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Viral Encephalitis. John Nuara, Salwa Touma, Kelly Wines Microbiology and the Control of Infectious Diseases April 22, 2003. Kelly. Introduction Transmission Outbreaks Reservoir. Introduction to Viral Encephalitis. Inflammation of brain tissue caused by a viral infection

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Viral Encephalitis

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Viral encephalitis

Viral Encephalitis

John Nuara, Salwa Touma, Kelly Wines

Microbiology and the Control of Infectious Diseases

April 22, 2003



  • Introduction

  • Transmission

  • Outbreaks

  • Reservoir

Introduction to viral encephalitis

Introduction to Viral Encephalitis

  • Inflammation of brain tissue caused by a viral infection

  • Can have acute viral encephalitis or post-infectious encephalomyelitis

    • Acute viral encephalitis is caused by direct viral infection of neural cells

    • Post-infection encephalomyelitis follows infection with various viral agents

Primary cause

Primary Cause

  • Arboviruses are the most common causes of viral encephalitis

  • Arbovirus stands for arthropod-borne viruses

  • There are 3 virus families associated with encephalitis

    • Togaviridae (Alphavirus) – most common

    • Flaviviridae (Flavavirus)

    • Bunyaviridae (Bunyavirus)

Alphavirus surface

Alphavirus Surface

  • Spike-like structures on virion surface

  • Spikes are used to attach to susceptible animal cells

  • RNA-containing viruses

Transmission of arbovirus

Most encephalitides are transmitted by mosquitoes

Other types are transmitted by ticks

Transmission of Arbovirus

Transmission cont

Transmission cont..

  • Infection occurs when the infected arthropod takes a blood meal

  • Most cases of arboviral encephalitis occur from June – September, when arthropods are most active

  • In warmer parts of the country, cases can occur year-round

Other causes

Other Causes

  • Herpes simplex, type 1 (HSV-1) and type 2 (HSV-2)

  • Enteroviruses – viruses that typically occur in the GI tract

  • Coltivirus

  • Measles

  • Mumps

  • Rabies

  • Rubella

  • Lymphatic choriomeningitis virus

Other cause cont

Other Cause cont...

  • Cytomegalovirus

  • Epstein-Barr virus (EBV)

  • Human immunodeficiency virus (HIV)

  • Varicella-zoster virus (VZV)

  • Influenza A and B

    *Viruses are not the only cause of encephalitis; bacteria, fungus, parasites, toxins, and allergic reactions can also cause it.

Most common types of arboviral encephalitis in the us

Most Common Types of Arboviral Encephalitis in the US

  • Eastern equine encephalitis

  • Western equine encephalitis

  • La Crosse encephalitis

  • St. Louis encephalitis

  • West Nile encephalitis

Eastern equine encephalitis eee

Eastern Equine Encephalitis (EEE)

  • Alphavirus

  • Mosquito-borne

  • First identified in the 1930’s

  • Found

    • Eastern Seaboard

    • Gulf Coast

    • Some inland Midwestern area

  • Most common during warm months

Eastern equine encephalitis cont

Eastern Equine Encephalitis cont..

  • 153 confirmed cases in US since 1964

  • One-third of infected individuals die; many others suffer permanent brain damage

  • In addition to humans, EEE virus can also produce severe disease in horses, some birds, and puppies

Eastern equine outbreaks in us

Eastern Equine Outbreaks in US

Western equine encephalitis wee

Western Equine Encephalitis (WEE)

  • Alphavirus

  • Mosquito-borne

  • First isolated in 1930’s from the brain of a horse with encephalitis

  • Found mainly in western parts of US

  • Most common in June and July

Western equine encephalitis cont

Western Equine Encephalitis cont..

  • 639 confirmed cases in US since 1964

  • Usually causes a mild infection in people

  • Children <1 yr. often suffer permanent complications

  • 3% of infected individuals die

Western equine outbreaks in us

Western Equine Outbreaks in US

La crosse encephalitis lac

La Crosse encephalitis (LAC)

  • Bunyavirus

  • Transmitted by the tree-hole mosquito

  • Principal vertebrate hosts

    • Eastern chipmunks

    • Gray Squirrels

    • Red Foxes

La crosse encephalitis cont

La Crosse Encephalitis cont..

  • Discovered in La Crosse, Wisconsin in 1963

  • Since then, the virus has been identified in several Midwestern and Mid-Atlantic states

  • Approximately 75 new cases of LAC are reported per year

  • Most cases occur in children <16 yrs.

  • Fatality ratio <1%

La crosse outbreaks in us

La Crosse Outbreaks in US

St louis encephalitis sle

St. Louis Encephalitis (SLE)

  • Flavavirus

  • Mosquito-borne

  • Discovered in St. Louis, Missouri in 1933

    • 1095 reported cases

    • 201 deaths

  • During the same year, human SLE epidemics also occurred in

    • Kansas City, Missouri

    • St. Joseph, Missouri

    • Louisville, Kentucky

St louis encephalitis cont

St. Louis encephalitis cont..

  • In hindsight, the first known SLE epidemic actually occurred in 1932 in Paris, Illinois

    • 38 reported cases

    • 14 deaths

  • Since 1933, there have been at least 41 outbreaks in US

  • Since 1964, there have been 4, 478 reported human cases; with an average of 128 cases/yr.

  • Outbreaks occurs intermittently

    • Up to 3,000 cases during large outbreaks

    • 20 in other years

St louis encephalitis cont1

St. Louis Encephalitis cont..

  • Outbreaks most commonly occur in the late summer or early fall; year-round in milder climate (such a St. Louis)

  • Can occur throughout most of the US

  • Less than 1% of SLE viral infections are clinically apparent, and most remain undiagnosed

  • Clinical disease most often occurs in children and elderly

    • 7% children die

    • 30% elderly die

St louis outbreaks in us

St. Louis Outbreaks in US

West nile virus

West Nile Virus

  • Flavavirus

  • First isolated in the West Nile Province of Uganda in 1937

  • First epidemic occurred in Israel in the 1950s

  • Primarily affects individuals in Africa, the Middle East, western Asia, and Europe

  • In late August and September 1999, there was an outbreak in New York City and neighboring counties

    • Outbreak was initially attributed to St. Louis Encephalitis

    • Most likely introduced to North America through international travel of infected persons to New York or by imported infected birds

West nile virus cont

West Nile Virus cont..

  • The virus can infect people, horses, many birds, and some other animals

  • Infection usually occurs in the late summer or early fall

  • Most people who become infected with West Nile Virus have either no symptoms or only mild ones.

  • A small percentage (<1%) of patients will develop more severe diseases from infection, including West Nile Encephalitis

  • Infections are most common in children and the elderly and as many as 10% of infected individuals will die

West nile outbreaks in us

West Nile Outbreaks in US

Mosquito breeding grounds

Mosquito Breeding Grounds

Discarded tires

Discarded Tires

Lakes and ponds

Lakes and Ponds



Plants are natural incubators

Plants are natural incubators



  • Structure

  • Pathogenesis

  • Symptoms

  • Diagnosis

Viral encephalitis


  • Treatment

  • Prevention

  • Genetic Engineering

  • Weaponization

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