2015
, DVM1,2,3,*; , PhD1,4,*; , DVM1,4; , MD1,2,3; , DVM3,6; , MD4; , MSN2; , MS2; , PhD5; , MSPH3; , MPH3; , MD4; , MBBS4
St. Louis encephalitis virus (SLEV) and West Nile virus (WNV) are closely related mosquito-borne flaviviruses that can cause outbreaks of acute febrile illness and neurologic disease. Both viruses are endemic throughout much of the United States and have the same Culex species mosquito vectors and avian hosts (1); however, since WNV was first identified in the United States in 1999, SLEV disease incidence has been substantially lower than WNV disease incidence, and no outbreaks involving the two viruses circulating in the same location at the same time have been identified. Currently, there is a commercially available laboratory test for diagnosis of acute WNV infection, but there is no commercially available SLEV test, and all SLEV testing must be performed at public health laboratories. In addition, because antibodies against SLEV and WNV can cross-react on standard diagnostic tests, confirmatory neutralizing antibody testing at public health laboratories is usually required to determine the flavivirus species (2). This report describes the first known concurrent outbreaks of SLEV and WNV disease in the United States.
During 2010–2014, 537 WNV disease cases and only one SLEV disease case were reported to the Arizona Department of Health Services. However, during 2015, by the end of July, SLEV infection had been confirmed in seven ill Arizona residents. In addition, the Maricopa County Vector Control Division identified 60 pools of Culex tarsalis or Culex quinquefasciatus mosquitoes that tested positive for SLEV RNA by reverse transcription polymerase chain reaction, and 97 pools that tested positive for WNV RNA. An investigation was initiated to ascertain the magnitude and describe the epidemiology of the outbreaks. Cases were defined according to national surveillance case definitions (3). If the patient had immunoglobulin M antibody against both WNV and SLEV, and insufficient sample or inconclusive results on neutralizing antibody testing, the case was classified as an unspecified flavivirus infection.
As of November 24, 2015, a total of 117 cases of flavivirus disease had been reported to the Arizona Department of Health Services, including 75 WNV, 19 SLEV, and 23 unspecified flavivirus disease cases. Laboratory testing is ongoing, and some cases will likely be reclassified. Among all cases, 103 (88%) occurred from July through September. Eight (53%) of 15 counties reported cases; 45 (60%) WNV and 18 (95%) SLEV disease cases were reported from Maricopa County. Overall, 77 (66%) patients were aged ≥50 years (median = 54 years, range = 21–89 years), and 61 (52%) were male. Seventy-nine (68%) patients had neuroinvasive disease (e.g., meningitis, encephalitis, or acute flaccid paralysis), including 47 (63%) with WNV infection, 17 (89%) with SLEV infection, and 15 (65%) with unspecified flavivirus infection. Among all 117 cases, 86 (74%) patients were hospitalized and five (4%) died.
This is the first known outbreak of concurrent WNV and SLEV disease. Enhanced clinical and laboratory surveillance activities in Arizona will continue through the end of the arboviral transmission season in late November to characterize the outbreak. WNV and SLEV disease cases will be compared to better understand differences in the epidemiology and outcomes of these diseases. Because of the similarity in clinical presentation for WNV and SLEV disease cases, cross reactivity between WNV and SLEV antibodies, and the lack of availability of a commercial SLEV test, SLEV disease cases could be incorrectly diagnosed as WNV disease cases or remain undetected if clinicians only request WNV testing and no confirmatory testing is conducted. Health care providers should consider both WNV and SLEV infections in the differential diagnosis of cases of aseptic meningitis and encephalitis and obtain appropriate cerebrospinal fluid, serum specimens, or both for laboratory testing (4). Confirmatory testing at state health departments or CDC will be required to distinguish these flavivirus infections. When feasible, vector control programs should test mosquitoes for SLEV in addition to WNV. Clinical management for both diseases involves supportive care. Because human vaccines against domestic arboviruses are not available, prevention of arboviral infection depends on local vector control, community, and household efforts to reduce vector populations (e.g., removal of standing water), and individual efforts to decrease exposure to mosquitoes (e.g., applying mosquito repellant and eliminating mosquito breeding sites).
Acknowledgments
Tammy Kafenbaum, Andrew Strumpf, Carrie Walker, Rebecca Sunenshine, Maricopa County Department of Public Health, Phoenix, Arizona; Jennifer Chevinsky, Kathryn Fitzpatrick, Jennifer Pistole, Arizona Department of Health Services; John Townsend, Maricopa County Environmental Services, Mesa, Arizona; Robert Lanciotti, Jennifer Lehman, Nicole Lindsey, Ingrid Rabe, Arboviral Diseases Branch, Division of Vector-Borne Diseases, National Center for Emerging and Zoonotic Infectious Diseases, CDC.
1Epidemic Intelligence Service, CDC; 2Maricopa County Department of Public Health, Phoenix, Arizona; 3Arizona Department of Health Services; 4Division of Vector-Borne Diseases, National Center for Emerging and Zoonotic Infectious Diseases, CDC; 5Maricopa County Environmental Services Vector Control Division; 6Career Epidemiology Field Officer Program, CDC.
Corresponding authors: Heather Venkat, HeatherVenkat@mail.maricopa.gov, 602-531-4422; Elisabeth Krow-Lucal, ekrowlucal@cdc.gov, 970-266-3565.
References
- Reimann CA, Hayes EB, DiGuiseppi C, et al. Epidemiology of neuroinvasive arboviral disease in the United States, 1999–2007. Am J Trop Med Hyg 2008;79:974–9.
- Martin DA, Noga A, Kosoy O, Johnson AJ, Petersen LR, Lanciotti RS. Evaluation of a diagnostic algorithm using immunoglobulin M enzyme-linked immunosorbent assay to differentiate human West Nile virus and St. Louis Encephalitis virus infections during the 2002 West Nile virus epidemic in the United States. Clin Diagn Lab Immunol 2004;11:1130–3.
- CDC. Arboviral diseases, neuroinvasive and non-neuroinvasive: 2015 case definition. Atlanta, GA: US Department of Health and Human Services, CDC; 2015. Available at http://wwwn.cdc.gov/nndss/conditions/arboviral-diseases-neuroinvasive-and-non-neuroinvasive/case-definition/2015/.
- CDC. West Nile virus—for healthcare providers: diagnostic testing. Atlanta, GA: US Department of Health and Human Services, CDC; 2015. Available at http://www.cdc.gov/westnile/healthCareProviders/healthCareProviders-Diagnostic.html.
* These authors contributed equally to this report.
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