Morphologyandlifecycle.C.hominisandC.parvuminhabitmainlythesmall
intestineandproduceoocysts 4 – 5 lmindiameter.Followingperoralinges-
tionofinfectiousoocysts,eachofwhichcontainsfoursporozoites,the
releasedsporozoitesinvadeenterocyteswhereeachstageresideswithina
parasitophorousvacuolejustbeneaththecellmembraneinthemicrovillus
regionofthehostcell.Thislocalizationistypicalofcryptosporidia(Fig.9. 15 ).
FollowingformationoftypeImerontswitheightmerozoites,thelattercan
infectnewcells.Inthefurthercourse,typeIImerontswithfourmerozoites
areproducedthatgiverisetosexualforms(gamogony).Thefertilizedzygote
encyststoproduceabout 80 %thick-walledand 20 %thin-walledoocysts.The
oocystssporulatewhilestillintracellularintheintestine.Eachsporulated
oocystcontainsfourfreesporozoites(i.e.,notenclosedinasporocyst).
Thin-walledoocystscanburstwithinthehost,releasingsporozoitesthat
causeendogenousautoinfections.Afterabriefprepatentperiod(twoto
four,sometimesupto 12 days),thick-walledoocystsareshedwithfeces
andcanimmediatelyinfectnewhosts.Itisassumedthatpersistentinfections
inimmunodeficientpersons areduetoendogenousautoinfections by
sporozoitesfromthin-walledoocystsorbymerozoitesfromtypeImeronts.
Epidemiology.C.hominisistransmittedwithinthehumanpopulation.Hu-
mansmayalsoacquirezoonoticinfectionswiththebovinetypeofC.parvum
(mainsourceofinfection:calves)orrarelywithotherspeciesorgenotypesof
animalorigin.
Transmissionoftheoocystsisbythedirectfecal-oralrouteorincontami-
natedfoodsordrinkingwater.TheoocystsofC.parvumremainviableincool
waterformonths.Thisexplainstheetiologyofmajorepidemicsduetofecal
contaminationandimproperprocessingofdrinkingwatersuchasoccurred
inMilwaukeein 1993 with 403000 personsinvolved.Sewagecontainedup
to 13000 oocystsperliter,surfacebodiesofwaterupto 112 oocystsperliter.
Asfewas 30 – 100 oocystsaresufficienttoinduceaninfectioninhumans.
Clinicalmanifestations.Cryptosporidiainhabitmainlythesmallintestine,
wheretheymaycausedestructionofmicrovilli,shortening,swelling,andfu-
sionofthevilliandcellularinfiltrationofthemucosa.Theseverityandcourse
ofaninfectiondependsontheimmunestatusoftheinfectedperson.
&Immunocompetentpersons.Infectionseithertakeaninapparentcourse
orresult,afterincubationperiodsoffiveto 28 days,inacute,self-limiting,in
mostcasesmildillnesseslastingoneto 26 dayswithdiarrheaandvarious
generalizedsymptoms.
&Immunodeficientpersons.Chronicinfectionswithseverediarrheaand
longperiodsofoocystexcretion,e.g.,inAIDSpatients.Thediarrheaiswatery,
voluminous,choleralikeandoftenassociatedwithothersymptoms(abdom-
inalpain,nausea,vomiting,mildfever,etc.).InHIVpatients,cryptosporidia518 9 Protozoa9
Kayser, Medical Microbiology © 2005 Thieme