Rift Valley fever: The Nigerian story

Abstract

Rift Valley fever (RVF) is an arthropod-borne zoonotic disease of livestock. It is characterised by fever, salivation, abdominal pain, diarrhoea, mucopurulent to bloody nasal discharge, abortion, rapid decrease in milk production and death in animals. Infected humans experience an influenza-like illness that is characterised by fever, malaise, headaches, nausea and epigastric pain followed by recovery, although mortality can occur. RVF was thought to be a disease of sub-Saharan Africa but with the outbreaks in Egypt and the Arabian Peninsula, it may be extending its range further afield. Virological and serological evidence indicates that the virus exists in Nigeria and, with the warning signal sent by international organisations to countries in Africa about an impending outbreak, co-ordinated research between veterinarians and physicians in Nigeria is advocated.

Full text

VeterinariaItaliana,2011,47(1),35‐40
©IstitutoG.Caporale2011www.izs.it/vet_italianaVol.47(1),VetItal35
RiftValleyfever:theNigerianstory
AdewaleA.Adeyeye(1),PiusS.Ekong(2)&NicholasN.Pilau(1)
Summary
RiftValleyfever(RVF)isanarthropod‐borne
zoonoticdiseaseoflivestock.Itischaracterised
byfever,salivation,abdominalpain,diarrhoea,
mucopurulenttobloodynasaldischarge,
abortion,rapiddecreaseinmilkproduction
anddeathinanimals.Infectedhumans
experienceaninfluenza‐likeillnessthatis
characterisedbyfever,malaise,headaches,
nauseaandepigastricpainfollowedby
recovery,althoughmortalitycanoccur.RVF
wasthoughttobeadiseaseofsub‐Saharan
AfricabutwiththeoutbreaksinEgyptandthe
ArabianPeninsula,itmaybeextendingits
rangefurtherafield.Virologicalandsero‐
logicalevidenceindicatesthatthevirusexists
inNigeriaand,withthewarningsignalsentby
internationalorganisationstocountriesin
Africaaboutanimpendingoutbreak,
co‐ordinatedresearchbetweenveterinarians
andphysiciansinNigeriaisadvocated.
Keywords
Abortion,Africa,RiftValleyfever,Livestock
handlers,Nigeria,Publichealth,Virus,
Zoonosis.
Rapportosullafebbredella
ValledelRiftinNigeria
Riassunto
LafebbredellaValledelRift(RVF)èunamalattia
zoonosicadelbestiameveicolatadaartropodi.È
caratterizzatadafebbre,salivazione,dolore
addominale,diarrea,scolonasaledamucopurulento
aematico,aborto,rapidadiminuzionenella
produzionelatteaemorteneglianimali.Isoggetti
umaniinfettimanifestanounamalattiasimile
all’influenzacaratterizzatadafebbre,malessere,
attacchidicefalea,nauseaedoloreepigastrico
seguitidallaguarigione,sebbenepossaverificarsi
unamortalità.LaRVFeraritenutaunamalattia
dell’Africasub‐sahariana,maconifocolaiepidemici
inEgittoenellapenisolaarabicaèpossibilechestia
ampliandoancoradipiùilsuoraggio.L’evidenza
virologicaesierologicaindicacheilvirusèpresente
inNigeriaeconsistemadiallertainviatidalle
organizzazioniinternazionaliallenazioniafricane
riguardanteunimminentefocolaioepidemico,è
urgenteinquestopaeseunaricercacoordinatafra
veterinariemedici.
Parolechiave
Aborto,Addettialbestiame,Africa,Febbre
dellaValledelRift,Nigeria,Salutepubblica,
Virus,Zoonosi.
Introduction
RiftValleyfever(RVF)isalsoknownas
enzootichepatitisofsheepandcattle.Itisan
acute,infectiousandzoonoticdiseaseof
predominantlycattle,sheep,goat,camels,
Africanbuffalo(Synceruscaffer)andhumans.
Thediseaseiscausedbyanarbovirusandis
associatedwithperiodicoutbreaksthatmostly
occurontheAfricancontinent.Itisafebrile
diseasethatisaccompaniedbyabortionin
livestockandaseverefatalhaemorrhagic
syndromeinhumanshasbeenobserved(11).It
isanotifiablediseaseandisspreadbythebite
ofmosquitoes.
Thediseasewasfirstreportedamongsheepin
KenyabyMontgomeryin1912andStordyin
1913(4),butthediseasewasnotisolateduntil
1931(25).Presently,virologicalandserological
evidencesuggeststhatthevirusexists
(1) Department of Veterinary Medicine, Surgery and Theriogenology, Usmanu Danfodiyo University, P.M.B. 2254,
Sokoto, Nigeria
ayo4wale@hotmail.com
(2) Central Diagnostic Department, National Veterinary Research Institute, P.M.B. 01, Vom, Jos, Plateau State, Nigeria

AreportonRiftValleyfeverinNigeriaAdewaleA.Adeyeye,PiusS.Ekong&NicholasN.Pilau
36 Vol.47(1),VetItalwww.izs.it/vet_italiana©IstitutoG.Caporale2011
throughoutsub‐SaharanAfricaandMadagascar
and,inthelightofitsrecurrenceinEgyptin
1993and2003(5,7,9,39),itmaybeextending
itsrangeevenfurther.InSeptember2000,
casesofunexplainedhaemorrhagicfeverin
humansandassociatedanimaldeathsin
south‐westernSaudiArabiaandYemenwere
confirmedasRVFand,bymid‐January2001,
thediseasehadclaimedseveralhumanlivesin
thesecountries(1);theoutbreakonthe
ArabianPeninsularepresentsthefirstcasesof
RVFoutsideAfrica.In2007,anoutbreak
occurredinKenyaandSomaliawhereover
404humancases,including118deaths,were
reported(10).InSouthAfrica,thelastoutbreak
occurredinMay2010;preliminary
investigationrevealedthat186humanswere
confirmedRVFcasesoutofwhich18died(40).
Anoverviewofthediseaseisnecessarygiven
climatechangesthatfavourpossibleoutbreaks
(20,24)andthewarningsignalsdespatchedto
countriesinAfricabytheFoodand
AgricultureOrganizationandWorldHealth
Organization(16).
Aetiology
TheRVFvirusisthecausalagentofthe
disease.ThevirusisofthegenusPhlebovirus,
familyBunyaviridae(26).Itisanenveloped,
single‐strandednegative‐senseRNAvirusthat
measuresbetween80nmand120nmin
diameterwiththreesegmentsnamedS(small),
M(medium)andL(large)(8).
Transmission
RVFisamosquito‐bornedisease(2).Aedesis
thespeciesofmosquitothatisincriminatedin
biologicaltransmission(36,37),although
Glossina,Culicoides,Culexspeciesandsandflies
mayplaylimitedrolesinbiologicaland
mechanicaltransmission(21,25).Apartfrom
thesevectors,thediseasehasbeenreportedto
spreadthroughneedleinoculation,contact
withinfectedanimalsorhumanswithhigh
prevalenceduringperiodsofheavyrainfall(2,
34,38,42).Experimentaltransmissioninsheep
frominfectedmouseserumhasbeenreported
(35).Riskgroupsincludeanimalhandlers,
particularlythoseassistinganimalsduring
abortions/parturition,thosetreatingsick
animalsandthosenursingaffectedpeople,
particularlyinareaswithoutbreaks(38).
Humansmaybeinfectedbydirectandindirect
contactwithinfectedanimalsaswellasby
bloodfromfoetusesorslaughteredanimals
(17).Transmissionisalsopossiblebymosquito
bite(6,25)andingestionofunpasteurisedor
uncookedmilkfrominfectedanimals(5,31).
Casesofverticaltransmissionhavealsobeen
reported(3,6).
Clinicalsigns
Theincubationperiodforthediseaseisshort.
Itrangesfrom1to6daysinanimals,although
itisshorterinyoungeranimals,andbetween4
to7daysinhumans(2,33).Inadultanimals,it
ischaracterisedbyfeverwithin24hof
infectionthatlastsbetween6and7daysbut
peaksbetweendays2and4postinfection;
viraemiacoincideswithfeverwhichpeakson
day3postinfection(29).Abortionsoccurin
pregnantanimalsandsigns,suchassalivation,
abdominalpain,diarrhoea,mucopurulentto
bloodynasaldischargeandrapiddecreasein
milkproductioninparticular,areobservedin
dairycows(2,25,35,41).Inyoungeranimals,
lambsandneonates,deathusuallyoccurs
within24hbutsignsoffever,depression,
anorexiaandabdominalpainareprominent
priortodeath(33).
Thediseaseinhumansisinfluenza‐likeandis
characterisedbyfever,malaise,headache,
nausea,epigastricpain,sensationoffullness
overtheregionoftheliver,followedby
completerecovery.However,itmayevolve
intoahaemorrhagicfeversyndrome,meningo‐
encephalitisorevenaffecttheeyes(2,19,25,
32,33).
Animalsthatdieofthediseaseshowextensive
livernecrosisandenlargementatpost‐mortem
examination,withoedemaandhaemorrhages
onthewallofthegallbladder.Innewborn
lambsinfectedinutero,thecontentsofthe
abomasumandsmallintestineareofa
chocolate‐browncolour(8).Inaddition,the
lambwillexhibitgeneralisedhepaticnecrosis.

AdewaleA.Adeyeye,PiusS.Ekong&NicholasN.PilauAreportonRiftValleyfeverinNigeria
©IstitutoG.Caporale2011www.izs.it/vet_italianaVol.47(1),VetItal37
Diagnosis
Diagnosisisperformedbyisolatingand
identifyingthevirusorbyobservinga
fourfoldriseinspecificneutralisingantibody
titrebetweenacuteandconvalescentsera.
Uponpost‐mortemexamination,characteristic
histopathologicalfindingsoflivernecrosisin
allsusceptibleanimalsoftenprovidethefirst
cluetothedisease.Confirmatorydiagnosisis
byserologyusingIgMorIgGantibody‐specific
enzyme‐linkedimmunosorbentassay(ELISA),
complimentfixation,viralneutralisation,
haemagglutinationinhibitionandtheplaque
reductiontest.
Differentialdiagnosisinanimalsincludesthe
following:
 bacterialsepticaemias
 poisoning
 rinderpest
 pestedespetitsruminants(PPR)
 bluetongue
 bovineephemeralfever
 enterotoxemiaofsheep
 brucellosis
 campylobacteriosis
 toxoplasmosis
 trichomoniasis
 heartwater
 otherabortificientcausalagents.
Epidemiology
RVFwasthoughtobeadiseaseofsub‐Saharan
Africa,butthediseasehasspreadtoEastand
CentralAfrica(14)andtowesternSaudi
ArabiaandYemen(25).Inadditiontocattle
andsheep,otheranimalshavebeenreported
tobesusceptibletothedisease.Antibodies
againstthediseasehavebeendetectedin
Africanbuffalo,blackrhino(Dicerosbicornis),
lesserkudu(Tragelaphusimberbis),impala
(Aepycerosmelampus),Africanelephant
(Loxodontaafricana),kongoni(Alcelaphus
buselaphuscokii),waterbuck(Kobusellipsiprymnus)
(11),goats,camelsandhorses(29)but
evidenceoftherolethattheseanimalsplayin
maintainingthevirusbetweenoutbreaksis
stillunknown.However,LaBeaudetal.(23)
reportedthatoldermaleslivinginrural
villagesandwhohaddisposedofanimal
abortuseswereresponsibleforinter‐epidemic
transmissioninKenya.
InNigeria,Fergusonfirstisolatedthevirus
fromanimals(15).Subsequentserological
evidencesuggeststhatthevirusmaybe
circulatingatlowlevelsindomesticlivestock
andinthehumanpopulation,particularly
amonglivestockworkersandwildliferangers
(28).Cattle,sheepgoatsandcamelsinthe
StatesofKadunaandSokotohaverevealed
significantantibodytitresintheirserum(12).
Serologicalprevalenceofthediseaseinthese
animalspeciesinIle‐IfeandIbadanwas
observedbyOlaleyeetal.(27)whoconfirmed
theexistenceofthediseaseinNigeria.Apart
fromtheseobservations,experimental
infectionwithdifferentstrainsofthediseasein
threeindigenousbreedsofsheepinNigeria,
namely:theWestAfricandwarf,Yankasaand
Oudahaveresultedinfataldisease(13).
Furtherstudiesarethereforerequiredto
determinethepresentstatusofthediseasein
Nigeria.
Treatment
Thereisnotreatmentforanimals,butaninitial
50mg/kgloadingdoseofribavirinfollowedby
10mg/kgat8hintervalfor9dayshasbeen
suggestedtocontributetothepreventionorto
providetherapyinhumans(30).
Preventionandcontrol
Preventionisbestachievedbyvaccination.A
live‐attenuatedandkilledvaccineisavailable
forlivestock.Thelive‐attenuatedSmithburn
strainprovideslong‐lastingimmunitybutit
producesateratogeniceffectinsheepwhen
administeredinthefirsttrimester(22)anditis
abortogenicinpregnantewes(25).Apartfrom
these,thevaccinehasbeendocumented
experimentallynottoprotectagainstthe
NigeriaandLunyostrainsofthevirusinWest
Africandwarfsheep(35).Thelivevirus
vaccinesarerecommendedforuseinendemic
areas,whilethekilledvaccinesare
recommendedforuseoutsideendemicareas.
Aformalin‐inactivatedvaccineissafefor

AreportonRiftValleyfeverinNigeriaAdewaleA.Adeyeye,PiusS.Ekong&NicholasN.Pilau
38 Vol.47(1),VetItalwww.izs.it/vet_italiana©IstitutoG.Caporale2011
pregnantewesbutprovidesonlyshort‐term
immunityandhencerequiresabooster
inoculationtomaintaindurableimmunity.The
onlyvaccineclearedforhumanuseisthe
killedproductavailableonlyfromtheUnited
StatesArmyMedicalResearchandMaterial
Command(USAMRMC).Itisaninitialthree‐
doseseriesforprotectiveimmunitywith
annualboosterinoculationsrequiredto
maintainthatimmunity(25).Thisvaccinehas
beenusedtoelicitimmuneresponseamong
laboratorystaffinNigeria(18).
Vectorcontrolofmosquitopopulationsand
restrictedmovementsduringoutbreakscanbe
effectivemeasuressinceanimalshavehigh
levelsofviraemiaandconsequentlyamplify
transmission.Educationalcampaignsto
preventhumanexposuretoinfectedanimal
tissuesorabortusesandexposuretomosquito
vectorsareeffective.Vaccinationofpeoplein
high‐riskoccupationsisequallyeffectivein
combatinganoutbreak.
Conclusion
Inconclusion,RVFisemerginginnewareas
andthereissomere‐emergenceofthevirusin
previouslyaffectedareas.Hence,thereisa
strongneedfortheAfricancontinentto
strengthenregionalandnationalwarning
systems,suchasco‐ordinatedresearch
betweenveterinariansandphysicians,which
willcontributetothepreventionofan
epidemic.
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©IstitutoG.Caporale2011www.izs.it/vet_italianaVol.47(1),VetItal39
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40 Vol.47(1),VetItalwww.izs.it/vet_italiana©IstitutoG.Caporale2011
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