inallesionsinexperimentalanimals.E.disparadherestohostcellsinvery
muchthesamewayasE.histolytica,butitproducesonlyverysmallamounts
ofamebaporeAandBandnoneoftheparticularlypotenttypeCatall.E.
disparislackingseveralgenesthatcodeforcertaincysteineproteases.
Also,theactivityofcertainproteasesinE.disparisgreatlyreducedcompared
toE.histolytica.
&Extraintestinalamebosis.E.histolyticacandisseminatetootherorgans
fromtheintestinalwall,mostparticularlytotheliver(Fig.9. 10 ).Asaresultof
thedestructionofparenchymalcells,smallnecroticfoci,so-calledabscesses,
formandgraduallybecomelargerandcanevenaffectmajorportionsofthe
organ.Bacteriaareinvolvedinonlyabout 5 %ofcases,sothattheinflamma-
toryreactionsattheedgesofthefociareusuallymild.Thedecomposingle-
sioncontainsabrownishoryellowish,puslikeliquid,inmostcasesbacte-
riologicallysterile,laterbecominganecroticmass;amebasareoftenonlyde-
tectableinthetransitionzonebetweenthelesionandintacthepatictissue.
Liverabscessessometimesperforateintothepleuralspaceorlung;lessoften
ahematogenousdisseminationofamebasresultsinaninvasionofthespleen,
brain,andotherorgans.Cutaneousamebosismostfrequentlyoccursinthe
perianalarea,associatedwithrectalchanges.
Epidemiology.HumansarethereservoirsforE.histolytica (rarelyalso:
monkeys,dogs,cats).Theinfectionisduetotransmissionofmaturecysts
withcontaminatedfoods(fruit,vegetables),drinkingwaterorfecallycon-
taminatedhands.Fliesandcockroachescanfunctionasintermediariesby
carryingcystsfromthefecesofanexcretortofoods.Incontrasttothe
vegetativeforms,thecystsarequiteresistantinamoistenvironment(i.e.,
theysurviveat 28 – 348 Cforabouteightdays,at 108 Cforaboutonemonth);
underconditionsofdesiccationandtemperaturesexceeding 558 Ctheyare
quicklykilled.Theamountsofchlorinenormallyaddedtodrinkingwaterare
insufficienttokillthecysts.MonkeyshavebeenshowntobehostsofE.his-
tolyticaandE.dispar.
Clinicalmanifestations.Clinicalsymptomscandevelopasearlyastwoto
fourweeksafterinfectionwithE.histolyticaorafterasymptomaticperiods
ofmonthsorevenyears.
&Intestinalforms
— Asymptomaticintestinalform.E.histolyticacancolonizetheintestinal
mucosa,reproduce,andpersistforlongperiodswithoutbecominginva-
siveorcausinganychanges.TheapathogenicE.disparismorefrequent
thanE.histolytica,sothatmostasymptomaticinfectionsareprobably
causedbytheformer.Trophozoites,andmorefrequentlycysts,ofthe
E.histolytica/E.dispartypeareexcreted,antibodiestoE.histolyticaanti-
gensareusuallynotfoundinserum.502 9 Protozoa9
Kayser, Medical Microbiology © 2005 Thieme