Medical Microbiology

9 Protozoa

J.Eckert

Generalinformationonparasites.Aparasite(fromtheGreekwordparasitos)

isdefinedasanorganismthatlivesinamoreorlesscloseassociationwith

anotherorganismofadifferentspecies(thehost),derivessustenancefromit

andispathogenictothehost,althoughthispotentialisnotalwaysexpressed.

Inthewidersense,thetermparasitereferstoallorganismswithsuchchar-

acteristics.Inmedicinethetermisusedinanarrowersenseanddesignates

eukaryoticpathogens,whichbelongtotheprotozoa(unicellularorganisms

Chapter9)andmetazoa,includinghelminths(parasitic“worms,”Chapter 1 0),

arthropods(Chapter 1 1),andsomeothergroupsoflowermedicalsignifi-

cance(Annelida,Pentastomida,notcoveredinthisbook).Parasitescause

numerousdiseases(parasitoses)inhumans,somebeingofextraordinary

significance(e.g.,malaria).OfpracticalconcernincentralEuropeareboth

autochthonousandimported(tropicalandtravelers’)parasiticinfections.

Auniformdiseasenomenclaturehasbeenadoptedinthisbookwiththe

soleuseofthesuffix–osis(plural–oses)—forexampletrypanosomosisand

nottrypanosomiasis.Thissystem,basedontheStandardizedNomenclature

ofParasiticDiseases(SNOPAD)(originallypublishedin 1988 andrecom-

mendedbytheInternationalSocietyofParasitologists)avoidstheinconsis-

tentusageofdiseasenames,suchasleishmaniasisontheonehandand

toxoplasmosisontheother.

Aselectionofthemostimportantparasitosesispresentedinthefollowing

chapters.InTable 1. 10 (p.28)zoonosescausedbyparasitesarelisted.

Parasitic protozoaare eukaryotic, single-celled microorganisms about

1–1 50 lminsizeandenclosedbyatrilaminatedcellmembrane.Theypossess

one,rarelytwonuclei(andmultinuclearreproductiveforms).Reproduction

isasexualbybinaryormultiplefissionofthecell,orsexual.Thecellularcon-

structionoftheprotozoaisgenerallythesameasinothereukaryotesbut

theyalsoexhibitsomespecialfeatures.Forexample,duringthecourseof

evolutionsomeprotozoa(Giardia,Entamoeba)havelostthemitochondria

secondarily,exceptseveralgenomictraitsthatwerelaterallytransferred

tothenuclei.TheapicoplastpresentinsomespeciesofApicomplexa(see

Toxoplasma)isaresidualofaformerplastidtypicalfortheirancestors.

Someprotozoacontainspecializedorganelles,suchasglycosomes(exclu-

sivelyintrypanosomatids),hydrogenosomes(trichomonadsandprotozoa

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Kayser, Medical Microbiology © 2005 Thieme