Medical Microbiology

Immunity.VariousToxoplasmaantigensinducehumoralandcellularim-

muneresponseswhich,followingaprimaryinfection,resultinantibodypro-

ductionandinhibitionoftachyzoitemultiplication.Toxoplasmasthen“es-

cape”fromtheimmunedefensesystembyencysting(immunoevasion).

Thisenablesthemtopersistinalatentstateformanyyearsinimmunocom-

petenthosts,atthesametimemaintaininganimmunestatusthatconfers

protectionfromnewinfectionsduetothecontinuouspresentationofanti-

gens.

Therelevantimmunedefensemechanismsincludemainlycellularmech-

anismsandproductionofIFNc.Bradyzoitescanalsomigrateoutofcysts

(withoutcystrupture),arethen,however,locallyinactivatedinimmunocom-

petentpersons,sometimesleadingtoformationofsatellitecysts.Incasesof

cellularimmunedeficiency,thiscontrolsystemislackingandthelatent

infectionprogressestobecomeanacute,manifesttoxoplasmosis.Similarly,

latentToxoplasmainfectionsinAIDSpatientsareusuallyactivatedandturn

symptomaticwhentheCD4+cellcountfallsbelow 20 0/ll.

Pathogenicityandclinicalmanifestations.Focalnecrotic,inflammatoryand

immunopathologicalprocessesarethebasisofthepathogenesisandvaried

clinicalmanifestationsobservedintoxoplasmosis.Casesaredifferentiatedas

totimeofacquisition,i.e.,postnatalandprenatalinfections.

FormsofPostnatalToxoplasmaInfection

&Primaryinfectioninimmunocompetentpersons.Thisisthemostfre-

quentformwithoutclinicalmanifestations,recognizablebythespecific

serumantibodies.Theinfectioncanpersistforthelifeofthehost,andit

mayexacerbateinresponseto immunosuppression. Subacute cervical

lymphadenitisoccursinabout 1 %ofinfectedpersons.

&Primaryinfectionduringpregnancy.Thismaycauseprenatalinfection

ofthefetusandthusbecomeasignificantthreat(seeprenataltoxoplasmosis

p. 51 3).

&Primaryinfectioninimmunosuppressedpersons.Incasesofimmune

deficiency(withsignificantdisturbanceofCD4+andCD8+cellfunctions)or

immunosuppressanttherapies(e.g.,inorgantransplantations)theinfection

givesrisetofebrilegeneralizedillnesswithmaculopapulousexanthema,

generalizedlymphadenitis,necrotizinginterstitialpneumonia,hepatosple-

nomegaly,mycocarditis,meningoencephalitis,eyedamage,andotherman-

ifestations.Thereisahighrateoflethalityifleftuntreated.

&Reactivationtoxoplasmosisincasesofimmunedeficiency.Localand

generalizedreactivationofaToxoplasmainfectionoriginatingfromtissue

cysts.Cerebralmanifestationsarethemostfrequent(upto 40 %ofpatients

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Kayser, Medical Microbiology © 2005 Thieme