First Concurrent St. Louis Encephalitis Virus and West Nile Virus Disease Outbreak in the United States — Arizona, 2015

BACKGROUND:  St. Louis encephalitis virus (SLEV) and West Nile virus (WNV) are closely related mosquito-borne flaviviruses that can cause acute febrile illness and neuroinvasive disease. No outbreaks involving both viruses circulating in the same location and at the same time have been identified previously. We describe the epidemiology of a concurrent WNV and SLEV disease outbreak that occurred in Arizona during 2015.

METHODS:  We reviewed medical records for WNV and SLEV disease cases reported to the Arizona Department of Health Services. We performed enhanced case finding by sending specimens with detectable anti-WNV immunoglobulin M antibodies identified at commercial laboratories to the Arizona Bureau of State Laboratory Services and CDC for WNV and SLEV testing. We defined cases according to national surveillance case definitions; cases indistinguishable by available testing were classified as unspecified flavivirus disease.

RESULTS: During 2015 in Arizona, we identified 75 WNV, 19 SLEV, and 23 unspecified flavivirus disease cases; 103 (88%) occurred during July–September. Eight (53%) of 15 counties reported cases; 45 (60%) WNV, 18 (95%) SLEV, and 17 (74%) unspecified flavivirus disease cases were reported from Maricopa County. Seventy-seven (66%) patients were aged ≥50 years (median: 54 years), and 61 (52%) were male. Seventy-nine (68%) patients had neuroinvasive disease, including 47 (59%) with WNV, 17 (22%) with SLEV, and 15 (19%) with unspecified flavivirus infections; 54 (68%) patients with neuroinvasive disease were aged ≥50 years, and 47 (59%) were male. Eighty-six (74%) patients were hospitalized; 5 (4%) died. 

CONCLUSIONS: This is the first known concurrent SLEV and WNV disease outbreak in the United States. Because these viruses can cocirculate, clinicians should consider both infections in the differential diagnosis of neuroinvasive disease.