Intestinal parasites
Introduction
These notes are a guide to the undergraduate medical student lecture series at McGill
University. They are not meant to replace a textbook or information that can only be
presented well in lecture or tutorial (problem solving) format.
Definitions
Medical parasitology: the study of the parasites of man and their medical consequences.
Parasite: living organism requiring intimate prolonged contact with another living organism
to meet some of its basic nutritional needs. In a more restricted definition, it refers to
organisms that are not viruses, bacteria, fungi, rickettsia, or chlamydia and obviously include
organisms of varying complexity from a unicellular protozoa to a complex multicellular
helminths.
Host: organism harbouring a parasite.
Definitive host: animal harbouring the adult or sexually mature stage of the parasite.
Intermediate host: animal in which development occurs but in which adulthood is not
reached.
Life cycle: for survival and reproduction reasons many parasites evolve through a number of
morphologic stages and several environments or different hosts. The sequence of
morphologic and environmental stages is referred to as the life cycle.
Parasitic infection: invasion by endoparasites (protozoa and helminths).
Parasitic disease: invasion and pathology produced by endoparasites.
Parasitic infestation: external parasitism by ectoparasites (arthropods).
Commensalism: the association of two different species or organism in which one is
benefited and the other is neither benefited nor harmed. (e.g. non pathogenic intestinal
protozoa)
Reservoir host: an animal that harbours a species of parasite that can be transmitted to and
infect man.
Vector: an arthropod or other living carrier that transports a pathogenic organism from an
infected to a non-infected host. This can be passive transport or as an essential host in the life
cycle of the pathogenic organism (i.e. a biologic vector).
Carrier: a host that harbours a parasite but exhibits no clinical signs or symptoms.
Zoonosis: a disease involving a parasite for which the normal host is an animal, and wherein
man can also be infected.
Protozoa: a subkingdom consisting of unicellular eukaryotic (Greek-karyon=nut=nucleus)
animals.
Eukaryote: a cell with a well-defined chromosome in a membrane bound nucleus. (versus
prokaryotic bacteria with nucleic acid material bound in a nuclear membrane).
World annual rates of morbiditv and mortality
Protozoa
Nematodes
Trematodes
Cestodes
malaria
amoeba
toxoplasma
trypanosoma
intestinal
nematodes
filaria
onchocerca
schistosoma
tapeworms
Infections
(millions)
800
480
24
Disease
(millions)
150
50
40
1.2
Deaths
(thousands)
1500
100
10
60
2400
2.6
80
250
30
200
2.5
3
5
20
<1
50
1000
Taxonomy
CLASSIFICATION
Kingdom
Subkingdom
Phylum
Subphylum
Subphylum
Phylum
Phylum
Phylum
CLASSIFICATION
Kingdom
Subkingdom
Phylum
Phylum
Class
Class
NAME
Animalia
Protozoa
Sarcomastigophora
Sarcodina
Mastigophora
Apicomplexa
Ciliophora
Microspora
NAME
Animalia
Metazoa
Nematoda
Platyhelminthes
Cestoidea
Trematoda
EXAMPLE GENUS
Entamoeba
Giardia
Plasmodium (malaria)
Balantidium
Enterocytozoan
(microsporidium)
EXAMPLE GENUS
Ancylostoma (hookworm)
Taenia (tapeworm)
Clonorchis (liver fluke)
Phylum
Arthropoda
Anopheles
mosquito)
(malaria
vector
.
Intestinal Protozoa
OVERVIEW
The following areas of knowledge are suggested as especially important for the beginner.
-biology: systematics, structural and motility features
- pathogenesis: in small intestine of Giardia, Microsporidia, Cryptosporidia,
Cyclospora
in large intestine of : Entamoeba histolytica
- epidemiology: zoonoses, carrier states, fecal-oral transmission
- clinical features: acute, chronic, asymptomatic carrier states, opportunism
in the immunocompromised
- diagnosis: stool examination, stains, transport preservatives
- treatment: metronidazole, diodoquine, trimethoprim/sulfa
- problems: drug resistance; pathogenesis; laboratory identification
Taxonomy
The classification of the medically important parasites is as follows (ref. Beaver-Clinical
Parasitology 1984) within the kingdom "Animalia".
Subkingdom Protozoa: 45,000 unicellular species, each defined in the phylum according
to organelles, locomotion, life cycle and type of reproduction.
Phylum Sarcomastigophora.
Subphylum - Mastigophora: movement with flagella - e.g. Trichomonas, Giardia
Subphylum - Sarcodina: pseudopodia, e.g. amoeba
Phylum Apicomplexa: apical complex, no locomotor apparatus; sexual
reproduction, e.g. malaria, Isospora, Toxoplasma
Phylum Ciliophora: movement with cilia, e.g. Balantidium.
Phylum Microspora: e.g. Enterocytozoon
trophozoites
cysts
Sarcodina
Ciliophora Mastigophora Apicomplexa Microspora
eg
eg.
eg.
eg
eg
E.
Balantidium Giardia
Cyclospora Enterocytozoon
histolytica
Subkingdom-Metazoa multicellular organisms.
Phylum-Nematoda: round worms, round in cross section; separate sexes; complete digestive
tract; 500,000 species only a few parasitic to man; e.g. hookworm.,
filaria
Phylum - Platyhelminthes: flat worms; incomplete or absent digestive tract; no body cavity
; mostly hermaphroditic.
Class Trematoda: flukes; leaf shaped unsegmented body, often complex life cycle;
e.g. lung fluke.
Class Cestoidea: tapeworms; segmented bodies each segment containing complete set of
male and female reproductive organs; no alimentary tract, nutrition by
absorption through body wall. e.g. beef tape worm.
PROTOZOA
There are 45,000 species of protozoa. A small number are parasites of man, some pathogenic
and others non-pathogenic (commensals).
The life cycles of the protozoa vary from simple binary fission (e.g. Entamoeba histolytica) in
one host to a complicated sequence of morphologic transformations through several hosts
(intermediate and definitive) e.g. malaria.
The biology of the organism, pathogenesis of disease and epidemiology will be discussed
with emphasis on the common or representative organisms.
A taxonomic approach to classification is of biological importance but a clinical classification
is useful for the physician.
Taxonomic or Clinical
Taxonomy
Mastigophora
Clinical
Intestinal protozoa (eg amoeba, Giardia)
Sarcodina
Apicomplexa
Ciliophora
Microsporidia
Tissue protozoa
Blood protozoa
(eg Toxoplasma)
(eg malaria)
Intestinal protozoa
The important intestinal protozoa that infect man fall within the following 5 catagories:
Sarcodina:
Entamoeba histolytica**
(**=pathogenic)
Entamoeba dispar
Iodomoeba butschlii
Dientamoeba fragilis**
Endolimax nana
Entamoeba coli
Entamoeba hartmani
Apicomplexa:
Cryptosporidium parvum.**
Isospora belli **
Cyclospora cayetanensis**
Mastigophora:
Giardia lamblia**
Trichomomas hominis
Chilomastix mesnili
Ciliophora:
Balantidium coli**
Microsporidia:
Enterocytozoon bienusi**
Only eight have been shown to be pathogenic and therein lies a major problem. Some
significant expertise is required to diagnose the genus or species of the protozoan in a
laboratory specimen; lacking this, diagnoses are very difficult.
The characteristics important to the clinical parasitology microscopist include nuclear shape
and size, chromatin distribution, the micrometer measured size of the protozoan, intracellular
organelles and locomotion.
Entamoeba histolytica
E.
Subphylum: Sarcodina
histolytica trophozoite
with ingested RBCs
E. histolytica trophozoite
Biology: Two morphological stages occur
Trophozoite - metabolically active invasive stage, moves with pseudopodia, ingests RBC,
lives in colon and is found in fresh diarrheal stool; divides by binary fission.
- trophozoite 10-60 µm
- cogwheel distribution of nuclear chromatin
- hematophagous
- unidirectional movement with pseudopodia
Cyst - "vegetative" inactive form resistant to unfavourable environmental conditions outside
human host;
- 4 nuclei
- this is the infective form resistant to stomach acid if swallowed
- survives up to 30 days; excyst to trophozoite on passing through stomach
- cyst 10-20 µm
- chromotoidal body
Pathogenesis: - Digests (liquifies) human host cells (colon wall, neutrophils, liver cells)
Disease states:
- asymptomatic carrier- symptomatic infection
- amoebic dysentery - mucoid bloody
- amoebic - liver or lung abscess
Diagnosis:
- stool examination - for trophozoites and cysts
- amoebic serology
- abscess aspirate
- Entamoeba dispar a non-pathogen is indistinguishable by microscopy and is a much more
common intestinal protozoan than Entamoeba histolytica. Antigen capture and PCR tests
can distinguish E. dispar from E. histolytica in heavier infections.
Treatment:
Invasive states (Dysentery, Liver abscess): metronidazole
Carrier states: diiodoquine, diloxanide furoate, or paromomycin
Giardia lamblia
Subphylum: Mastigophora
Biology:
- occurs as both a flagellated trophozoite and a non-flagellated cyst form
- trophozoite (9-21 µm long), motile, with 8 long flagella, ventral sucker which attaches to
duodenal mucosa; lives only in small intestine; non invasive.
- cyst (8-12 µm); resistant to external environment, to municipal chlorination; intermittently
expelled in stool.
Giardia trophozoites
Giardia cyst
http://www.dpd.cdc.gov/dpdx/HTML/ImageLibrary/Giardiasis_il.htm
Epidemiology:
- faecal oral spread
- prevalence about 3-5% in Canada; increased in some travelers, backpackers, institutions,
day cares and any groups with increased fecal-oral spread.
- zoonosis - found in most mammals; esp. beaver, cattle, cats, dogs, etc.
Pathogenesis: - mechanism unknown; toxin? host immunity?
Giardia trophozoites in small intestine
- more common in IGA deficient
- some immunity occurs post infection
- pathology: villus atrophy and crypt hyperplasia
Clinical:
- 90% of infected are asymptomatic carriers
- acute giardiasis includes diarrhea, gas, anorexia for 1-2 weeks
- chronic giardiasis - diarrhea, malabsorption
Diagnosis:
- stool examination
- duodenal fluid (aspirate or string test)
- giardia antigen detection in stool
Treatment: - metronidazole, atabrine.