WELCOME
TO
MEDICAL
PARASITOLOGY II

1. INTRODUCTION TO
MEDICAL
PROTOZOOLOGY
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Learning Objectives
Up on completion of this unit, you should be able
to:
Describe the general characteristics of
protozoans
Describe the basic structure of protozoa
Describe the basic properties of protozoa
Describe the classification of parasitic protozoa
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OUTLINE
 General Introduction to protozoa
 Definition
 Diversity and importance of protozoa
 General morphology of protozoa
 Basic structure of protozoa
 Basic properties of protozoa
 Protozoan life cycle stages
 Protozoan taxonomy
 General characteristics of the five main groups of
protozoa
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What are Protozoa?

Definition
 single-celled eukaryotic organisms
 kingdom Protista
 Vary in size (1 -150um )
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General introduction to protozoa

=Unknown until the invention of the microscope in 1675
=First recognized by antony van leewenhoek in 1676
= He described it as little animal or animacula
Int…
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Diversity
 protozoa are extremely diverse organisms and
found in a variety of niches
Diversity and importance of protozoa
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Div…
 200,000 named species
 Most species are free-living in
=Freshwater
=marine environments
=decaying organic matter and soil
 Few are adapted to a parasitic life but all plant and
animal species have at least one protozoan parasite
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Div…
=~10,000 are parasites in a wide range of hosts
=Vertebrate
= invertebrate
= Plants
 ~20 human pathogens
 Adapted to life in a wide range of sites within the
host
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Ecological Niches in the Human Body
 Skin: Leishmania
 Eye: Acanthamoeba
 Mouth: Amoebae and flagellates (usually non-
pathogenic)
 Gut: Giardia, Entamoeba (and invasion to liver),
Cryptosporidium, Isospora, Balantidium
 G.U. tract: Trichomonas
 Bloodstream: Plasmodium, Trypanosoma
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Div…
 Spleen: Leishmania
 Liver: Leishmania, Entamoeba
 Muscle: Trypanosoma cruzi
 CNS: Trypanosoma, Naegleria, Toxoplasma,
Plasmodium
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Importance of protozoa
 Medical importance:
 Cause of more sickness and death than any other
disease-causing organisms
 significant damage to specific organs
 brain (African sleeping sickness), eyes
(acanthamoeba keratitis), heart (toxoplasmosis)
 reduced working capacity
 Loss of productivity
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 Veterinary importance:
 Death
 Potential losses
 coccidiosis costs £8 million/yr.. Prophylactics
 Animal trypanosomiasis
 Zoonoses
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Imp…

General morphology of protozoa
 Protozoa exhibit a wide variety of morphologies
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morp…
 Size: range from 1-150um
 The smaller members, 1-10um
 include most of the intracellular parasites (e.g.
plasmodium, Leishmania …
 The biggest in size belongs to ciliate
e.g. Balantidium coli
 Shape:
 No single shape that represent all
 Ranges from amorphous shapeless amoeba to
relatively rigid forms
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Basic structure of protozoa
1. Cell membrane
2. Cytoplasm: ectoplasm & endoplasm
3. Nucleus: two kinds of nuclei:
 Vesicular & compact
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1. Cell membrane: chemically similar to all eukaryotic
cells
 Thin & flexible layer called plasmalemma (sarcodina)
 Also called pellicle or ectoderm or outer membrane
 More rigid wall, usually supported by microtubules
(sub-pellicular microtubules)
 Results in more constant and uniform shape than
that of amorphous amoeba
 Characteristics of some flagellates and most
ciliates
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Stru …

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2. Cytoplasm: divided in two areas
 Ectoplasm=outer transparent layer
 Locomotion
 protection
 Sensation, respiration
 Excretion, Ingestion of food & etc
 Endoplasm= inner granular layer
 More fluid than ectoplasm, contains nuclei,
mitochondria, vacuoles & etc
 metabolism
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Stru…

3. Nucleus
 well defined nuclei bounded by nuclear envelop
 Most easily identifiable structure in protozoan cell
 Single/multiple
 Important for reproduction
 Similar in size or not
E.g. G. lamblia, ciliate (micronuclei & macronuclei)
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Stru…

Stru…
 Two morphologically distinct nuclei:
 Vesicular:
 with a clearly defined internal space resulting
from irregular distribution of chromatin
 Nucleoplasm contains one or more Karyosome
 Karyosome
nucleoli-like body
 Most protozoa of humans exhibit the vesicular
type
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Stru…
 Compact:
 appears to be a solid mass
 densely packed chromatin
 Larger than vesicular nucleus
 Are found in ciliates
 Other structures like;
 Mitochondria: sites for aerobic metabolism
 Lysosomes: degradation of ingested materials
 Ribosome: sites for protein synthesis
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A. Locomotion: Structures that mediate
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B. Reproduction
-General Protozoan Reproduction
Asexual
Reproduction
Sexual
Reproduction
Binary Fission
Mutiple Fission
Endodyogeny
Conjugation
Singamy
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Prop…

Prop…
1. Asexual reproduction
 Fission
 Division follows a sequence:
Organelles nucleus cytoplasm
A. Binary fission
 Each parent gives rise to two progeny
 Seen in amoeba, flagellates, ciliates
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Prop…
B. Multiple fission/ Schizogony/ Merogony
 Rapid organell and nuclear divisions occur at the
cell periphery
 Cytoplasmic segmentation to form separate
organisms called merozoites
 The parent or multinucleated cell is called the
schizont or segmenter and the daughter cells are
merozoites
 Seen in apicomplexan
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C. Endopolyogeny
 Daughter cells form in the center of the mother
cell (internal budding) rather than at the
periphery
 Endodyogeny is a form of endopolyogeny
where two daughters are formed
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Prop…

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2. Sexual Reproduction
A. Conjugation
 Specialized sexual reproduction (mostly in ciliates)
 Involves nuclear exchange and union after cells join
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Sexual Recombination in Paramecium
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B. Syngamy
Union of the entire cell (gametes fuse), seen in
apicomplexan
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C. Feeding
 Most parasitic protozoa are heterotrophic
 ingest particulates (phagotrophy)
 Phagocytosis:
 Engulfment of solid material
 predation on bacteria or other protozoa
 Pinocytosis:
 Invagination of membrane surrounding liquids
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 Peristome or cytostomal feeding
 Food is ingested at a definite site, using a
specialized feeding structure
 Diffusion =absorb solutes (osmotrophy) through
cell membrane
 Food may be retained in special intracellular reserves,
or vacuoles
 Some protozoan are photosynthetic (autotrophy) or
combination
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D. Excretion
 Undigested particles and wastes are extruded at the
cell surface by mechanisms that are the reverse of
those used in ingestion
 diffusion (primary mechanism)
 contractile vacuole (osmo-regulation)
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E. Respiration
 Both aerobic (Malaria) and anaerobic respiration
(dysenteric amoeba)
 Most parasitic protozoa are facultative anaerobes
 They can live in reduced oxygen environments
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Prop…

Protozoan life cycle stages/terms
1. Vegetative form: trophozoites
 stage of active feeding and reproduction
 commonly the pathogenic protozoan form
 Excystation: For the completion of life cycle, cysts
ingested by the definitive host has to change to
vegetative form
2. Resístant form: cyst
 Stage of inactivity
 Most resistant stage = hard to kill (no medication)
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Encystation
 Many protozoa form cysts – round cytoplasmic mass
surrounded by a rigid or semi-rigid cyst wall secreted
by organism
 Function:
 Protect against unfavorable external environmental
conditions
 Are the infective or transmissible form of parasite
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Conditions for encystment:
 Deficiency of host nutrient essential to the parasites
 Desiccation/dryness
 increase in osmotic pressure
 Accumulation of waste product in the medium
 Crowding
 Low pH, Temperature change
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3. Oocyst:
 results from fusion of gametes in the Sporozoa
 Infective stage in most cases:
 passed in host feces in case of intestinal
protozoans or
 in gut of the mosquito vector in the case of
Plasmodium
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4. Sporozoite:
 Formed within oocyst via asexual fission & infects
new host cells of
 intestinal epithelial cells in intestinal protozoans or
 hepatocytes in case of Plasmodium
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5. Merozoite:
 Resulted from schizogony of sporozoites
 May infect:
 intestinal epithelial cells in intestinal coccidian or
 Red blood cells in the case of Plasmodium. …
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6. Gametocyte:
 result of merozoite cell fission
 Fusion of these results in formation of oocyst
 Other life cycle stages:
 Amastigote, Promastigote, Epimastigote ,
Trypomastigote & Metacyclic Trypomastigote
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Example of Protozoan life cycle
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Intestinal Coccidian Life cycle
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Life cycle of Plasmodium species

Protozoan Taxonomy
• Confused, controversial and constantly changing
• No universal agreement = many systems & frequent
changes
• Several bases of classification;
• light microscope = Originally based on motility
• electron microscope =Ultra-structural criteria used
since 1960’s;
 4-6 phyla? In the kingdom protista
• Molecular technique: DNA sequences
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General Characteristics of five main groups
of protozoa
1. Amoebas (Subphylum Sarcodina)
= Move by extending blunt, lobe like projections -
pseudopodia (false feet)
= Amoebas engulf food with pseudopods &
phagocytize it
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Char…
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 Several genera:
•Entamoeba
•Iodamoeba
•Endolimax
•Naegleria (CNS)
•Acanthamoeba (CNS)
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Char…

= Move by means of rotation of a whip-like organelle
called a Flagellum (plural: flagella)
= The flagella can also be used for gathering and sorting
of food
= Some species have organelles for the purpose of food
ingestion (gullet or cytostome), and for excretion
(cytopyge)
2. Flagellates (Mastigophora)
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Char…

 several genera:
 Giardia
 Chilomastix
 Dientamoeba
 Trypanosoma
 Leishmania
 Trichomonas
Char…
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Giardia intestinalis
Cyst Trophozoite
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Char…

Trichomonas
T. intestinalis
T. vaginalis
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Char…

Trypanosoma
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Char…

Leishmania
Amastigote
Promastigote
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Char…

C. Apicomplexans (Phylum
Apicomplexa)
 Have specialized
organelles at tip (apex) of
cells that penetrate host
tissues
 Locomotion of mature form
by body flexion or gliding
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Char…

 Obligate intracellular parasites
 usually 2 or more hosts required
 Their life cycles have asexual and sexual phases
 several forms: sporozoites, trophozoites, merozoites,
gametocytes, schizont, oocyst & cyst.
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Char…

Toxoplasma gondii (Toxoplasmosis)
Bradyzoite
Gamete
Merozoite
Oocyst
Sporulated
oocyst
Sporocyst
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Char…

Char…
 Includes:
1. Intestinal coccidian
 Cryptosporidium
 Isospora
 Cyclospora
2. Blood and tissue coccidian
 Plasmodium
 Toxoplasma
 Babesia
 Sarcocystis
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D. Ciliates (Phylum Ciliophora)
 Move and obtain food using
cilia
 Distinct cytostome (mouth)
Structure
 Dimorphic nuclei, typically
larger macronuclei and
smaller micronuclei
 Only known human pathogen
is Balantidium coli
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Char…

E. Microsporans (Phylum Microsporidia)
 Obligate intracellular spore forming parasites
 Discovered in 1984 to cause chronic diarrhea and
conjunctivitis, mainly in AIDS patients
 at least 14 microsporidian species identified as human
pathogens: Enterocytozoon bieneusi, Encephalitozoon
intestinalis, Encephalitozoon hellem ….
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Char…

1.7. Protozoans associated with HIV/AIDS
 C. parvum, I. belli and C. cayatenensis: causing
enteritis
 Microsporidia species: causing a diarrhea with
wasting, eye disease, and disseminated disease
 Pneumocystis carnii: causing life threatening
pneumonia
 Blastocystis hominis: cause severe enteritis
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Pro …
T. gondii: causing cerebral toxoplasmosis
Leishmania species: emerging as a major
pathogens in HIV infected persons
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