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Keyfactsaboutequineherpesvirus(EHV)
Q.WhatisequineherpesvirusandEHV-1?
Equineherpesvirusisacommonvirusthatoccursinhorse
populationsworldwide.Oneofthevirus’mostcommon
strainsisequineherpesvirustype1(EHV-1)that’sknown
tocauserespiratorydiseasesaswellasoutbreaksof
abortionsandneurologicdisease.
brainbarrier.EHMcancausesinglecasesofdisease,butit
hasalsobeenidentifiedasacauseofoutbreaksaffecting
20to50percentofaffectedpopulations.EHMoutbreaks
mayormaynotbeassociatedwithpreviousorconcurrent
respiratorydisease.
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Q.Whichhorsesaremostsusceptible?
AllhorsesaresusceptibletoinfectionwithEHV,butclinical
signsandseverityofdiseasemaydiffer;somehorsesmay
evenbecomeinfectedandtransmittheviruswithout
showingclinicalsigns.Therespiratorydiseasecausedby
EHVismostcommoninweanlingsandyearlings.Pregnant
maresaresusceptibletoabortion.Theneurologicformof
diseasemayoccurmorecommonlyinagedhorses. !
Q.Whataretheclinicalsigns?
Fevercommonlyprecedesotherclinicalsigns,butitmay
betheonlysignofinfection.Typicalsignsofrespiratory
diseaseincludecoughingandnasaldischarge.Abortions
causedbyEHV-1typicallyoccurlateinpregnancy,often
withoutwarningsigns.Fortheneurologicdisease,see
below.Differentformsofthediseasecanoccur
concurrently.
Neonatalfoalsmaybeinfectedinuteroandareusually
abnormalfrombirth.Weakness,jaundice,respiratory
distressandneurologicsignsareamongtheclinicalsigns.
Affectedfoalstypicallydiewithinseveraldays.Older
infectedfoalsgenerallyhavenasaldischargeorothersigns
ofrespiratorydisease.
Q.WhatisEHM?
Equineherpesvirusmyeloencephalopathy(EHM)is
anothernamefortheneurologicdiseasecausedbyEHV-1
infection.Itresultsfromwidespreaddamagetoblood
vesselendothelium—includingdamagetotheblood-
DuringanEHVoutbreak,
knowledgeandawarenessare
powerfultoolsincontrollingthe
spreadofthevirusamonghorse
populations.
Q.WhataretheclinicalsignsofEHM?
Neurologicdiseasemaybeprecededbyfeverand
respiratorysigns.EHMtypicallyaffectsthehind
limbsandurinarytract.Commonsignsinclude
inco-ordination,urinaryincontinenceandbladder
distension.Severelyaffectedhorsesmaybecome
recumbentandunabletorise.“Dog-sitting”maybe
observed.Horsesgenerallyremainbrightandoften
continuetoeatanddrink.
Q.What’sthevirus’incubationperiod?
Whilethetimebetweeninfectionandonsetofclinical
signscanbeasshortas24hours,theincubationperiod
typicallylastsbetweenfourandsixdays—andcan
extendlonger.EHVabortionscanoccurfromtwo
weekstoseveralmonthsfollowinginfection.
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Q.Howisthevirustransmitted?
•Directcontact:Thevirusisprimarilytransmitted
throughdirectcontactbetweenhorses.Virussheddingby
therespiratoryroutetypicallylastsforsevento10days
butcanpersistlonger.Abortedfetuses,fetalmembranes
andfetalfluidsarehighlyinfectious.Maresthathave
abortedalsoshedvirusintheirrespiratorysecretions.
•EHV-relatedabortions:Abortedfetuses,fetal
membranesandfetalfluidsareinfectious.Maresthat
haveabortedalsoshedvirusintheirrespiratory
secretions.
•Indirecttransmission:Theviruscanbeindirectly
transmittedbetweeninfectedanduninfectedhorses
whennasalsecretionsorfluidsfromabortionsaremoved
viapeopleorinanimateobjects.
•Poorhygiene:Alackofhandwashingandsharingof
equipmentcanpotentiallytransmitthevirusto
uninfectedhorses.Peoplewhoareincontactwith
infectedhorsesshouldchangetheirclothesand
thoroughlycleananddisinfecttheirhandsbefore
handlingotherhorses.Equipment(bucketsandhalters,
forexample)shouldnotbesharedbetweeninfectedand
uninfectedhorses.
Q.HowisEHVinfectiondiagnosed?
Veterinariansoftensuspectthediseasebasedonclinical
signs,butit’srecommendedthatfurthertesting(serology,
virusisolationandmoleculartesting)bedonetoestablish
thediagnosis.
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Q.What’sthetreatmentforEHM?
•Treatmentismostlysupportiveandincludesantiinflammatorymedicationsuchascorticosteroids.Some
horsesmayrequireIVfluidtherapy.
•Placeaffectedhorsesinasafe,well-beddedstall,
especiallyifthey’reseverelyunco-ordinatedandhave
troublerising.Recumbenthorsesmaybenefitfromthe
useofasling.
•Horsesthathavedifficultyurinatingmayneedtobe
catheterizedseveraltimesdailyandmaybenefitfrom
medicationtosupportbladderfunction.
•Specifictreatmentsdirectedagainstthevirusareunder
investigation.Preliminaryresearchsuggestsapotential
benefitofantiviralmedicationssuchasacyclovirand
valacyclovir.
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Q.What’stheprognosis?
Whiletheprognosisisgenerallyfavourableformildly
affectedhorses,theoutcomeispoorforthosethat
becomerecumbentforprolongedperiods.Itmaytake
severalweekstomonthsbeforeneurologicdeficitsresolve
inrecoveringhorses,andsomemayhavepersistent
deficitsfortherestoftheirlives.
Q.Howlongareaffectedhorsesstillinfectious?
Affectedhorsesusuallyshedthevirusforsevento20days
—possiblylonger—aftertheonsetoffeverorneurologic
disease.Infectedanimalstypicallydeveloplatent
infections,andtheycanshedvirus(followingvirus
reactivation)withorwithoutshowingsignsofthedisease
fortherestoftheirlives.Latently-infectedhorsestypically
shedthevirusduringperiodsofstress,andre-activation
ofthevirusisresponsibleforthespreadandsurvivalof
thevirusinhorses.Thisalsoprovidesalogicalexplanation
foroccasionaloutbreaksobservedinisolatedherds.
Q.CanownersvaccinatehorsesagainstEHV?
SeveralvaccinesagainstEHVareavailable.Vaccination
shouldbebasedontheperceivedriskofinfection,butit’s
generallyrecommendedforbroodmares.Horseowners
shouldtalktotheirveterinariansaboutvaccination
programs.Horseownersandveterinarianscanalso
consulttheAmericanAssociationofEquinePractitioners
(AAEP)vaccinationguidelines.Noneofthecurrently
availablevaccinesstatesanyclaimforprotectionagainst
theneurologicformofEHVinfection.
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Q.CanvaccinespreventfurthercasesduringanEHV
outbreak?
Thebenefitofvaccinationinthefaceofanoutbreakis
questionable.Boostervaccinationmayreducevirus
spreadandithasn’tbeenassociatedwithdetrimental
effects.
Q.WhatcanIdotocontrolEHV’sspread?
•IfyoususpectyourhorseisinfectedwithEHV,ask
yourveterinariantoexamineyourhorseandconduct
appropriatetesting.
•Isolateanyinfectedhorsesimmediatelyandavoidall
directandindirectcontactwithotherhorses.Exposed
horses(thosethathavebeenincontactwithaknown
infectedhorses)shouldalsobeisolatedasaprecautionary
measure.
•Stopallhorsetrafficonandoffpremiseswhereinfected
horseshavebeenidentified.
•Isolateinfectedandexposedhorsesfromthegeneral
populationfor28daysfollowingtheonsetofthelast
identifiedcase.Veterinarianscanconducttestingto
determinewhetherhorsesarestillsheddingthevirus.
•Paystrictattentiontohygieneandwashyourhands
properly.
Q.WhatshouldbedoneafteranEHVoutbreak?
Topreventfurthercases,it’simportanttothoroughly
cleananddisinfectyourfacilitiesandallequipment.EHVis
susceptibletomanydisinfectants,butasaprecaution,
readtheinformationprovidedwithdisinfectantproducts
orcontactthemanufacturers.Beforedisinfectingyour
facilities,makesuretoremovemanure,nasalsecretions
andotherorganicmaterialsfromthesite.
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Q.HowcanyoupreventEHVoutbreaks?
Boardingfacilities,showgrounds,auctionbarnsandother
publicfacilitiesshoulddiscussbiosecuritywiththeir
attendingveterinariananddevelopabiosecurityprotocol
specifictotheirsituation.Biosecuritymeasuresinclude
isolationofnewhorsesuponarrival,testingofnewhorses
uponarrival,andrequirementsforhealthcertificatesand/
orvaccination.
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Originalsource:AmericanAssociationofEquinePractitioners
(www.aaep.org),withadditionalinformationprovidedbyDr.
KatharinaLohmann,DepartmentofLargeAnimalClinical
Sciences,WesternCollegeofVeterinaryMedicine.