withintwotofourdays,producingtwosporocystswithfoursporozoiteseach.
Sporulatedoocystsareinfectiveforhumansandanimals.
Lifecycle.ThedevelopmentalcycleofT.gondiiinvolvesthreephases:intesti-
nal,external,andextraintestinal(Fig.9. 13 ).
&Theintestinalphasewithproductionofsexualforms(gamogony)takes
onlyplaceinenterocytesofdefinitivehosts.Onlydomesticcats,andseveral
otherfelidspeciesoflittleepidemiologicalsignificance,canfunctionasde-
finitivehostsforT.gondii.Onlyextraintestinaldevelopmentisseenininter-
mediatehosts(pigs,sheep,andmanyotheranimalspecies)aswellasin
dead-endhosts(humans).FollowingprimaryinfectionofacatwithToxoplas-
macystsinrawmeat,asexualreproductiveformsatfirstdevelopinthesmall
intestineepithelium,withsexuallydifferentiatedstagesandoocystsfollow-
inglater.Theoocystsareshedinfecesafteraprepatentperiodofthreetonine
days.Whencatsareinfectedwithsporulatedoocysts,theprepatentperiodis
extendedto 20 – 35 daysbecauseinthesecasestheintestinaldevelopmentof
Toxoplasmaisprecededbyanextraintestinalasexualreproduction(seebe-
low).Oocystsheddinglastsforonlyafewdaystoamaximumofthreeweeks,
butcanbehighlyintensive(upto 600 millionoocystspereatduringthe
patentperiod!).
&Externalphase.Oocystsexcretedincatfecessporulateatroomtempera-
turewithintwotofourdays,renderingtheminfective.Keptmoist,theyre-
maininfectiveforuptofiveyearsandarenotkilledbystandarddisinfectant
agents.Theydiewithinafewminutesattemperaturesexceeding 558 C.
&Extraintestinalphase.Thisphasefollowsaperoralinfectionwithoocysts
orcystsandisobservedinintermediatehosts(dogs,sheep,pigs,otherver-
tebrates,birds)anddead-endhosts(humans),aswellasinthedefinitive
hosts(cats).Startingfromtheintestine,theToxoplasmaorganismstravel
inbloodorlymphtovariousorgansandmultiplyinnucleatehostcells,espe-
ciallyinthereticulohistiocyticsystem,inmusculature,andintheCNS.Re-
peatedendodyogeniccyclesproduceasmanyas 32 individualdaughtercells
intheexpandinghostcellbeforeitbursts.Thetachyzoitesthusreleasedat-
tackneighboringcells.
Theseprocessesresultinfocalnecrosesandinflammatoryreactionsinaf-
fectedtissues.Generalizationoftheinfectioncanleadtocolonizationof
theplacentaand,aboutthreetofourweekslater,infectionofthefetus.Cysts
thatelicitnoinflammatoryreactionsinthenearvicinityareproducedearlyin
thecourseoftheinfection.Suchcysts(tissuecysts)arefoundaboveallinthe
CNS,intheskeletalandheartmusclesaswellasintheretina,theuterinewall,
andotherorgans.Theycanremainviableforyearswithoutcausingnotice-
abledamagetothehost.510 9 Protozoa9
Kayser, Medical Microbiology © 2005 Thieme