Parasitic Infections: an overview Colin MacLeod Microbiology Department, RPAH

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Parasitic Infections an overviewColin
MacLeodMicrobiology Department, RPAH
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ParasitesEucaryotic organisms living in/on
defined host for all or part of life cycleHosts
identified as definitive or secondary
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Protozoa (protozoans) single-celledMetazoa
(metazoans) multi-celled
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A simple functional classificationEctoparasites
colonise body surfaceEndoparasites colonise
tissues or hollow organs
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Ectoparasites LeechesMites (scabies)LiceMagg
ots (myiasis) - all are metazoans
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Parasitic infections endemic in
Australiarelatively uncommonmainly gut
protozoaoverall not very important
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Butmajor cause of mortality, morbidity
worldwide found in migrants, visitors,
refugees, Australians travelling abroad
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ProtozoaMainlyblood malaria, babesiosis,
trypanosomestissues toxoplasmosis,
leishmaniasisluminal (gut, genitourinary)
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MetazoaMainlyFlukes (trematodes)Flatworms (
cestodes)Roundworms (nematodes)
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Peripheral blood eosinophiliaRarely seen with
protozoaSometimes seen with metazoa (partly
depends on stage of life cycle)Resolution of
eosinophilia may indicate cure
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DiagnosisConfirmatory laboratory testing
usually necessaryVisual examination
macroscopic (worms, ectoparasites), microscopic
(direct, with or without staining, processing,
histopathological) Culture occasionallyAntigen,
PCR testingSerology
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Malaria4 human speciesPlasmodium falciparum
vivax malariae
ovale
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MalariaMust have been in endemic area (NB
recurrences)Incubation period 8-21 days (min 5)
Fever is cardinal symptom, sign /- sweating,
rigorsMyalgia, headache, malaiseExamination
often normal /- hepatosplenomegaly
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Main DD is other infectionsviruses esp.
arbovirusestyphoid feveretc
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Diagnosis can be made only by blood
testingblood film examinationdetection of
malarial Ag in blood(PCR) N.B. WCC usually
normalSpeciation necessary to dictate
treatment P falciparum has different
mortality/morbidity, different drug treatment
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Complications usually only with Pfrenal
failurecerebral infectionpulmonary
oedemapregnancy infants
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Exoerythrocytic cycle occurs only with P vivax,
P ovaleAdditional treatment necessary to
eradicate persisting liver forms
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Pathogenic luminal protozoans

• GIT
• Entamoeba histolytica
• Giardia intestinalis
• Cryptosporidium, Microsporidium
• Most other protozoa are nonpathogenic
• Vagina
• Trichomonas vaginalis

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E histolytica

• Inhabits colon only some strains pathogenic
• Causes amoebic dysentery, liver abscess
• Diagnosed by microscopy (stool, pus), serology

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Toxoplasma gondiiPrimary parasite of cats
(kittens)Man other mammals 2º hostsInfection
from cat faeces, eating meatMain
syndromesasymptomatic ocular (retinal
lesions)mononucleosis syndromepregnancyimmune
suppression
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Hydatid diseaseEchinococcus granulosus, dog
tapewormEndemic in many parts of
AustraliaInfection from ingesting eggs (dog
faeces)Cysts mainly in liver, usually
asymptomaticPressure effects, may rupture
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Wuchereria bancrofti lymphatic filariasis

• Spread by mosquitoes (Aedes spp)
• Adult worms develop near lymph nodes (esp groin)
• Recurrent cellulitis common
• Lymphatic obstruction a late complication

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Diagnosisperipheral blood eosinophiliadetect
microfilariae in (nocturnal) bloodhistology of
lymph node (not recommended)filaria serology
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Flukes (trematodes)

• Blood Schistosoma spp
• Liver (biliary) Chlonorchis, opisthorchis,
  Fasciola
• Lung Paragonimus spp

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Schistosomiasis (bilharziasis)Mainly Africa
parts of Asia, Central S AmericaLife cycle
involves humans, snails, fresh waterCercaria
penetrate skin, enter circulation, settle in
abdominal veins.Adult flukes lays eggs,
penetrate adjacent tissue (bowel, bladder),
liverGranulomatous response fibrosis,
haemorrhage portal hypertension, vesical
contraction, Ca
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