eMedicine Specialties > Infectious Diseases > CNS Infections

St. Louis Encephalitis

Author: Charurut Somboonwit, MD, Assistant Professor of Medicine, Division of Infectious Disease and International Medicine, University of South Florida College of Medicine; Senior Physician, Polk County Health Department
Coauthor(s): Emad Soliman, MD, MSc, Consulting Staff, Department of Neurology, St John's Riverside Hospital; Eduardo Gotuzzo, MD, Adjunct Professor, Department of Medicine, University of Alabama School of Medicine; Norvin Perez, MD, Clinical Assistant Professor of Emergency Medicine, Albert Einstein College of Medicine; Consulting Staff, Department of Emergency Medicine, Montefiore Medical Center; Fariba M Donovan, MD, PhD, Fellow, Department of Internal Medicine, Division of Infectious Diseases, University of South Florida
Contributor Information and Disclosures

Updated: Jan 29, 2009

Introduction

Background

St. Louis encephalitis virus (SLEV) belongs to the family Flaviviridae (group B arborviruses). The principal reservoirs of SLEV include wild birds and domestic fowl, and the virus is transmitted to humans by mosquitos (Culex tarsalis, Culex quinquefasciatus, Culex pipiens). The clinical manifestations of SLEV infection range from mild flulike syndromes to fatal encephalitis.

Pathophysiology

A primary viremia follows reproduction of the SLEV at the site of inoculation. In cases of subclinical SLEV infection, the pathogen is cleared by the reticuloendothelial system (the liver, spleen, and lymph nodes) before invasion of the CNS can occur.

Continued viral replication gives rise to a secondary viremia. Saturation of the filtering capacity of the reticuloendothelial system enables invasion of the CNS. The probability of CNS infection depends on the efficiency of viral replication at the extraneural sites and the degree of viremia. The virus enters the CNS either through the cerebral capillary endothelial cell/astrocyte complex (the blood-brain barrier) or across fenestrated endothelium in areas of the CNS that do not have the usual blood-brain barrier capacity (ie, choroid plexus). Rarely, SLEV tracts retrograde from a peripheral site (the olfactory nerve) that was infected during the viremia.

Many of the flaviviruses exhibit various types of neurotropism. The specific neurotropic mechanisms in SLEV have not been established.

Focal neuronal degeneration with necrosis occurs, leading to the development of glial nodules. Upon healing, spongiform changes occur. The perivascular inflammatory infiltrates are made up of activated T cells and macrophages.

Frequency

United States

SLEV is widely distributed in the United States, and infections occur as periodic focal outbreaks of encephalitis in the midwestern, western, and southwestern United States, followed by years of sporadic cases. SLEV infections have caused large urban epidemics of encephalitis. Outbreaks occur from August through October. During the last 5 decades, 10,000 cases were reported.

St. Louis encephalitis is mostly sporadic. The annual incidence is 0.003-0.752 cases per 100,000 population. The median frequency is 35 cases per year. In the United States, SLEV exhibits an endemic pattern primarily along the western coast and sporadic infections in the east.

International

Outbreaks of SLEV infection have occurred in Canada and Mexico. Sporadic cases have occurred in South America and the Caribbean.

Mortality/Morbidity

St. Louis encephalitis carries a mortality rate of 5-30% (higher among older individuals). Of patients with clinical illness, 20% develop sequelae, including irritability, memory loss, various types of movement disorders, and motor deficits. Seizures and coma are unusual. Focal deficits are uncommon, except for cranial nerve palsies.

Sex

SLEV infection is reported more often in males than in females, probably due to more outdoor exposure.

Age

The severity of symptoms of SLEV infection increases with age. Older patients are at greater risk of developing clinical illness (especially those >60 y). The literature has reported that up to 90% of elderly individuals who are infected with SLEV develop clinical illness.

Clinical

History

After an incubation period of 4-21 days, St. Louis encephalitis virus (SLEV) infection can cause mild febrile illness, aseptic meningitis, or encephalitis. A prodrome of malaise and fever accompanied by cough and sore throat characterizes the onset of St. Louis encephalitis. Headache, nausea, vomiting, confusion, disorientation, irritability, tremors, and, occasionally, convulsions follow.

  • Several days after the onset of infection, the patient will defervesce, with gradual neurological improvement over several days.
  • Chronic infection does not occur, and relapsing infection has not been reported.
  • Risks factors for clinical SLEV infection include the following:
    • Individuals older than 70 years have a 10-fold increased risk of clinical illness.
    • Exposure to endemic areas or outdoor activities increases risk.
    • Male sex is a risk factor.
    • Most SLEV infection outbreaks occur in people of low socioeconomic status, which is probably due to less access to heath care and poor environmental control of mosquitos.
    • Comorbidity with atherosclerosis, heart disease, and hypertension is associated with an increased mortality rate of St. Louis encephalitis.

Physical

  • Most patients with SLEV infection develop a significant fever.
  • Meningismus may or not be present.
  • Photophobia is seldom present.
  • The neurological examination findings are usually normal.
    • Five percent of patients with SLEV infection present with deep coma, and 25% develop cranial nerve palsies. Fewer exhibit ataxia.
    • Seizures are unusual and occur more frequently in children.

Causes

St. Louis encephalitis is caused by an enveloped, single-stranded, positive-sense RNA virus of the Flaviviridae subgroup.

  • SLEV has a relatively conserved nucleotide sequence.
  • SLEV is an arbovirus transmitted via a mosquito vector from wild birds to humans. Birds, primarily passerine birds such as finches and sparrows, are the principal hosts of the virus. Vectors include the mosquitoes C pipiens, C tarsalis, and C quinquefasciatus.

More on St. Louis Encephalitis

Overview: St. Louis Encephalitis
Differential Diagnoses & Workup: St. Louis Encephalitis
Treatment & Medication: St. Louis Encephalitis
Follow-up: St. Louis Encephalitis
References
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References

  1. Rahal JJ, Anderson J, Rosenberg C, Reagan T, Thompson LL. Effect of interferon-alpha2b therapy on St. Louis viral meningoencephalitis: clinical and laboratory results of a pilot study. J Infect Dis. 2004;15;190(6):1084-7. [Medline].

  2. Centers for Disease Control and Prevention. Arboviral disease--United States, 1994. MMWR Morb Mortal Wkly Rep. Sep 8 1995;44(35):641-4. [Medline].

  3. Centers for Disease Control and Prevention. From the Centers for Disease Control and Prevention. Arboviral disease-- United States, 1994. JAMA. Oct 11 1995;274(14):1110-2. [Medline].

  4. Cerna F, Mehrad B, Luby JP. St. Louis encephalitis and the substantia nigra: MR imaging evaluation. AJNR Am J Neuroradiol. Aug 1999;20(7):1281-3. [Medline].

  5. Chandler LJ, Nordoff NG. Identification of genetic variation among St. Louis encephalitis virus isolates, using single-strand conformation polymorphism analysis. J Virol Methods. Jul 1999;80(2):169-78. [Medline].

  6. Chiles RE, Reisen WK. A new enzyme immunoassay to detect antibodies to arboviruses in the blood of wild birds. J Vector Ecol. Dec 1998;23(2):123-35. [Medline].

  7. Chu CT, Howell DN, Morgenlander JC. Electron microscopic diagnosis of human flavivirus encephalitis: use of confocal microscopy as an aid. Am J Surg Pathol. Oct 1999;23(10):1217-26. [Medline].

  8. Day JF. Predicting St. Louis encephalitis virus epidemics: lessons from recent, and not so recent, outbreaks. Annu Rev Entomol. 2001;46:111-38. [Medline].

  9. Day JF, Stark LM. Avian serology in a St. Louis encephalitis epicenter before, during, and after a widespread epidemic in south Florida, USA. J Med Entomol. Sep 1999;36(5):614-24. [Medline].

  10. Day JF, Stark LM. Frequency of Saint Louis encephalitis virus in humans from Florida, USA: 1990-1999. J Med Entomol. Jul 2000;37(4):626-33. [Medline].

  11. Day JF, Stark LM. Transmission patterns of St. Louis encephalitis and eastern equine encephalitis viruses in Florida: 1978-1993. J Med Entomol. Jan 1996;33(1):132-9. [Medline].

  12. Halperin JJ. Encephalitis: Diagnosis and Treatment. New York, NY: Informa Healthcare; 2008.

  13. Glaser C, Lewis P, Wong S. Pet-, animal-, and vector-borne infections. Pediatr Rev. Jul 2000;21(7):219-32. [Medline].

  14. Kramer LD, Presser SB, Hardy JL. Genotypic and phenotypic variation of selected Saint Louis encephalitis viral strains isolated in California. Am J Trop Med Hyg. Aug 1997;57(2):222-9. [Medline].

  15. Naidech A, Elliott D. St. Louis encephalitis with focal neurological signs. Clin Infect Dis. Nov 1999;29(5):1334-5. [Medline].

  16. Pelegrino JL, Suarez M, Guzman MG. [Surveillance of St. Louis encephalitis, Eastern equine and Western equine in the province of Ciego de Avila]. Rev Cubana Med Trop. 1996;48(2):109-13. [Medline].

  17. Reisen WK, Lundstrom JO, Scott TW. Patterns of avian seroprevalence to western equine encephalomyelitis and Saint Louis encephalitis viruses in California, USA. J Med Entomol. Jul 2000;37(4):507-27. [Medline].

  18. Rocco IM, Santos CL, Bisordi I, Petrella SM, Pereira LE, Souza RP, et al. St. Louis encephalitis virus: first isolation from a human in São Paulo State, Brazil. Rev Inst Med Trop Sao Paulo. Sep-Oct 2005;47(5):281-5. [Medline].

  19. Sejvar JJ, Bode AV, Curiel M, Marfin AA. Post-infectious encephalomyelitis associated with St. Louis encephalitis virus infection. Neurology. Nov 9 2004;63(9):1719-21. [Medline].

  20. Steele MT. Update on emerging infections from the Centers for Disease Control and Prevention. Arboviral infections of the central nervous system--United States, 1996-1997. Ann Emerg Med. Mar 1999;33(3):365-7. [Medline].

  21. Venugopal K, Jiang WR, Gould EA. Immunity to St. Louis encephalitis virus by sequential immunization with recombinant vaccinia and baculovirus derived PrM/E proteins. Vaccine. Aug 1995;13(11):1000-5. [Medline].

  22. Booss J, Esiri MM. Viral Encephalitis in Humans. Washington, DC: ASM Press; 2003.

  23. Vodkin MH, Szymzcak L, Koll M. Mosquito productivity and surveillance for St. Louis encephalitis virus in Chicago during 1993. J Am Mosq Control Assoc. Sep 1995;11(3):302-6. [Medline].

  24. Wasay M, Diaz-Arrastia R, Suss RA. St. Louis encephalitis: a review of 11 cases in a 1995 Dallas, Tex, epidemic. Arch Neurol. Jan 2000;57(1):114-8. [Medline].

  25. Wegbreit J, Reisen WK. Relationships among weather, mosquito abundance, and encephalitis virus activity in California: Kern County 1990-98. J Am Mosq Control Assoc. Mar 2000;16(1):22-7. [Medline].

  26. Wootton SH, Kaplan SL, Perrotta DM, Martin DA, Campbell GL. St. Louis encephalitis in early infancy. Pediatr Infect Dis J. Oct 2004;23(10):951-4. [Medline].

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Further Reading

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Keywords

St. Louis encephalitis, Saint Louis encephalitis, SLEV, mosquito-borne virus, Culex tarsalis, C tarsalis, Culex quinquefasciatus, C quinquefasciatus, Culex pipiens, C pipiens, encephalitis, flavivirus, Flaviviridae, arbovirus, avian virus, bird virus, passerine bird virus, St Louis encephalitis, arbovirus encephalitis, arboviral encephalitis

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Contributor Information and Disclosures

Author

Charurut Somboonwit, MD, Assistant Professor of Medicine, Division of Infectious Disease and International Medicine, University of South Florida College of Medicine; Senior Physician, Polk County Health Department
Charurut Somboonwit, MD is a member of the following medical societies: American College of Physicians, American Medical Association, and Infectious Diseases Society of America
Disclosure: Nothing to disclose.

Coauthor(s)

Emad Soliman, MD, MSc, Consulting Staff, Department of Neurology, St John's Riverside Hospital
Emad Soliman, MD, MSc is a member of the following medical societies: American Academy of Neurology and American Medical Association
Disclosure: Nothing to disclose.

Eduardo Gotuzzo, MD, Adjunct Professor, Department of Medicine, University of Alabama School of Medicine
Disclosure: Nothing to disclose.

Norvin Perez, MD, Clinical Assistant Professor of Emergency Medicine, Albert Einstein College of Medicine; Consulting Staff, Department of Emergency Medicine, Montefiore Medical Center
Norvin Perez, MD is a member of the following medical societies: American College of Emergency Physicians and American Medical Association
Disclosure: Nothing to disclose.

Fariba M Donovan, MD, PhD, Fellow, Department of Internal Medicine, Division of Infectious Diseases, University of South Florida
Fariba M Donovan, MD, PhD is a member of the following medical societies: American College of Physicians and American Society for Microbiology
Disclosure: Nothing to disclose.

Medical Editor

Mary Nettleman, MD, MS, Chair, Department of Medicine, Michigan State University
Mary Nettleman, MD, MS is a member of the following medical societies: American College of Physicians, American Federation for Medical Research, American Society for Microbiology, Central Society for Clinical Research, Infectious Diseases Society of America, and Society of General Internal Medicine
Disclosure: Nothing to disclose.

Pharmacy Editor

Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine
Disclosure: Nothing to disclose.

Managing Editor

John L Brusch, MD, FACP, Assistant Professor of Medicine, Harvard Medical School; Consulting Staff, Department of Medicine and Infectious Disease Service, Cambridge Health Alliance
John L Brusch, MD, FACP is a member of the following medical societies: American College of Physicians and Infectious Diseases Society of America
Disclosure: Nothing to disclose.

CME Editor

Eleftherios Mylonakis, MD, Clinical and Research Fellow, Department of Internal Medicine, Division of Infectious Diseases, Massachusetts General Hospital
Eleftherios Mylonakis, MD is a member of the following medical societies: American Association for the Advancement of Science, American College of Physicians, American Society for Microbiology, and Infectious Diseases Society of America
Disclosure: Nothing to disclose.

Chief Editor

Burke A Cunha, MD, Professor of Medicine, State University of New York School of Medicine at Stony Brook; Chief, Infectious Disease Division, Winthrop-University Hospital
Burke A Cunha, MD is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, and Infectious Diseases Society of America
Disclosure: Nothing to disclose.

 
 
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