Venezuelan equine encephalitis virus: Difference between revisions

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{{Viral Biorealm Family}}
{{Viral Biorealm Family}}


[[Image:honeybees.jpg|thumb|200px|right|<i>Apis mellifera</i>, a common host of Kashmir bee virus. [http://www.ars.usda.gov/IS/pr/2007/071119.htm].]]
[[Image:Venezuelan_equine_encephalitis_virus_fig1.jpg|thumb|400px|right|
 
Electron micrograph of Venezuelan Equine Encephalitis Virus in infected <I>Aedes albopictus</I> cells.  
 
[http://www.microbelibrary.org/asmonly/details.asp?id=2407].]]
 
<br><br>
<br><br>
==Baltimore Classification==
==Baltimore Classification==
Line 12: Line 17:
<br> Family: <i>Togaviridae</i>
<br> Family: <i>Togaviridae</i>
<br> Genus: <i>Alphavirus</i>
<br> Genus: <i>Alphavirus</i>
<br> Complex: New World


==Description and Significance==
==Description and Significance==
<br>
Venezuelan equine encephalitis virus (VEEV) is an important animal and human pathogen found only in the Americas (the New World). VEEV is actually a complex of 7 different species as well as multiple subtypes and varieties [3]. Depending on the subtype, VEEV maintains either an enzootic or an epizootic life cycle. Enzootic species sustain a chain of transmission between a rodent reservoir and mosquito vector. Epizootic species cause epidemics in horses and other animals of agricultural importance. Most human cases of VEEV result from spill-over into the human population from an epizootic outbreak. Human are only incidental and usually dead-end hosts.
Venezuelan equine encephalitis virus (VEEV) is an important animal and human pathogen that was weaponized by the U.S. military in the 1960’s. [http://books.google.com/books?id=fGp53P6ijqsC&pg=PA30&lpg=PA30&dq=chemical+corps+korean+war&source=web&ots=jp78usJez9&sig=1mLxK5_APeDB5fxAh9J3W1ouyMg&hl=en&sa=X&oi=book_result&resnum=6&ct=result#v=onepage&q=chemical%20corps%20korean%20war&f=false]. Although it was weaponized it is still only considered a category B bioterrorism threat by the Centers for Disease Control and Prevention because it causes only moderate morbidity and low mortality rates. It is categorized as biosafety level 3 (BSL 3) agent (Bio safety level 3) which requires researchers to work in a negative pressure laboratory with extensive safety protocols. [http://www.bt.cdc.gov/agent/agentlist-category.asp]
<br><br>
In the 1960's VEEV was weaponized by the U.S. and the U.S.S.R. [4]. The virus is considered a category B select bioterrorism threat by the U.S. government because it causes only moderate morbidity and low mortality in humans but severe morbidity and mortality in animals.
<br><br>
The virus has a biosafety level of 3 (BSL 3). It can only be worked with inside of specially equipped laboratories designed to deal with BSL 3 agents [5]. although VEEV is transmitted by mosquitoes in nature it is a very good aerosol. The virus has been implicated in more than 150 cases of accidental laboratory exposures that were not associated with cuts or needle pricks [6].
<br><br>
VEEV is an arbovirus (arthropod-virus), a general term for a virus that is transmitted via a hematophagous arthropod vector such as mosquitoes, flies and ticks. To be considered a true arbovirus there must be replication of the virus within the vector and VEEV is no exception [3].
<br><br>
<br><br>
VEEV is part of a group of alphaviruses that cause arthritic and encephalitic disease in birds, horses, rodents and humans. This group includes O'nyong'nyong virus, Chikungunya virus, Western equine encephalitis virus, and Eastern equine encephalitis virus. Alphaviruses are not just importance human and animal pathogens but have also been used as a model system in the study of enveloped virus structure and as viral vectors in gene therapy [add the gene therapy thing].[1] (sounds weird)
The genus alphaviruses is included in the general group of Alphaviruses cause arthritic and encephalitic disease in mammals. This group includes the pathogens O'nyong'nyong virus, Chikungunya virus, Western equine encephalitis virus, Eastern equine encephalitis virus, and Venezuelan equine encephalitis. Alphaviruses are not only important human and animal pathogens but have also been used as model systems for the study of enveloped virus structure and as viral vectors in gene therapy research [2].
 
==Genome Structure==
==Genome Structure==
<br>
[[Image:Genome_organization_of_VEEV.jpg|thumb|600px|center|
VEEV has an 11.5 kb (+) sense ssRNA genome. The genome contains two reading frames that produce two different polyproteins. The structural polyprotein contains 5 proteins related to capsid formation and envelope proteins. The other polyprotein encodes four proteins responsible for genome replication including a viral RNA-dependent RNA polymerase. (MORE HERE!)more about the genome and the stuff it codes for.
 
Venezuelan equine encephalitis virus (VEEV) genome  
 
[http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TD4-4XG52JN-M&_user=7774802&_coverDate=11%2F05%2F2009&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_acct=C000062877&_version=1&_urlVersion=0&_userid=7774802&md5=8024782bd6bb05c40cdbb28a254145b9&searchtype=a].]]


The genome contains two open reading frames each coding for a polyprotein. These ORFs are separated by and intergenic region and flanked with short non-coding regions. [6]  
VEEV has a non-segmented 11.4 kb (+) sense ssRNA genome with a poly (A) tail. The genome contains two reading frames that encode for two different polyproteins. Two-thirds of the genome starting from the 5’ end encode for a polyprotein that contains the four nonstructural proteins nsP1, nsP2, nsP3, and nsP4 (viral RNA-dependent RNA polymerase) that are required for genome replication. The last third of the genome encodes for a polyprotein with the capsid and envelope proteins CP, pE2, 6K, and E1 [7].
<br><br>
<br><br>
The 5’ ORF codes the non-structural polyprotein which includes three helicase domains, a 3C-protease domain, and 8 RNA polymerase domains (including an RNA-dependent RNA polymerase). The 3’ ORF codes for the structural polyprotein which contains two picornavirus-like capsid protein domains and other capsid proteins. [6]
The viral RNA itself contains four conserved sequence elements (CSEs) that interact with various viral and host proteins involved in the replication, transcription, and packaging of viral RNA. A small non-translated region at the beginning of the 5’ end of the genome (CSE1) appears to fold up into a stem-loop structure that acts as a promoter for (+) RNA strand replication from a (-) RNA template. CSE2 is found near the 5’ end as well but it is within the coding region for nsP1 and may be a promoter for the synthesis of (-) strand template RNA from genomic (+) strand RNA. CSE4, located just upstream of the poly (A) tail near the 3’ end appears to be involved in promoting the synthesis of template RNA as well. CSE3 is located between the two ORFs and is required for transcription of subgenomic RNA (smaller sections of mRNA that encode for only part of the genome). Just upstream Near CSE4 there are also repeated sequence elements 40-60-nt in length that bind to host proteins involved in the translation of viral RNA [2].


==Virion Structure of Venezuelan equine encephalitis virus==
==Virion Structure of Venezuelan equine encephalitis virus==
<br>Venezuelan equine encephalitis virus is an enveloped isohedral virion. (sounds bad) consists of a non-enveloped, icosohedral capsid. The capsid is constructed from 3 structural proteins [5]. The capsid appears round and is approximately 30 nm in width [8].
<br>Venezuelan equine encephalitis virus has an enveloped isohedral nucleocapsid. The capsid has a triangulation of t=4. Virions appear round and are 65-70 nm in diameter. The envelope surrounding the nucleocapsid is derived from host lipid bilayer embedded with approximately 240 copies of E1 and E2 glycoproteins. The glycoproteins form heterodimer spikes that link up in an isohedral lattice that also displays t=4 symmetry like the Nucleocapsid [2].


==Reproductive Cycle of Venezuelan equine encephalitis virus in a Host Cell==
==Reproductive Cycle of Venezuelan equine encephalitis virus in a Host Cell==
[[Image:Alphavirusreplicationincell.jpg|thumb|400px|center|
Venezuelan equine encephalitis virus (VEEV) replication in host cell.
[http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2762864/?tool=pubmed].]]
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<br>
Little is specifically known about the KBV reproductive cycle. Some insight into the process can be gained by examining related viruses and examining the patterns of viral RNA and protein expression in infected bees.
VEEV virions attach to a host receptor via the glycosylated envelope protein E2 and possibly E1. The exact host receptor for VEEV is unknown but it is probably fairly common because the virus can infect vertebrate (mammals) as well as invertebrate (mosquitoes) cells. One hypothesis is that single amino acid substitutions in the E2 protein allow the virus to attach to multiple cell surface receptors.
<br><br>
Once attached to the receptor the virus is endocytosed and the pH of the endosome drops and causes the E1-E2 dimer to destabilize and then trimerize with the late endosomal membrane. This trimerization allows for the fusion of the viral envelope with the endosome membrane and the eventual escape of the nucleocapsid into the cytoplasm. The exact mechanism of nucleocapsid uncoating in the cytoplasm is not known but may be related to some sort of destabilization by the initial drop in pH in the endosome.
<br><br>
<br><br>
The infective ability and life cycle of Rhopalosiphum padi virus (RhPV), also of the <i>Cripavirus</i> genus, has been studied to some extent. In cell culture, viral protein is localized to the cytoplasm, as visualized by immunogold labeling. This study also observed clusters of single membrane vesicles associated with virions in the early stages of infection. Crytalline arrays of virus particles and dense amorphous cytoplasmic structures were also observed.[3] Similar observations have been made for related viruses so it is reasonable to think that KBV infection may follow a similar scheme.
(+) Genomic RNA is translated using host ribosomes and the nonstructural polyprotein is processed and the resulting proteins form replication complexes within vacuoles derived from lysosomal and endosomal membranes. Replication complexes begin synthesis of (-) template RNA from which both genomic RNA and subgenomic RNA are transcribed. The subgenomic RNA only encodes for the structural polyprotein that contains envelope and capsid components. The capsid subunit CP autoproteolyses and self assembles into nucleocapsids in the cytoplasm and genomic RNA is encapsidated by an unknown mechanism [2].
<br><br>
<br><br>
Another clue can be taken from the apparent ability of KBV to maintain a latent infection. Since KBV does not have a DNA intermediate, it cannot enter true lysogeny and splice into the host genome but it can remain in the cell without any observable pathology. Molecular evidence for latent infection was found when ELISA (immuno assay)results for viral-capsid proteins and RT-PCR amplification of viral RNA where compared as diagnostic techniques. In a majority of cases, viral RNA was present with little or no viral-capsid proteins.[9] This suggests that the viral RNA can persist in infected cells with limited replication and formation of new virus particles.
The remaining segment of polyprotein translocates into the ER.  E1 and pE2 are folded and glycosylated in the ER and then taken to the golgi body where pE2 is cleaved to form E2 and E3. E3 seems to be involved with the dimerization between E1 and E2 but its exact role in viral replication has yet to be elucidated [2]. From the golgi the E1-E2 heterodimer is transported to the plasma membrane where E2 is required for nucleocapsid fusion with the plasma membrane and eventual viral budding.
<br><br>
During infection VEEV turns off cellular transcription and translation which leads to a decreased response from the innate immune system. Infection of the cell also initiates apoptosis by some unknown mechanism [2].


==Viral Ecology & Pathology==
==Viral Ecology & Pathology==
<br>
<br>
animal hosts
In nature VEEV is only transmitted by mosquitoes but aerosol transmission has been observed in the laboratory. Mosquitoes become infected after feeding on infected blood. The virus eventually escapes the gut of the insect and makes its way to the salivary glands where it replicates. The new host is infected when virus laden saliva is injected into the skin during feeding [3].
skeeters are vectors-what species specifically
what it does-mostly flu-like illness, no biggie BUT can cause encephalitis and death
KBV is known to infect many species of bees across a large geographic distribution. Cases of <i>A. melifera</i> infection have been reported worldwide, although cases involved in CCD have only been reported in the US. KBV is also known to infect <i>A. cerana</i> (Asiatic honey bee) in parts of Southeast Asia and India, where the virus is believed to have originated. KBV has also been reported in populations of <i>Bombus spp.</i> (bumble bees) in New Zealand and <i>Vespula germanica</i> (European wasp) in Australia. [6] Some evidence shows that mites such as <i>Varroa destructor</i> not only act as vectors for viral sread, but also as hosts in which viral replication occurs [9].
<br><br>
<br><br>
KBV is spread between bees in a colony and between colonies through many methods of transmission. KBV can be introduced to hives by mites such as <i>Varroa destructor</i> [4] and can also be spread between worker bees and from the queen bee to her offspring. Viral RNA has been amplified from queens and their eggs suggesting that the virus can be transferred via a transovarial route. Viral RNA has also been amplified from food sources such as honey, brood food, and royal jelly. Since these food sources are partially composed of secretions from worker bees, this is a likely method of bee-to-bee transmission. [9]
VEEV can either sustain an enzootic or epizootic life cycle. Enzootic subtypes have a smaller host range and are transmitted between reservoir rodent hosts by mosquitoes of the genus <I> Culex </I>. These subtypes (II-IV) rarely cause disease unless a susceptible human or horse ends up in the middle of the transmission cycle. In order for an enzootic strain to start an epizootic There must be changes in genotype as well as geographic cross-over between susceptible animals and mosquito vectors. This can occur when there is a dramatic change in mosquito habitat such as an unusually wet season that allows the insects to move beyond their normal geographic range. Susceptible hosts must be within geographic range of the vector swamps and forests (this often times occurs when land is cleared for agricultural use) [3].
<br><br>
<br><br>
As mentioned previously, KBV infection of shows no visually detectable signs of pathology [9]. KBV is commonly found in bees co-infected with other related viruses.
The epizootic subtypes of VEEV are IAC and IC. They are responsible for most major human and equine (horse) epidemics. Epizootic subtypes arise from changes in the amino acid sequence of the E2 envelop protein. Even single amino acid substitutions can result in epizootic strains. E2 protein is responsible for binding to host receptors so changes in the structure of this protein affect host range which in turn allows for zoonotic outbreaks (epidemics in animals) that can incidentally involve humans [8]. Changes in E2 structure allow mosquitoes of the genera <I>Ochlerotatus</I> and <I>Psorphora</I> to transmit the disease to horses and other mammals that display high titres of virus in their blood (viremia). This viremia is important in maintaining an epidemic. Luckily for the animals that survive infection (mortality is high) the antibodies produced by the immune systems are strong enough to prevent reinfection.
<br><br>
VEEV in humans presents mostly as a flu-like illness and is often times missed because of a lack of surveillance and laboratory diagnosis. Disease mortality is less than 1% and (encephalitic) only 14% of cases involve the central nervous system. In fatal human cases there is edema, congestion, hemorrhages, meningitis, and encephalitis of the brain and spinal cord. The lungs, liver and lymphoid tissue are also ravaged by the virus [8].
<br><br>
In horses, epizootic VEEV has very high mortality (50-90%). It causes swelling of the brain (encephalitis) that often-times result in fatal seizures [1].
 
==Vaccines and Treatments==
<br>
Currently treatment of VEEV infection is mostly supportive because there are no specific drugs for alphaviruses.
<br><br>
There is currently a vaccine available for both humans and horses. The live attenuated vaccine known as TC-83 is a strain of VEEV that was passed 83 times in guinea pig heart cells [7]. There is also an inactivated form of the vaccine known as C-84 derived from the TC-83 strain. Currently only the C-84 vaccine is licensed for use in horses in the U.S. although countries such as Mexico and Colombia still produce the live vaccine for horses. In the U.S. only at risk military and laboratory personal are vaccinated with the TC-83 strain and some receive C-84 boosters if initial vaccination did not produce sufficient immunity. The vaccine does have side effects that ranged from mild to moderate and did not provided full protection of nonhuman primates challenged by aerosol exposure the route of transmission most likely if VEEV were to be used in a biological terrorist attack [7].<br>


==References==
==References==


[1] Jose, J., J.E. Snyder, R.J. Kuhn. “A structural and functional perspective of alphavirus replication and assembly.” Future Microbiology 4 (2009): 837-856
[1] Walton, T., E., Suchman. “Venezuelan Equine Encephalitis (VEE) Virus” American Society for Microbiology. MicrobeLibrary.org. http://www.microbelibrary.org/asmonly/details.asp?id=2407
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[2] Berenyi, O., T. Bakonyi, I. Derakhshifar, H. Koglberger, N. Nowotny. “Occurrence of Six Honeybee Viruses in Diseased Australian Apiaries.” Applied and Environmental Microbiology 72.4 (2006): 2412-2420.
[2] Jose, J., J.E. Snyder, R.J. Kuhn. “A structural and functional perspective of alphavirus replication and assembly”. Future Microbiology 4 (2009): 837-856
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<br><br>
[3] Boyapalle, S., N. Pal, W.A. Miller, B.C. Bonning. “A glassy-winged sharpshooter cell line supports replication of Rhopalosiphum padi virus (Dicistroviridae). Journal of Invertebrate Pathology 94 (2007): 130-139.
[3] Weaver, S.C., W.K. Reisen. “Present and future arboviral threats”. Journal of Antiviral Reseach 85 (2010): 328-345
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<br><br>
[4] Chen, Y.P., J.S. Pettis, A. Collins, M.F. Feldaufer. “Prevalence and Transmission of Honeybee Viruses.” Applied and Environmental Microbiology 72.1 (2006): 606-611.
[4] Croddy, Eric C. and Hart, C. Perez-Armendariz J., <I>Chemical and Biological Warfare,</I> (Google Books), Springer, 2002, pp. 30-31, (ISBN 0387950761)
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[5] “Cripavirus.” ICTVdB Descriptions <www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/00.101.0.01.htm>
[5] CDC list of Bioterrorism Agents/Diseases http://www.bt.cdc.gov/agent/agentlist-category.asp
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<br><br>
[6] de Miranda, J.R., M. Drebot, S. Tyler, M. Shen, C.E. Cameron, D.B. Stoltz, S.M. Camazine. “Complete nucleotide sequence of Kashmir bee virus and comparison with acute bee paralysis.” Journal of General Virology 85 (2004): 2263-2270.
[6] Zacks, M.A., S., Paessler. “Encephalitic alphaviruses”. Journal of Veterinary Microbiology 140 (2010): 281-286
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[7] “Kashmir bee virus.” NCBI Taxonomy browser <www.ncbi.nlm.nih.gov/Taxonomy/Browser>
[7] Slobodan, P., S.C., Weaver “Vaccines for Venezuelan equine encephalitis” Vaccine 27 (2009): D80-D85
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<br><br>
[8] Oldroyd, Benjamin P. “Unsolved Mystery: What’s Killing American Honey Bees?” PLoS Biology 5.6 (2007).
[8] Weaver, S.C., A.D.T., Barrett “Transmission cycles, host range, evolution and emergence of arboviral disease” Nature Reviews; Microbiology 2 (2004): 789-801
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[9] Shen, M., L. Cui, N. Ostiguy, D. Cox-Foster. “Intricate transmission routes and interactions between picorna-like viruses (Kashmir bee virus and sacbrood virus) with the honeybee host and the parasitic varroa mite.” Journal of General Virology 86 (2005): 2281-2289.
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[10] Slonczewski, J.L., J.W. Foster. “Chapter 6: Virus Structure and Function.” Microbiology: An Evolving Science (2009): 181-217.
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Page authored for [http://biology.kenyon.edu/courses/biol375/biol375syl08.htm BIOL 375 Virology], September 2008
Page authored for [http://biology.kenyon.edu/courses/biol375/biol375syl08.htm BIOL 375 Virology], September 2008


<!--Do not edit or remove this line-->[[Category:Pages edited by students of Joan Slonczewski at Kenyon College]]
<!--Do not edit or remove this line-->[[Category:Pages edited by students of Joan Slonczewski at Kenyon College]]

Latest revision as of 06:48, 10 September 2010

Venezuelan equine encephalitis virus (VEEV)

This is a curated page. Report corrections to Microbewiki.

A Viral Biorealm page on the family Venezuelan equine encephalitis virus

Electron micrograph of Venezuelan Equine Encephalitis Virus in infected Aedes albopictus cells. [1].



Baltimore Classification


Group IV: (+) sense single-stranded RNA viruses

Higher Order Categories


Family: Togaviridae
Genus: Alphavirus
Complex: New World

Description and Significance

Venezuelan equine encephalitis virus (VEEV) is an important animal and human pathogen found only in the Americas (the New World). VEEV is actually a complex of 7 different species as well as multiple subtypes and varieties [3]. Depending on the subtype, VEEV maintains either an enzootic or an epizootic life cycle. Enzootic species sustain a chain of transmission between a rodent reservoir and mosquito vector. Epizootic species cause epidemics in horses and other animals of agricultural importance. Most human cases of VEEV result from spill-over into the human population from an epizootic outbreak. Human are only incidental and usually dead-end hosts.

In the 1960's VEEV was weaponized by the U.S. and the U.S.S.R. [4]. The virus is considered a category B select bioterrorism threat by the U.S. government because it causes only moderate morbidity and low mortality in humans but severe morbidity and mortality in animals.

The virus has a biosafety level of 3 (BSL 3). It can only be worked with inside of specially equipped laboratories designed to deal with BSL 3 agents [5]. although VEEV is transmitted by mosquitoes in nature it is a very good aerosol. The virus has been implicated in more than 150 cases of accidental laboratory exposures that were not associated with cuts or needle pricks [6].

VEEV is an arbovirus (arthropod-virus), a general term for a virus that is transmitted via a hematophagous arthropod vector such as mosquitoes, flies and ticks. To be considered a true arbovirus there must be replication of the virus within the vector and VEEV is no exception [3].

The genus alphaviruses is included in the general group of Alphaviruses cause arthritic and encephalitic disease in mammals. This group includes the pathogens O'nyong'nyong virus, Chikungunya virus, Western equine encephalitis virus, Eastern equine encephalitis virus, and Venezuelan equine encephalitis. Alphaviruses are not only important human and animal pathogens but have also been used as model systems for the study of enveloped virus structure and as viral vectors in gene therapy research [2].

Genome Structure

Venezuelan equine encephalitis virus (VEEV) genome [2].

VEEV has a non-segmented 11.4 kb (+) sense ssRNA genome with a poly (A) tail. The genome contains two reading frames that encode for two different polyproteins. Two-thirds of the genome starting from the 5’ end encode for a polyprotein that contains the four nonstructural proteins nsP1, nsP2, nsP3, and nsP4 (viral RNA-dependent RNA polymerase) that are required for genome replication. The last third of the genome encodes for a polyprotein with the capsid and envelope proteins CP, pE2, 6K, and E1 [7].

The viral RNA itself contains four conserved sequence elements (CSEs) that interact with various viral and host proteins involved in the replication, transcription, and packaging of viral RNA. A small non-translated region at the beginning of the 5’ end of the genome (CSE1) appears to fold up into a stem-loop structure that acts as a promoter for (+) RNA strand replication from a (-) RNA template. CSE2 is found near the 5’ end as well but it is within the coding region for nsP1 and may be a promoter for the synthesis of (-) strand template RNA from genomic (+) strand RNA. CSE4, located just upstream of the poly (A) tail near the 3’ end appears to be involved in promoting the synthesis of template RNA as well. CSE3 is located between the two ORFs and is required for transcription of subgenomic RNA (smaller sections of mRNA that encode for only part of the genome). Just upstream Near CSE4 there are also repeated sequence elements 40-60-nt in length that bind to host proteins involved in the translation of viral RNA [2].

Virion Structure of Venezuelan equine encephalitis virus


Venezuelan equine encephalitis virus has an enveloped isohedral nucleocapsid. The capsid has a triangulation of t=4. Virions appear round and are 65-70 nm in diameter. The envelope surrounding the nucleocapsid is derived from host lipid bilayer embedded with approximately 240 copies of E1 and E2 glycoproteins. The glycoproteins form heterodimer spikes that link up in an isohedral lattice that also displays t=4 symmetry like the Nucleocapsid [2].

Reproductive Cycle of Venezuelan equine encephalitis virus in a Host Cell

Venezuelan equine encephalitis virus (VEEV) replication in host cell. [3].



VEEV virions attach to a host receptor via the glycosylated envelope protein E2 and possibly E1. The exact host receptor for VEEV is unknown but it is probably fairly common because the virus can infect vertebrate (mammals) as well as invertebrate (mosquitoes) cells. One hypothesis is that single amino acid substitutions in the E2 protein allow the virus to attach to multiple cell surface receptors.

Once attached to the receptor the virus is endocytosed and the pH of the endosome drops and causes the E1-E2 dimer to destabilize and then trimerize with the late endosomal membrane. This trimerization allows for the fusion of the viral envelope with the endosome membrane and the eventual escape of the nucleocapsid into the cytoplasm. The exact mechanism of nucleocapsid uncoating in the cytoplasm is not known but may be related to some sort of destabilization by the initial drop in pH in the endosome.

(+) Genomic RNA is translated using host ribosomes and the nonstructural polyprotein is processed and the resulting proteins form replication complexes within vacuoles derived from lysosomal and endosomal membranes. Replication complexes begin synthesis of (-) template RNA from which both genomic RNA and subgenomic RNA are transcribed. The subgenomic RNA only encodes for the structural polyprotein that contains envelope and capsid components. The capsid subunit CP autoproteolyses and self assembles into nucleocapsids in the cytoplasm and genomic RNA is encapsidated by an unknown mechanism [2].

The remaining segment of polyprotein translocates into the ER. E1 and pE2 are folded and glycosylated in the ER and then taken to the golgi body where pE2 is cleaved to form E2 and E3. E3 seems to be involved with the dimerization between E1 and E2 but its exact role in viral replication has yet to be elucidated [2]. From the golgi the E1-E2 heterodimer is transported to the plasma membrane where E2 is required for nucleocapsid fusion with the plasma membrane and eventual viral budding.

During infection VEEV turns off cellular transcription and translation which leads to a decreased response from the innate immune system. Infection of the cell also initiates apoptosis by some unknown mechanism [2].

Viral Ecology & Pathology


In nature VEEV is only transmitted by mosquitoes but aerosol transmission has been observed in the laboratory. Mosquitoes become infected after feeding on infected blood. The virus eventually escapes the gut of the insect and makes its way to the salivary glands where it replicates. The new host is infected when virus laden saliva is injected into the skin during feeding [3].

VEEV can either sustain an enzootic or epizootic life cycle. Enzootic subtypes have a smaller host range and are transmitted between reservoir rodent hosts by mosquitoes of the genus Culex . These subtypes (II-IV) rarely cause disease unless a susceptible human or horse ends up in the middle of the transmission cycle. In order for an enzootic strain to start an epizootic There must be changes in genotype as well as geographic cross-over between susceptible animals and mosquito vectors. This can occur when there is a dramatic change in mosquito habitat such as an unusually wet season that allows the insects to move beyond their normal geographic range. Susceptible hosts must be within geographic range of the vector swamps and forests (this often times occurs when land is cleared for agricultural use) [3].

The epizootic subtypes of VEEV are IAC and IC. They are responsible for most major human and equine (horse) epidemics. Epizootic subtypes arise from changes in the amino acid sequence of the E2 envelop protein. Even single amino acid substitutions can result in epizootic strains. E2 protein is responsible for binding to host receptors so changes in the structure of this protein affect host range which in turn allows for zoonotic outbreaks (epidemics in animals) that can incidentally involve humans [8]. Changes in E2 structure allow mosquitoes of the genera Ochlerotatus and Psorphora to transmit the disease to horses and other mammals that display high titres of virus in their blood (viremia). This viremia is important in maintaining an epidemic. Luckily for the animals that survive infection (mortality is high) the antibodies produced by the immune systems are strong enough to prevent reinfection.

VEEV in humans presents mostly as a flu-like illness and is often times missed because of a lack of surveillance and laboratory diagnosis. Disease mortality is less than 1% and (encephalitic) only 14% of cases involve the central nervous system. In fatal human cases there is edema, congestion, hemorrhages, meningitis, and encephalitis of the brain and spinal cord. The lungs, liver and lymphoid tissue are also ravaged by the virus [8].

In horses, epizootic VEEV has very high mortality (50-90%). It causes swelling of the brain (encephalitis) that often-times result in fatal seizures [1].

Vaccines and Treatments


Currently treatment of VEEV infection is mostly supportive because there are no specific drugs for alphaviruses.

There is currently a vaccine available for both humans and horses. The live attenuated vaccine known as TC-83 is a strain of VEEV that was passed 83 times in guinea pig heart cells [7]. There is also an inactivated form of the vaccine known as C-84 derived from the TC-83 strain. Currently only the C-84 vaccine is licensed for use in horses in the U.S. although countries such as Mexico and Colombia still produce the live vaccine for horses. In the U.S. only at risk military and laboratory personal are vaccinated with the TC-83 strain and some receive C-84 boosters if initial vaccination did not produce sufficient immunity. The vaccine does have side effects that ranged from mild to moderate and did not provided full protection of nonhuman primates challenged by aerosol exposure the route of transmission most likely if VEEV were to be used in a biological terrorist attack [7].

References

[1] Walton, T., E., Suchman. “Venezuelan Equine Encephalitis (VEE) Virus” American Society for Microbiology. MicrobeLibrary.org. http://www.microbelibrary.org/asmonly/details.asp?id=2407

[2] Jose, J., J.E. Snyder, R.J. Kuhn. “A structural and functional perspective of alphavirus replication and assembly”. Future Microbiology 4 (2009): 837-856

[3] Weaver, S.C., W.K. Reisen. “Present and future arboviral threats”. Journal of Antiviral Reseach 85 (2010): 328-345

[4] Croddy, Eric C. and Hart, C. Perez-Armendariz J., Chemical and Biological Warfare, (Google Books), Springer, 2002, pp. 30-31, (ISBN 0387950761)

[5] CDC list of Bioterrorism Agents/Diseases http://www.bt.cdc.gov/agent/agentlist-category.asp

[6] Zacks, M.A., S., Paessler. “Encephalitic alphaviruses”. Journal of Veterinary Microbiology 140 (2010): 281-286

[7] Slobodan, P., S.C., Weaver “Vaccines for Venezuelan equine encephalitis” Vaccine 27 (2009): D80-D85

[8] Weaver, S.C., A.D.T., Barrett “Transmission cycles, host range, evolution and emergence of arboviral disease” Nature Reviews; Microbiology 2 (2004): 789-801


Page authored for BIOL 375 Virology, September 2008