Clinical Evaluation & Disease

Diagnosis & Reporting

St. Louis encephalitis (SLE) virus disease should be considered in any person with a febrile or acute neurologic illness who has had recent exposure to mosquitoes, blood transfusion, or potentially organ transplantation, especially during the summer months in areas where virus activity has been reported.

In addition to other more common causes of encephalitis and aseptic meningitis (e.g., herpes simplex virus and enteroviruses), other arboviruses (e.g., West Nile, La Crosse, Eastern equine encephalitis, and Powassan viruses) should also be considered in the differential etiology of suspected SLE illness.

SLE virus disease is a nationally notifiable condition. All cases should be reported to local public health authorities in a timely manner. Reporting can assist local, state, and national authorities to recognize outbreaks and to implement control measures to reduce future infections.

Clinical Signs & Symptoms

Less than 1% of SLE virus SLEV infections are clinically apparent, and the vast majority of infections remain undiagnosed. The incubation period for SLE virus disease ranges from 4 to 14 days. Initial symptoms include fever, headache, vomiting, and malaise. The disease can progress to encephalitis, meningoencephalitis, or aseptic meningitis. Symptoms of encephalitis may include altered mental status, seizures, speech problems (aphasia, dysarthria), paresis or paralysis, movement disorders, and cranial nerve palsies.

About 40% of children and young adults with SLE virus disease develop only fever and headache or aseptic meningitis; almost 90% of elderly persons with SLE virus disease develop encephalitis. The overall case-fatality is 5 to 20%. The risk of fatal disease increases with age.

Clinical Evaluation

In acute SLE virus neuroinvasive disease cases, cerebrospinal fluid (CSF) examination shows a moderate (typically lymphocytic) pleocytosis. CSF protein is elevated in about a half to two-thirds of cases. Computed tomography (CT) brain scans are usually normal; electroencephalographic (EEG) results often show generalized slowing without focal activity.


Approximately 5 to 20% of SLE virus disease cases are fatal. One-third to half of patients with neuroinvasive SLE virus disease can experience ongoing symptoms such as lethargy, irritability, tremors, memory loss, or headaches but most symptoms resolve within months to years. However, 5 to 10% of patients with neuroinvasive disease might have symptoms that persist for longer, including gait and speech disturbances, tremors, or psychological/behavioral problems.