The first known concurrent outbreaks of St. Louis encephalitis virus (SLEV) and West Nile virus (WNV) occurred this year in Arizona, according to a report by Heather Venkat, D.V.M., of the Arizona Department of Health Services and her colleagues.
SLEV and WNV are closely related mosquito-borne flaviviruses that can cause outbreaks of acute febrile illness and neurologic disease. They are endemic in the United States and have the same Culex species mosquito vectors and bird hosts.
As of Nov. 24, 2015, a total of 117 cases of flavivirus disease had been reported to the Arizona Department of Health Services: 75 WNV, 19 SLEV, and 23 unspecified flavivirus disease cases. The vast majority of these cases (103) occurred from July through September. Seventy-nine of the flavivirus patients had neuroinvasive disease (for example, meningitis, encephalitis, or acute flaccid paralysis): 47 with WNV infection 17 with SLEV infection, and 15 with unspecified flavivirus infection. Among all flavivirus patients, 86 patients were hospitalized and 5 patients died.
“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 [request only] WNV testing and no confirmatory testing is conducted. Health care providers should consider both WNV and SLEV infection in the differential diagnosis of cases of aseptic meningitis and encephalitis and obtain appropriate cerebrospinal fluid, serum specimens, or both for laboratory testing,” the authors wrote.
Read the full study in MMWR.