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| − | Also known as: '''''EEV — Alphavirus — Eastern Equine Encephalitis Virus, EEE — Western Equine Encephalitis Virus, WEE — Venezuelan Equine Encephalitis Virus, VEE
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| − | ==Introduction== | + | ====Viral Family==== |
| | + | Members of the Togaviridae are small, lipid- and protein-enveloped RNA viruses. Within this family are disease-causing arboviruses (insect-borne virsuses) of the ''Alphavirus'' genus. |
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| − | '''Family [[Togaviridae - Overview|Togaviridae]]'''
| + | ====Viral Genus==== |
| | + | Alphaviruses are single-stranded, linear, positive-sense RNA viruses, 60-70nm in diameter. |
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| − | '''Genus ''Alphavirus'''''
| + | ====Important Serotypes==== |
| − | Single-stranded, linear, positive-sense RNA viruses, 60-70nm in diameter.<ref name="multiple">Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref>[[Image:Alphavirus.gif|300px|thumb|right|''Surface of an Alphavirus'' JoelMills 2007, Wikimedia Commons]]
| + | During equine and human epidemics of encephalitis in the Western Hemisphere, the most frequently isolated alphaviruses have included: |
| | + | *[[Equine Togaviral Encephalitis|Eastern Equine Encephalitis virus (EEV)]] |
| | + | *[[Equine Togaviral Encephalitis|Western EEV]] |
| | + | *[[Equine Togaviral Encephalitis|Venezuelan EEV]] |
| | + | EEE and WEE are specific and discrete species. There are North and South American antigenic variants of EEE. WEE is a recombinant between an EEE-like virus and a Sindbis-like virus. There are two antigenic subtypes of WEE virus: WEE and Highlands J viruses. Extensive geographical overlap occurs between the various strains which have equivocal differences in antigenic properties and biological behaviour. There are 6 distinct subtypes of VEE virus. Over the last 20 years, large outbreaks of equine encephalitis in the Western Hemisphere have been caused by subtypes IAB, IC and IE. ID and IF variants from Central America and Brazil respectively, type II (Everglades) virus in Florida and types II, IV, V ad VI viruses are considered endemic and are of low pathogenicity for horses under most circumstances. |
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| − | ===Important Serotypes=== | + | ====Diseases==== |
| − | Several alphavirus strains have been isolated during equine and human epidemics of encephalitis in the Western Hemisphere. These epidemics have most often been attributed to:
| + | [[Equine Togaviral Encephalitis]] |
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| − | *Eastern Equine Encephalitis virus (EEV)
| + | ====Epizootiology==== |
| − | *Western EEV
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| − | *Venezuelan EEV.<ref name="multiple">Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref>
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| − | Eastern EEV has North and South American antigenic variants.<ref name="multiples"> Casal, J (1964) Antigenic variants of equine encephalitis virus, ''J Exp Med'' 119:547-565. In: Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref> Western EEV is a recombinant between an Eastern EEV-like virus and a Sindbis-like virus.<ref> Hahn, C.S, Lustig, S, Strauss, E.G, Strauss, J.H (1988) Western equine encephalitis virus is a recombinant virus, ''Proc Natl Acad Sci USA'' 85(16):5997-6001. In: Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref> Western EEV also has two antigenic subtypes - WEE and Highlands J viruses.<ref name="multiple">Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref> Considerable overlap exists between the various strains in terms of their geography, and potentially also in their antigenic properties and biological behaviour.<ref name="multiples"> Casal, J (1964) Antigenic variants of equine encephalitis virus, ''J Exp Med'' 119:547-565. In: Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12.</ref> Of the 6 subtypes of Venezuelan EEV (I-VI), significant outbreaks of equine encephalitis in the Western Hemisphere over the last two decades have been caused by IAB, IC and IE. Variant ID from Central America and variant IF from Brazil are considered endemic and typically demonstrate low pathogenicity for horses. These features are also typical of subtype II (Everglades) virus in Florida and types II, IV, V and VI viruses.<ref name="multiple">Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref>
| + | =====Distribution===== |
| | + | =====Epidemic===== |
| | + | =====Reservoirs===== |
| | + | Togaviridae generally persist by asymptomatically infecting wild animals (sylvatic hosts) such as birds, small mammals and reptiles. Viruses overwinter in sylvatic populations. |
| | + | =====Vectors===== |
| | + | The major disease vectors for each serotype of veterinary significance are: |
| | + | *EEE: ''Aedes'' spp. |
| | + | *WEE: ''Culex tarsalis'' |
| | + | *VEE: ''Culex melanconium'', ''Aedes'' spp., ''Phosphora'' spp. |
| | + | ''Culiseta melanura'' is a vector for EEE. It is largely confined to freshwater swamps, feeds primarily on swamp birds and is rarely found in areas of increased horse density. This mosquito generally serves as a vector for the enzootic cycle involving swamp birds. ''Aedes'' spp. are more important in epizootics and epidemics. |
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| − | ===Diseases=== | + | =====Seasonal Incidence===== |
| − | Arboviruses are the most common cause of [[Equine Togaviral Encephalitis|equine encephalitis]].<ref name="duplicate">Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition), Merial found at http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/100900.htm&word=Equine%2cencephalitis, accessed July 2010</ref>
| + | =====Zoonology===== |
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| − | ==Distribution== | + | ====Pathogenesis==== |
| − | Virus distribution is largely dictated by the distribution of appropriate vectors. Thus Eastern EEV can be found in a region extending from eastern Canada at its northern extremity, down throughout the Caribbean and in parts of Central and South America.<ref name="multiple">Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref>. The virus has also been identifed in the Philippines<ref> Livesay, H.R (1949) Isolation of eastern equine encephalitis virus from naturally infected monkey (''Macacus philippensis''), ''J Infect Dis'' 84:306-309. In: Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref> and may be present in Europe.<ref> Gibbs, E.P.J (1976) Equine viral encephalitis, ''Equine Vet J'', 8:66-71. In: Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref> Over the last two decades, Western EEV has rarely caused disease in the western United States, although it is found here in avian reservoir hosts.<ref name="multiple">Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref> As well as producing disease outbreaks in Mexico, Venezuela and Colombia, Venezuelan EEV has recently been recognized in Peru, Brazil, French Guiana and Trinidad.<ref name="multiple">Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref>.
| + | After inoculation into an equine host, viruses multiply in the muscle, enter the lymphatic circulation and localize in lymph nodes. Viral replication occurs in macrophages and neutrophils with subsequent shedding and significant clearance of viral particles. No further clinical signs develop if clearance is successful but neutralizing Abs are still produced. Viral immunological avoidance mechanisms include erythrocyte and leukocyte absorption. After incomplete elimination, residual virus infects endothelial cells and concentrates in highly vascular organs such as the liver and spleen. In these organs, viral replication produces circulating virus. The second viraemic period is typically associated with early clinical signs. CNS infection occurs within 3-5 days. |
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| − | ===Reservoirs===
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| − | Togaviridae survive by asymptomatically infecting sylvatic hosts such as birds, small mammals, reptiles and amphibians.<ref name="multiple">Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref> Overwintering occurs in these wild populations.
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| − | ===Vectors===
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| − | During a blood meal, [[Culicidae|mosquito]] vectors transmit viral particles between wild animal hosts. Viruses that are able to penetrate the mosquito's gut can pass via the haemolymph to the oral glands. Here, viral replication is followed by the shedding of viral particles into the saliva and other oral secretions. Viral multiplication may not be necessary for transmission if the original blood meal contained sufficient numbers of viral particles.<ref name="multiple">Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref>. It is thought that the mosquito remains permanently infected.<ref> Crans, W.J, McNeilly, J, Schulze, T.L, Main, A (1986)Isolation of eastern equine encephalitis virus from ''Aedes solicitans'' during an epizootic in Southern New Jersey, ''J Am Mosq Control Assoc'' 2:68-72. In: Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref>
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| − | The most significant disease vectors for each viral serotype are:
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| − | *Eastern EEV: ''Aedes'' spp.
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| − | *Western EEV: ''Culex tarsalis''
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| − | *Venezuelan EEV: ''Culex melanconium'', ''Aedes'' spp., ''Phosphora'' spp.<ref name="multiple">Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref>.
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| − | ''Culiseta melanura'' is another vector for Eastern EEV. It feeds mostly on swamp birds, completing an enzootic cycle of viral transmission. ''C.melanura'' is thus an inhabitant of freshwater swamps and is not usually found in areas densely populated by equids.<ref> Hoff, G.L, Bigler, W.J, Buff, E.E, Beck, E (1978) Occurrence and distribution of western equine encephalomyelitis in Florida, ''J Am Vet Med Assoc'', 172:351-352. In: Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref> Epizootics and epidemics of Eastern EEV disease are propagated by ''Aedes'' spp. Western EEV persists in an enzootic cycle with passerine birds, transmitted by ''C.tarsalis''.<ref name="multiple">Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref>. Other vectors or overwintering hosts for this serotype may include [[Dermacentor spp.|''Dermacentor andersoni'']] ticks<ref> Syverton, J.T, Berry, G.P (1937) The tick as a vector for the virus disease equine encephalomyelitis, ''J Bacteriol'', 33:60. In: Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref>, ''Triatoma sanguisuga'' (the assassin bug)<ref> Kitselman, C.H, Grundman, A.W (1940) Equine encephalomyelitis virus isolated from naturally infected ''Triatoma sanguisuga'', ''Kans Agric Exp Station Tech Bull'', 50:15. In: Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref>, and the cliff swallow bug (''Oeciacus vicarius'').<ref> Hayes, R.O, Francy, D.B, Lazuick, J.S (1977) Role of the cliff swallow bug (''Oeciacus vicarius'') in the natural cycle of a Western equine encephalitis-related alphavirus, ''J Entomol'', 14:257-262. In: Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref> Epidemic strains of Venezuelan EEV have infected mosquito species from several genera and this viral serotype may also be transmitted by ticks.<ref name="multiple">Bertone, J.J (2010) Viral Encephalitis in Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) '''Equine Internal Medicine''' (Third Edition), ''Saunders'', Chapter 12</ref>.
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| − | ==Virus Identification==
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| − | Viruses can be identified by complement fixation, immunofluorescence, PCR, ELISA, or virus isolation.
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| − | *The complement fixation test can be used to identify Eastern or Western EEV in infected mouse or chicken brains, cell culture fluid or amniotic-allantoic fluid.<ref name="again">''Manual of Diagnostic Tests and Vaccines for Terrestrial Animals'' found at http://www.oie.int/eng/normes/mmanual/A_00081.htm, accessed July 2010.</ref>
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| − | *Virus may be identified in brain tissue or cell culture using direct immunofluorescent staining.<ref name="again">''Manual of Diagnostic Tests and Vaccines for Terrestrial Animals'' found at http://www.oie.int/eng/normes/mmanual/A_00081.htm, accessed July 2010.</ref>
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| − | *EEE and WEE viral RNA in mosquitoes and equine tissues may be detected by reverse-transcription PCR.<ref>Vodkin, M.H, McLaughlin, G.L, Day, J.F, Shope, R.E, Novak, R.J (1993) A rapid diagnostic assay for eastern equine encephalomyelitis viral-RNA, ''Am J Trop Med Hyg'', 49, 772-776. In: ''Manual of Diagnostic Tests and Vaccines for Terrestrial Animals'' found at http://www.oie.int/eng/normes/mmanual/A_00081.htm, accessed July 2010.</ref>
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| − | *ELISA can be used to detect virus in brain tissue. An antigen-capture ELISA, developed for EEE surveillance in mosquitoes, can be used where virus isolation and PCR facilities are unavailable. <ref>Brown, T.M, Mitchell, C.J, Nasci, R.S, Smith, G.C. and Roehrig, J.T. (2001). Detection of eastern equine encephalitis virus in infected mosquitoes using a monoclonal antibody-based antigen-capture enzyme-linked immunosorbent assay, ''Am J Trop Med Hyg'', 65, 208-213. In: ''Manual of Diagnostic Tests and Vaccines for Terrestrial Animals'' found at http://www.oie.int/eng/normes/mmanual/A_00081.htm, accessed July 2010.</ref>
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| − | *Virus isolation is the most definitive diagnostic method for EEE or WEE.<ref name="again">''Manual of Diagnostic Tests and Vaccines for Terrestrial Animals'' found at http://www.oie.int/eng/normes/mmanual/A_00081.htm, accessed July 2010.</ref>. Brain is preferred, but virus has also been isolated from the liver and spleen.<ref name="again">''Manual of Diagnostic Tests and Vaccines for Terrestrial Animals'' found at http://www.oie.int/eng/normes/mmanual/A_00081.htm, accessed July 2010.</ref>. Samples of these tissues should be taken in duplicate, one set for virus isolation and the other placed in formalin for histopathology.<ref name="again">''Manual of Diagnostic Tests and Vaccines for Terrestrial Animals'' found at http://www.oie.int/eng/normes/mmanual/A_00081.htm, accessed July 2010.</ref>. Viral isolation specimens should be sent frozen unless they can be received refrigerated within 48 hours of sampling.<ref name="again">''Manual of Diagnostic Tests and Vaccines for Terrestrial Animals'' found at http://www.oie.int/eng/normes/mmanual/A_00081.htm, accessed July 2010.</ref>. Unless clinical signs persist for more than 5days prior to death, EEE virus is frequently isolated from equine brain tissue. WEE virus, however, is rarely isolated from tissues of infected horses. Newborn mice, chicken embryos and a number of cell culture systems can be used for virus isolation.<ref name="again">''Manual of Diagnostic Tests and Vaccines for Terrestrial Animals'' found at http://www.oie.int/eng/normes/mmanual/A_00081.htm, accessed July 2010.</ref>. Virus may also be isolated from cerebrospinal fluid (CSF) of acutely infected horses.<ref name="duplicate">Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition), Merial found at http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/100900.htm&word=Equine%2cencephalitis, accessed July 2010</ref>
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| − | {{Learning
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| − | |literature search = [http://www.cabdirect.org/search.html?q=title%3A%28%22Equine+Encephalitis+Virus%22%29+AND+od%3A%28horses%29 Equine encephalitis virus in horses]
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| − | [http://www.cabdirect.org/search.html?rowId=1&options1=AND&q1=%22Equine+Encephalitis+Virus%22&occuring1=title&rowId=2&options2=NOT&q2=horses&occuring2=od&rowId=3&options3=AND&q3=&occuring3=freetext&x=42&y=9&publishedstart=yyyy&publishedend=yyyy&calendarInput=yyyy-mm-dd&la=any&it=any&show=all Equine encephalitis virus in other species, including laboratory animals]
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| − | ==References==
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| | [[Category:Togaviridae]] | | [[Category:Togaviridae]] |
| − | | + | [[Category:Horse]] |
| − | [[Category:Expert_Review]] | + | [[Category:To_Do_-_Viruses]] |
| − | [[Category:Neurological Diseases - Horse]] | + | [[Category:To_Do_-_Nina]] |
