Key Points
- St. Louis encephalitis virus can cause an acute febrile illness or neurologic disease, including meningitis and encephalitis.
- The incubation period ranges from 4–14 days.
- The overall fatality in persons with St. Louis encephalitis is approximately 5–20%, and 5–10% have long-term neurologic sequelae.
Clinical presentation
St. Louis encephalitis should be considered in any person with an acute febrile or neurologic illness who has had recent exposure to mosquitoes, especially during the summer months in areas where virus activity has been reported.
Other causes of encephalitis and aseptic meningitis should also be considered, as appropriate (e.g., herpes simplex viruses, enteroviruses, West Nile virus, eastern equine encephalitis virus, La Crosse virus, Powassan virus).
Signs and symptoms
The clinical presentations of St. Louis encephalitis and West Nile virus disease are similar. Fewer than 1% of St. Louis encephalitis virus infections are clinically apparent, and the vast majority of infections remain undiagnosed. The incubation period for St. Louis encephalitis ranges from 4–14 days.
Initial symptoms include fever, headache, vomiting, and malaise. The disease can progress to encephalitis, meningoencephalitis, or aseptic meningitis. Symptoms of encephalitis might include altered mental status, seizures, speech problems (aphasia, dysarthria), paresis or paralysis, movement disorders, and cranial nerve palsies. Older age is a risk factor for severe disease.
Clinical assessment
In acute St. Louis encephalitis, 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.
Outcomes
Approximately 5–20% of St. Louis encephalitis cases are fatal. One-third to half of patients with St. Louis encephalitis can experience ongoing symptoms such as lethargy, irritability, tremors, memory loss, or headaches, but most symptoms resolve within months to years. However, 5–10% of patients with neuroinvasive disease might have symptoms that persist for longer, including gait and speech disturbances, tremors, or psychological/behavioral problems.
- Curren EJ, Lindsey NP, Fischer M, Hills SL. St. Louis encephalitis virus disease in the United States, 2003-2017. Am J Trop Med Hyg. 2018 ;99(4):1074-1079. doi: 10.4269/ajtmh.18-0420
- Venkat H, Krow-Lucal E, Hennessey M, Jones J, Adams L, Fischer M, et al. Concurrent outbreaks of St. Louis encephalitis virus and West Nile virus disease – Arizona, 2015. MMWR Morb Mortal Wkly Rep. 2015;64(48):1349-1350. doi: 10.15585/mmwr.mm6448a5
- Venkat H, Krow-Lucal E, Kretschmer M, Sylvester T, Levy C, Adams L, et al. Comparison of characteristics of patients with West Nile virus or St. Louis encephalitis virus neuroinvasive disease during concurrent outbreaks, Maricopa County, Arizona, 2015. Vector Borne Zoonotic Dis. 2020;20(8):624-629. doi: 10.1089/vbz.2019.2572
- Sejvar JJ, Bode AV, Curiel M, Marfin AA. Post-infectious encephalomyelitis associated with St. Louis encephalitis virus infection. Neurology. 2004;63(9):1719-1721. doi: 10.1212/01.wnl.0000143061.63041.07
- Wootton SH, Kaplan SL, Perrotta DM, Martin DA, Campbell GL. St. Louis encephalitis in early infancy. Pediatr Infect Dis J. 2004;23:951-954. doi: 10.1097/01.inf.0000141739.70357.db
- Centers for Disease Control and Prevention. St. Louis encephalitis outbreak – Arkansas, 1991. MMWR Morb Mortal Wkly Rep. 1991;40(35):605-607.
- Centers for Disease Control and Prevention. Update: St. Louis encephalitis – Florida and Texas, 1990. MMWR Morb Mortal Wkly Rep. 1990;39(42):756-759.