Neuropathogenic Viruses Eastern Equine Encephalitis virus , Western Equine Encephalitis virus, West Nile virus

1. Bilal Sami Al-Moshaigah
Neuropathogenic Viruses

2. Learning Objectives
Neuropathogenic Viruses
Eastern Equine Encephalitis virus
Western Equine Encephalitis virus
West Nile virus
Main properties Pathogenic mechanism Clinical significance Laboratory diagnosis

3. Introduction
Arbovirus Acronym for arthropod-borne virus and highlights the fact that these viruses are
transmitted by arthropods
Primarily mosquitoes and ticks
Blood-filled proboscis and the abdomen engorged with blood
It is a collective name for a large group of diverse viruses, more than 600
Important Properties
Most arboviruses are classified in three families, namely, Togaviruses,
Flaviviruses, and bunyaviruses

4. Important Properties
Togaviridae Flavivirdae
 Icosahedral nucleocapsid
 Enveloped
 Single-stranded
 Positive-polarity RNA genome
Alphaviruses
70 nm in diameter 40 to 50 nm in diameter
Flaviviruses
Family
Genus
West Nile virus
 EEEV
 WEEV

5. Transmission
Humans are dead-end hosts ?
Must be present in the bloodstream of the
vertebrate host (viremia) in sufficiently high titer
Extrinsic incubation period, must pass before
the virus has replicated sufficiently for the saliva
to contain enough virus
Only the female ?
Only she requires a blood meal in
order for progeny to be produced
Ranges from 7 to 14 days
Some are transmitted by vertical “transovarian”
passage from the mother tick to her offspring.
Value?

6. Pathogenic mechanism
On biting a host, the mosquito regurgitates virus-containing saliva
into the bloodstream
female
Circulates freely in the host’s plasma and Comes
into contact with susceptible target cell
Flulike symptoms
The virus gains access to the brain by infecting
the endothelial cells lining the small vessels of
the brain or the choroid plexus

7. Clinical significance
Most human arboviral infections are asymptomatic
A minority of infections cause neuroinvasive disease, such as encephalitis and meningitis
The clinical picture usually fits one of three categories
Encephalitis
Hemorrhagic fever
Fever with myalgias, arthralgias, and nonhemorrhagic rash
The pathogenesis of these diseases involves not only the cytocidal effect of the
virus, but also, in some, a prominent immunopathologic component
Eastern equine encephalitis virus
Western equine encephalitis virus
West Nile virus
Immunity following the
infection is lifelong

8. Eastern Equine Encephalitis Virus
Causes the most severe disease and is associated with the highest fatality rate (approximately 50%)
Encephalitis
Characterized by the sudden onset of :
 Severe headache
 Nausea and vomiting
 Fever
 Changes in mental status
 Rapidly progressive downhill course with nuchal rigidity
 Seizures
 Coma
The diagnosis is made by either isolating the virus or demonstrating a rise in antibody titer
Atlantic and Gulf states
If the patient survives, the central nervous system sequelae are usually severe occur in one third of survivors
The disease does not occur in the winter ?

9. Western Equine Encephalitis Virus
Causes disease more frequently than does EEE virus
The virus is transmitted primarily by Culex mosquitoes among the wild bird
The clinical picture of WEE virus infection is similar but less severe, the fatality rate is roughly 2%.
The diagnosis is made by isolating the virus or observing a rise in antibody titer
West of Mississippi
Sequelae are less common
Wild birds

10. West Nile Virus
The clinical picture is one of encephalitis with or without signs of meningitis, typically in a person over 60
years of age
Less than 1% of those infected have symptomatic disease, the mortality rate is roughly 12%.
The laboratory diagnosis can be made by either isolation of the virus from brain tissue, blood, or spinal
fluid or by detection of antibodies in spinal fluid or blood and PCR-based assays are also available.
Detection of IgM antibody is the most efficient diagnostic method
Endemic in Africa; Widespread in United States
The most common cause of neuroinvasive (encephalitis, meningitis) arboviral disease

11. Laboratory
diagnosis
Clinical significance
Main properties
Virus
isolating the virus
or demonstrating a
rise in antibody
titer
 The majority of infections are
subclinical
 May cause acute encephalitis
(most severe disease)
 Family Togaviridae
 Genus Alphavirus
 Enveloped, icosahedral viruses
 Positive sense
 Single-stranded RNA genome
 70 nm in diameter
 Transmitted to humans by mosquitoes
Eastern Equine
Encephalitis virus
 The majority of infections are
subclinical
 May cause acute encephalitis
(less severe disease)
Western Equine
Encephalitis virus
Isolation of the
virus from brain
tissue, blood, or
spinal fluid or by
detection of
antibodies in spinal
fluid or blood and
PCR-based assays
are also available
 Usually self-limited illness
 May result in aseptic meningitis or
meningoencephalitis, especially in
the elderly
 Family Flaviviridae
 Genus flavivirus
 Enveloped, icosahedral viruses
 Positive sense
 Single stranded RNA genome
 40 to 50 nm in diameter
 Transmitted to humans by mosquito
West Nile virus

12. Any Question

13. Reference
Warren E. Levinson (2014) Review of Medical Microbiology and Immunology, 13th edn., New
York, United States: McGraw-Hill Education - Europe.
Neal R. Chamberlain ( 2009) Medical Microbiology: The Big Picture, New York, United States:
McGraw-Hill Education - Europe.
Richard A. Harvey , Cynthia Nau Cornelissen (2012) Lippincott Illustrated Reviews:
Microbiology, Third edn., Philadelphia, United States: Lippincott Williams and Wilkins.