Medical Microbiology

Nematoda(Roundworms) 599

intoorgansandbodytissuesbymeansoflymphogenousandhematogenous

migration.Furtherdevelopmentoccursonlyinstriatedmusclecellsthatthey

reachfivetosevendaysp.i.attheearliest.

Thelarvaepenetrateintomusclefibers,whicharenormallynotdestroyed

intheprocess,buttransformedinto“nurse”cellsprovidingasuitableenvi-

ronmentfortheparasite.Themusclecellbeginstoencapsulatetheparasite

abouttwoweeksp.i.bydepositinghyalineandfibrousmaterialwithinthe

sarcolemma.Encapsulationiscompletedafterfourtosixweeks.Thecapsules

areabout0.2–0.9mmlongwithanovalformresemblingalemon.Granula-

tiontissueorfatcellsformatthepoles(Fig. 1 0. 16 e).Thecapsulemayalso

graduallycalcifybeginningatthepoles.

TheTrichinellalarvaeatfirstliestretchedoutstraightwithinthemuscle

cell,butbythethirdweekp.i.theyrollupintoaspiralform(notobservedin

Trichinellapseudospiralisandsomeotherspecies,seeTable 1 0. 5 ).Theydiffer-

entiatefurtherduringthisperiodtobecomeinfective.Theencapsulated

Trichinellaremainviableforyearsinthehost(demonstratedforupto 31

yearsinhumans).Thedevelopmentalcycleiscompletedwheninfectious

muscleTrichinellaareingestedbyanewhost.

Epidemiology.Inmanycountriestrichinellosisexistsinnaturalfociwith

sylvaticcyclesinvolvingwildanimals,inparticularcarnivores.Suchcycles

areknowntooccurinmostoftheTrichinellaspeciesbutT.spiralisispredom-

inantlyperpetuatedinasynanthropiccycle(Table 1 0. 5 ).Humanscanacquire

theinfectionfromsylvaticcyclesbyeatingundercookedmeatofwildboar,

bearetc.containinginfectiveTrichinellalarvae.Sylvaticcyclesmayremain

restrictedtonaturalfociwithoutspreadingtodomesticpigsorotherdomes-

ticanimals.ThisisapparentlythecasewithT.britoviinSwitzerland,for

example.Humaninfectionsaremostfrequentlyderivedfromthesynanthro-

piccycleofT.spiralis.

Encapsulatedmusclelarvaeareveryresistant.Theyremaininfectiousfor

atleastfourmonthsinrottingmeat.Cooledto 2 – 48 C,theysurviveinmus-

culaturefor 300 days.Theyaregenerallykilledbydeep-freezingto–25 8 C

within 10 – 20 days,althoughmusclelarvaeofthecold-resistantspecies

T.nativamayremaininfectiveformanymonthsat–20 8 C(Table 1 0. 5 ).

Heatisrapidlylethal,butthelarvaecansurvivedryingandpickling.

Thesourcesofhumaninfectionarerawandinsufficientlycookedor

frozenmeatproductsfromdomesticpigsandwildboars,horses,andless

frequentlyfrombears,dogs,andotheranimalspecies.Driedandpickled

meatcontainingtrichinellaecanalsobeinfective.

Clinicalmanifestations.Theseverityanddurationofclinicalmanifestations

dependontheinfectivedoseandtherateofreproductionofthetrichinellae.As

fewas 50 – 70 T.spiralislarvaecancausediseaseinhumans.Thepathogenicity

oftheotherspeciesisapparentlylower.Infectionsrunatwo-phasecourse:

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Kayser, Medical Microbiology © 2005 Thieme