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The
name ‘proto-zoa’ literally means ‘first
animals’ and early classification systems grouped the
protozoa as basal members of the animal kingdom. However,
they were recognized as a discrete assemblage on the basis
of their unicellularlity and were assigned to the taxon Protozoa
(but still invariably figured as the trunk of the animal tree
of life). Members of the subkingdom Protozoa are quite disparate;
indeed the taxon has never been considered a natural assemblage
of organisms but rather one of convenience. More recently,
the protozoa have been classified together with several algal
and fungal groups in the kingdom Protista (protozoa representing
the motile protists). Irrespective of contemporary classification
systems, most parasitological texts continue to use the name
protozoa for historical reasons.
Protozoa
are eukaryotic organisms (with a membrane-bound nucleus) which
exist as structurally and functionally independent individual
cells (including those species which are gregarious or form
colonies). None have adopted multicellular somatic organisation
characteristic of metazoan organisms. Instead, protozoa have
developed relatively complex subcellular features (membranes
& organelles) which enable them to survive the rigours
of their environments. Most protozoa are microscopic organisms,
only a few grow to a size large enough to be visible to the
naked eye. As unicellular eukaryotes, protozoa display all
the same essential life activities as higher metazoan eukaryotes:
they move about to survive, feed and breed.
Biodiversity
Four
main groups of protozoa are recognized on the basis of their
locomotion using specialized subcellular and cytoskeletal
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Amoebae
use
pseudopodia (singular: pseudopodium) to creep or crawl
over solid substrates. Pseudopodia (or ‘false
feet’) are temporary thread-like or balloon-like
extensions of the cell membrane into which the protoplasm
streams. Similar amoeboid motion has been observed in
cells of many life-forms, especially phagocytic cells
(e.g. human macrophages).
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Flagellates
use
elongate flagella (singular: flagellum) which undulate
to propel the cell through liquid environments. Flagella
are ‘whip-like’ extensions of the cell membrane
with an inner core of microtubules arranged in a specific
2+9 configuration (2 single central microtubules surrounded
by 9 peripheral doublets). This configuration is conserved
throughout eukaryotic biology, many organisms produce
flagellated cells (e.g. human spermatozoa). |
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Ciliates
use
numerous small cilia (singular: cilium) which undulate
in waves allowing cells to swim in fluids. Cilia are
‘hair-like’ extensions of the cell membrane
similar in construction to flagella but with interconnecting
basal elements facilitating synchronous movement. Ciliated
cells are found in specialized tissues and organs in
many other higher life-forms (e.g. human bronchial epithelial
cells). |
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Sporozoa
(‘spore-formers’)
were originally recognized not on the basis of their
locomotion, but because they all formed non-motile spores
as transmission stages. Recent studies, however, have
shown that many pre-spore stages move using tiny undulating
ridges or waves in the cell membrane imparting a forward
gliding motion, but the actual mechanisms involved are
not yet known. |
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Protozoan
biodiversity (or species richness) includes counts (or estimates)
of some 32,000 extant (living) species and another 34,000
extinct (fossil) species (especially foraminifera). Of those
alive today, some 21,000 species occur as free-living organisms
in aquatic or terrestrial environments, whereas the remaining
11,000 species are parasitic in vertebrate and invertebrate
hosts. There are approximately 6,900 flagellate species (1,800
parasitic, 5,100 free-living), 11,550 amoebae species (250
parasitic, 11,300 free-living), 7,200 ciliate species (2,500
parasitic, 4,700 free-living) and 5,600 sporozoan species
(all parasitic).
Life-cycles
Most
protozoa have enormous reproductive potential because they
have short generation times, undergo rapid sequential development
and produce large numbers of progeny by asexual or sexual
processes. These characteristics are responsible for many
protozoan infections rapidly causing acute disease syndromes.
Parasites may multiply by asexual division (fission/splitting
or internal/endogenous budding) or sexual reproduction (formation
of gametes and fertilization to form zygote, or unique process
of conjugation where ciliates exchange micronuclei).
Protozoan
developmental stages occurring within hosts generally consist
of feeding trophozoites, and they may be found intracellularly
(within host cells) or extracellularly (in hollow organs,
body fluids or interstitial spaces between cells). While trophozoites
are ideally suited to their parasitic mode of existence, they
are not very resistant to external environmental conditions
and do not survive long outside of their hosts. To move from
host-to-host, protozoan parasites use one of four main modes
of transmission: direct, faecal-oral, vector-borne and predator-prey
transmission. |
direct
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faecal-oral
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vector-borne
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predator-prey
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direct
transmission
of
trophozoites through intimate body contact, such as
sexual transmission (e.g. Trichomonas spp.
flagellates causing trichomoniasis in humans and bovine
infertility in cattle). |
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faecal-oral
transmission
of
environmentally-resistant cyst stages passed in faeces
of one host and ingested with food/water by another
(e.g. Entamoeba histolytica, Giardia duodenalis
and Balantidium coli all form faecal cysts
which are ingested by new hosts leading to amoebic dysentery,
giardiasis and balantidiasis, respectively). |
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vector-borne
transmission
of
trophozoites taken up by blood-sucking arthropods (insects
or arachnids) and passed to new hosts when they next
feed (e.g. Trypanosoma brucei flagellates transmitted
by tsetse flies to humans where they cause sleeping
sickness, Plasmodium spp. haemosporidia transmitted
by mosquitoes to humans where they cause malaria). |
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predator-prey
transmission
of
zoites encysted within the tissues of a prey animal
(e.g. herbivore) being eaten by a predator (carnivore)
which subsequently sheds spores into the environment
to be ingested by new prey animals (e.g. tissue cysts
of the sporozoan Toxoplasma gondii being ingested
by cats, and tissue cysts of the microsporan Thelohania
spp. being ingested by crustaceans). |
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Taxonomic
overview
Flagellates
and amoebae are considered to be closely related, because
some amoebae form transient flagellated stages (to aid
in dispersal) and some flagellates exhibit intermittent
amoeboid motion. Two groups of flagellates are recognized
on the basis of the presence or absence of chloroplasts:
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Phytoflagellates
with
chloroplasts derive energy by photosynthesis. Most are
free-living aquatic organisms and some exhibit periodic
blooms (e.g. red tides). Others contain potent neurotoxins
and cause paralytic shellfish poisoning. |
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Zooflagellates
without
chloroplasts derive energy by the absorption of nutrients
or the ingestion of food particles. Many species occur
as free-living aquatic organisms whereas others live
in insects and some vertebrates as symbiotes, commensals
or parasites (several species cause major human diseases
such as sleeping sickness, Chagas disease, kala azar
and diarrhoea). |
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phytoflagellates |
zooflagellates |
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Two
groups of amoebae are recognized on the basis of the
types of pseudopodia formed with or without regular
microtubule arrays:
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Rhizopod
amoebae
produce broad lobopodia, fine filopodia or net-like
reticulopodia which do not contain regular microtubule
arrays. Many aquatic species contribute to water quality
by consuming bacteria and algae whereas terrestrial
species contribute to soil health via nutrient cycling.
Some species, such as foraminifera, build unique tests
(shells) which contribute to fossil records.
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Actinopod
amoebae
form radial axopodia which are stiffened by internal
arrays of microtubules arising from an organizing centre.
All species are free-living planktonic organisms, marine
species known as radiolaria, and freshwater species
known as heliozoa (or sun animacules). |
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rhizopods |
actinopods |
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The ciliates
are regarded to be quite separate from other groups,
more because they possess 2 types of nuclei (vegetative
macronuclei and reproductive micronuclei) than because
they possess cilia. Three groups are recognized on the
basis of their patterns of somatic (body) and buccal
(oral) ciliature: |
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Lower
holotrichs
have simple body and oral ciliature. Most are free-living
aquatic species but some are highly specialized symbionts
aiding cellulose digestion in herbivores.
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Higher
holotrichs
have simple body ciliature but more specialized oral
ciliature forming membranelles. Most occur as free-living
aquatic organisms but some live as commensals or parasites
in a range of animals. |
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Spirotrichs
have
reduced body ciliated but well developed oral ciliature
forming an adoral zone of membranelles. The majority
are bactivores living in aquatic and terrestrial habitats. |
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lower
holotrich |
higher
holotrich |
spirotrich |
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All
sporozoa are obligate parasites, they form temporary
non-motile spores which contain infective cells. Four
major groups are recognized on the basis of different
spore morphology: |
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Apicomplexan
parasites form distinctive oocysts containing infective
sporozoites. Many species occur only in invertebrates
whereas others may infect vertebrates causing severe
diseases (such as malaria, tick fever, diarrhoea or
abortion).
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Microsporan
parasites
form unicellular spores containing coiled polar tubes
used to infect host cells. Most species infect invertebrates
(especially insects) although some form cysts in vertebrates
(mainly fish). |
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Haplosporidian
parasites form unicellular spores without polar filaments
in the tissues of aquatic invertebrates. They cause
significant morbidity and mortality in oysters throughout
the world. |
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Paramyxean
parasites
form unique spore-within-spore arrangements within the
tissues of bivalves and polychaetes. They cause QX and
Aber disease in oysters. |
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apicomplexa |
microspora |
haplospora |
paramyxea |
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