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Protozoans are an informal group of “animal-like” unicellular organisms

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Important note: Protozoans are an informal group of “animal-like” unicellular organisms. For many years the four main groups of protozoa were amoebae, ciliates, flagellates, and spore formers. Because the true evolutionary relationships of many protozoan groups have yet to be elucidated, the taxonomic scheme that we will use continues to follow the principles of traditional taxonomy rather than cladistic taxonomy. Consequently, some of the following taxa may be either para- or polyphyletic (if those terms are not familiar to you, let me know so we can remedy that). The reason for this approach is that it is a compromise between reasonably current evolutionary thinking and the practical need for a system of nomenclatures that allow us to communicate with each other and retrieve information from the older literature.


protozoans – over 64,000 species in 7-30 phyla (depending on system of classification); unicellular and eukaryotic

outermost layer is plasma membrane (lipid bilayer with various proteins), next is ectoplasm (gel), both enclose endoplasm (sol)

pellicle – rigid body wall of some protozoans; supported by microtubules

nucleus – all protozoans have membrane-bound nucleus; some have single, others two or more, others (ciliophorans) two different types: macronucleus & micronucleus (sexual reproduction)

endosome = nucleolus (unlike nucleoli, do not disappear during mitosis)

lysosomes – contain hydrolytic enzymes; intracellular digestion

basal body (also called kinetosome, blepharoplast); anchors flagellum in cytoplasm; 9 + 2 arrangement of microtubules (morphologically identical to centriole)

parabasal body – homologous to Golgi complex

mastigont (whip) = flagellum + parabasal body (Golgi) + basal body

mitochondria – sites of aerobic metabolism; tubular-shaped cristae; kinetoplast: giant mitochondrian in some flagellated protozoans; some parasitic protozoans (eg, Entomoeba histolytica) lack mitochondria (associated with anaerobic metabolism)

locomotor organelles

flagellate: flagella and undulating membrane

ciliate: cilia (miniature flagella)

amoeba: pseudopods – constant flow of endoplasm in direction of extension (sol/gel transformations)

lobopodia type of pseudopodia most common form among parasitic amoebae (pseudopods lack internal support structures)

protozoans excrete ammonia; contractile vacuole is absent in marine and most parasitic protozoans except Balantidium

encystment and excystment:

trophozoites, trophs, or vegetative state - active, motile form (as opposed to resistant cyst)

(a) trophozoite (b) cyst
reproduction – asexual (binary fission, multiple fission) and sexual

Flagellated Protozoans – move by flagella

Class Zoomastigophorea – one to many flagella; no chloroplasts

Order Kinetoplastida – one or two flagella; kinetoplast (a large specialized mitochondrion)

different morphological types of Trypanosoma and Leishmania

amastigote, promastigote, epimastigote, trypomastigote

Trypanosoma – in all vertebrate classes; usually in blood, spleen, lymph nodes, or tissue fluids;

most transmitted by blood-feeding invertebrates; usually do not enter or live in cells (exception: T. cruzi); does not produce cysts; no sexual reproduction; amastigote and trypomastigote form found in vertebrate host; amastigote, promastigote, and epimastigote form found in the invertebrate host (infective trypomastigote form also found in the invertebrate host)

African trypanosomiasis

T. brucei – tropical Africa, transmitted by tsetse flies (Glossina); 3 anatomically indistinguishable subspecies (T. b. brucei, T. b. gambiense, T. b. rhodesiense); anterior station development in tsetse fly (in front/middle portion of digestive tract)

T. b. brucei – normal hosts are native grazers, called “nagana”; does not infect humans; deadly to domestic livestock

life cycle:

trypomastigote enters fly in blood meal and duplicates in midgut - moves to foregut - enters

salivary glands and develops to epimastigote - duplicates as epimastigote - transforms into the infective trypomastigote form - enters vertebrate via bite of tsetse fly - duplicates in blood and lymph - invades CNS; host physiological processes are disrupted; lysis of host cells; anemia, edema, fever, watery eyes and nose; death can be in few days or recovery may occur depending on host species and susceptibility

control - removal of brush, brushless belts, reservoir elimination, night grazing (flies feed during day), insecticides

T. b. gambiense (chronic form) and T. b. rhodesiense (acute form) – African Sleeping Sickness

in blood, lymph nodes, spleen, cerebrospinal fluid; after the bite, all organs are invaded; intermittent fevers, sleepiness, coma, death in 4 months to 7 years depending on form; Winterbottom’s sign – swollen neck lymph nodes; observation diagnosis, blood tests; drug treatments of varying success

VATs (variant antigenic types) - trypanosomes use a genetic set of programmed successive antigenic types (~1000 known to exist) which cause repeated remissions until death; antigenic series sequence is predictable

American trypanosomiasis

life cycle:

T. cruzi – Chagas’ Disease; South and Central America

transmitted by kissing bug, Triatoma; posterior station development in kissing bugs (in hindgut); bugs bite at night; infective forms in bug feces, usually scratched into bite site or eyes; trypomastigotes in blood, transform into amastigotes in host macrophages, muscle and other tissue cells, duplicate in amastigote form - pseudocysts rupture - nearby nerve cells are destroyed; favorite site is cardiac muscle, but also in cells of other muscles (smooth and skeletal), spleen, liver, and lymphatics; many wild and domestic animals (dogs and cats) are reservoirs; organism can cross the placenta

diagnosis - find blood forms, lab tests, culture in clean bugs (xenodiagnosis)

there is no effective treatment

disease - edema of eyelids and face, megacolon, megaesophagus; death in 3 to 4 weeks from heart failure
prevention - bug control, modern housing, do not scratch bites

T. lewisi – cosmopolitan in blood of rats; intermediate is rat flea, Nosopsyllus fasciatus; posterior

station development; rat is infected by ingesting flea feces or the flea itself; ablastin - a rat-

produced antibody that inhibits parasite reproduction

– leishmaniasis; in tissue cells; anterior station development in sand fly (Phlebotomus and Lutzomyia) intermediate; duplicate in amastigote form in vertebrates (human, dog, rodent); duplicate in promastigote form in sand flies; promastigotes inoculated via “throat clearing” as sand fly feeds; disease may be visceral, cutaneous, or mucocutaneous depending on species; no cysts; no sexual reproduction; diagnosis is by finding the flagellates; antimony compounds used as treatment (antimonials are toxic and dangerous)

Leishmaniasis is presently a big problem for our troops serving in Iraq

L. tropica & L. major – Oriental Sore; Jericho boil; cutaneous; Africa, Middle East, Asia, India

invades the reticuloendothelial system and skin; secondary infections are common

Phlebotomus intermediate; only symptom is sore; self-healing, provides immunity

L. donovani – kala-azar; visceral; Mediterranean, India, Africa, Central and South America

in cells of reticuloendothelial system (spleen, liver, lymph nodes)

slowly destroys body’s defense system; liver and spleen enlarge; death in 6 mo. to 3 yrs.

Phlebotomus intermediate; dogs and cats are reservoirs

diagnosis - spleen puncture, blood tests; prevention is reservoir and vector control

treatment not always successful; inadequate treatment - post-kala-azar dermal leishmanoid

L. braziliensis – espundia; uta; chiclero ulcer(?); mucocutaneous; Northern Argentina to Central Mexico

secondary lesion up to 30 years later ulcerates and destroys lips, palate, pharynx, tongue

Lutzomyia intermediate; primarily sylvatic - little chance for control

L. mexicana – almost always cutaneous; Central America, Mexico, Texas

Lutzomyia intermediate; a zoonose - main reservoirs are rodents

(recent research – parasite that causes chiclero ulcer is probably L. mexicana)

Classification of this group in flux

Order Diplomonadida – two nuclei

Giardia lamblia (= intestinalis = duodenalis) – giardiasis; cosmopolitan; inhabits small intestine

most common flagellate of human digestive tract

dorsoventrally flattened; ventral surface is a bilobed adhesive disc

4 pair of flagella; trophozoite has 2 nuclei, fully developed cyst has 4 nuclei

interferes with fat absorption in small intestine; many symptomless cases

if pathogenic - diarrhea, weight loss, abdominal pain, flatulence

many reservoirs including dogs, rats, rabbits, others

observation diagnosis - stool exam; drug treatment is Flagyl (=metronidazole)

prevention is sanitation and water treatment

Order Trichomonadida – 4 to 6 flagella, axostyle, no cysts

in reproductive and intestinal tracts of hosts

Trichomonas – undulating membrane, 4 anterior flagella, asexual fission only

T. tenax - cosmopolitan; commensal in mouth of humans, dogs, probably many others

T. vaginalis - cosmopolitan in vagina, prostate, urethra of humans; normally spread by sexual

contact, also in hot tubs; pathogenicity varies; observation diagnosis; drug treatment is

Flagyl (Flagyl + alcohol = violent nausea); this parasite survives in frozen semen

Pentatrichomonas hominis – cosmopolitan; commensal in humans

5 anterior flagella; inhabits large intestine and caecum

Tritrichomonas foetus – abortion disease in cattle; reproductive tract of cattle; cosmopolitan

3 anterior flagella; observation diagnosis; very difficult to treat successfully

cow recovery provides immunity; survives in frozen semen

Histomonas meleagridis – blackhead disease of turkeys; cosmopolitan

H. meleagridis transmitted between birds within eggs of nematode

inhabits ceca and liver of turkeys, chickens and related birds; uses cecal nematode as an intermediate host; this flagellate parasitizes the bird and the nematode (Heterakis) in the bird; H. meleagridis develops in the nematode and multiplies, eventually entering ovaries of the adult worm - invades the nematode embryo within the eggs - eggs containing juvenile worms parasitized by these protozoans are shed in the bird feces - eggs hatch when eaten by another bird, which releases H. meleagridis

earthworms also accidentally ingest these eggs and become paratenic hosts; in the earthworm the nematode eggs hatch and the juveniles released become dormant in the tissues of the earthworm; birds eat these earthworms

survival provides immunity; millions are lost annually to blackhead disease


Amoeboid Protozoans – move by pseudopodia

Entamoeba – in many hosts; asexual multiple fission in cysts; binary fission in trophozoites

young cysts contain chromatoidal bars (packaged RNA) which disappear in older cysts

E. histolytica – amoebic dysentery; third most common parasite cause of death in the world

cosmopolitan, but most prevalent in the tropics and subtropics; inhabits large intestine

trophozoites are 20 to 30 micrometers; cysts are 10 to 20 micrometers

trophs have central endosome and commonly contain ingested RBCs and intestinal cells

normal fully formed stool contains cysts, but not trophozoites

loose stool contains trophs, but cysts are rare

early cyst has cigar shaped chromatoid bodies; mature cyst 4 nuclei, no chromatoid bodies; pathogenic and non-pathogenic strains related to host diet and intestinal bacterial composition; ulcers erode intestinal wall, blood carries trophs to other body parts causing abscesses; cysts are not produced in abscesses outside of the large intestine

proteolytic enzymes cause ulceration, secondary bacterial invasion, colon perforation

cyst viability is 12 days if moist and cool, up to 30 days if in water

cysts are resistant to usual levels of chlorine used in water treatment facilities

symptoms - bloody stool, abdominal pain, nausea, flatulence, headache, fatigue

carriers are non-dysenteric

diagnosis is by observation of trophs and/or cysts in stool

treatment - first stop dysentery, then treat intestine, then treat parenterally

control - sewage and water treatment; fix defective plumbing; stop use of night soil;

prevent carriers from working in food handling jobs; control reservoirs (dogs, pigs, monkeys, others)

also spread by flies, cockroaches, colonic irrigation

E. hartmanni – commensal; identical to E. histolytica, but smaller

trophozoites are 12 to 15 micrometers; cysts are 5 to 9 micrometers

E. coli – cosmopolitan commensal in large intestine of humans; 100% rate in some areas

eccentric endosome; 8 nuclei in cysts; chromatoidal bodies with splintered ends;

trophs contain intestinal contents; E. coli is easily confused with E. histolytica

E. gingivalis – cosmopolitan commensal in mouth of humans, dogs, cats, primates

75% rate in some areas in people over 40; does not produce cysts

trophs contain WBCs and host cells

Order Schizopyrenida – uninucleate; amoeboid; most have temporary flagellate stages

Naegleria fowleri – facultative in humans; meningoencephalitis; almost always fatal

enters nose from swimming pools, lakes, thermal springs - follows olfactory nerve to brain

death can be in just a few days; seldom diagnosed before death

Acanthamoeba spp. – facultative in various human tissues; treatment is difficult

most common amoeba in soil and freshwater

usually enters eye in contaminated contact lens saline solution; corneal ulcers

Phylum Apicomplexa

protozoans possessing structures known collectively as the apical complex

apical complex found in sporozoite and merozoite stages of life cycle

all parasitic

schizogony (=merogony) - asexual multiple fission - produces merozoites

gametogony - merozoites become gametocytes - gametocytes produce microgametocyte and macrogametocyte which become or produce gametes

sporogony - multiple fission of a zygote - produces sporozoites

Class Gregarinea - spores (=oocyst) simple; contain sporozoites;

Order Eugregarinida – gametogony, sporogony, schizogony absent

parasites of annelids and arthropods

Monocystis lumbrici - life cycle in earthworm seminal vesicle

Class Coccidea, Order Eimerida - schizogony, gametogony, sporogony

called coccidians; disease is coccidiosis; treated with anticoccidials

usually a one host life cycle; self limiting; alternate sexual and asexual phases

coccidiosis is the greatest protozoan loss to domestic and game animals

Eimeria - oocyst has 4 sporocysts each with 2 sporozoites; probably 45,000 species; pathogenicity depends on number of oocysts ingested, number of generations, immunity of host, toxicity of the parasite; highly host specific

E. tenella - in ceca of chickens

E. stiedai - in liver of rabbits

E. bovis - cattle intestine; one schizont has 100,000 merozoites

Isospora -oocyst has 2 sporocysts each with 4 sporozoites

most are parasites of birds

I. belli - a severe disease in humans

diarrhea; self limiting

Toxoplasma gondii - toxoplasmosis; cosmopolitan in birds and mammals

rate in humans is about 13%; most human cases are without symptoms

symptoms - swollen lymph nodes and/or lesions in the eye, lung, liver, heart, brain

intracellular - schizogony produces “tissue cysts” or pseudocysts (=zoitocysts) that may persist for years; tissue cyst rupture causes flare up of symptoms

cats are the only final host, all others are intermediates; cats show few symptoms

gametogony, schizogony, and oocyst formation are in cat small intestinal cells

only schizogony occurs in non-cat hosts; oocyst like that of Isospora

prenatal infections are a major cause of birth defects; anyone who eats rare meat and/or

has a pet cat should read the text material on toxoplasmosis

transmission is via oocysts from cats and uncooked flesh containing the “tissue cysts”

also transmitted by fecal contamination, nursing, placental crossing, flies, earthworm pilings

lab test diagnosis; drug treatments

Cryptosporidium and Pneumocystis – opportunistic coccidians

Cryptosporidium – infects microvilli of small intestine; unlike other coccidians, lack host specificity; transmission by ingestion of oocyst, usually in contaminated drinking water; AIDS patients develop cholera-like symptoms

Pneumocystis – extracellular parasite in alveoli of the lungs; of the opportunistic diseases associated with AIDS, PCP (Pneumocystis carinii pneumonia) is the most common cause of death; asymptomatic in immunocompetent individuals


Order Haemosporida – no sporocysts, motile zygote (ookinete)

malaria is one of the most important human diseases in the world today

127 species of Plasmodium cause malaria in vertebrates (humans and many others)

four species in humans are spread by 390 species of female Anopheles mosquitoes

malaria as a disease has been known since antiquity

1902 Nobel Prize in Medicine went to Ronald Ross for his work on malaria

the liver cycle part of the overall life cycle was not worked out until 1948

symptoms occur when RBCs rupture releasing contents

acute - chills, fever (104o to 106o F), sweating, headache; lasts 8 to 12 hours

chronic - anemia, enlarged spleen and lymph nodes; hemozoin buildup

malaria RBC stages break down hemoglobin producing a pigment called hemozoin

diagnosis is by finding RBC stages in blood smears (see Plates 1 - 7)

malaria life cycle: mosquito bite injects sporozoites into blood stream - sporozoites

flow to liver and enter liver cells - schizogony - schizont - cell rupture releases merozoites

whether they can enter other liver cells is uncertain - merozoites enter RBCs - schizogony (ring stage, schizont, merozoites) in RBCs - RBC rupture releases merozoites which enter other RBCs - schizogony - RBC cycle can go on and on - eventually some merozoites become gametocytes in RBCs - RBCs containing gametocytes taken in blood meal by mosquito - male microgametocyte produces 6 to 8 microgametes by exflagellation - female macrogametocyte becomes a single macrogamete - gamete formation and fertilization occur in mosquito stomach - zygote in stomach becomes motile ookinete which penetrates to outer stomach wall - develops cyst wall, now called an oocyst - nuclear divisions in the oocyst produce sporoblasts - sporozoites form from sporoblasts – sporoblasts produce thousands of sporozoites which fill the oocyst - rupture of the oocyst wall releases the sporozoites which enter the salivary glands of the mosquito; the mosquito is infective for life

some sporozoites of relapsing species (P. vivax, P. ovale) become dormant in liver cells and are

called hypnozoites (a true relapse is due to hypnozoites becoming reactivated)

immunity develops in vertebrate hosts

Plasmodium - while life cycles of species that infect humans are basically similar, there are a number of differences:

P. malariae - 72 hour cycle; 7% of the world’s malaria; widespread; continuous blood cycle up to 53 years; blood forms do not re-invade fixed cells; no true relapses, just recrudescence

P. vivax - 48 hour cycle; 43% of the world’s malaria; Asia, North Africa; continuous liver cycle up to 8 years; blood forms can re-invade fixed cells; true relapses occur

P. falciparum - 48 hour cycle; 50% of the world’s malaria; tropics; blood forms do not re-invade

fixed cells; no true relapses, just recrudescence; most virulent of the human malarias

brain, spleen, and bone marrow cells are also attacked; blackwater fever

P. ovale - 48 hour cycle; rare; mostly in tropics; blood forms re-invade fixed cells; true relapses

control - destroy mosquito breeding places; biological controls; DDT

treatment - quinine, used since 1640

chloroquine - effective on blood forms

primaquine - effective on liver forms including hypnozoites

Babesia bigemina - Texas cattle fever, or red-water fever; in cattle

larval tick (called a seed tick), Boophilus annulatus, injects sporozoites while sucking blood -

indefinite schizogony cycles destroy up to 75% of RBCs (in fatal cases) - red urine is excess

hemoglobin from RBC destruction - when uninfected larval seed ticks feed on infected cattle,

gametocytes are taken in the blood meal and resulting sporozoites enter all cells of the tick -

ticks develop to adults and mate, females fall off - all eggs laid by female are infected (called

transovarial transmission) - all hatching seed tick larvae are infected

B. annulatus is a one-host tick; infected seed ticks transmit the disease

sporogony and gametogony occur in the tick, schizogony occurs in cattle

control - cattle quarantine; cattle dips in tickicides

Babesia microti - voles, rodents, pets and Ixodes scapularis ticks

hundreds of human cases in northeast US - transmitted by nymph stage of Ixodes scapularis

adults of I. scapularis feed on deer

Phylum Myxozoa (probably cnidarians) - over 1200 species; mostly parasites of fishes

spores of multicellular origin containing sporoplasms and one or more polar capsules enclosing

polar filaments which explode punching an anchoring hole into intestinal epithelium through

which the sporoplasm enters the cell

Myxobolus cerebralis - in head and spine cartilage of salmonids (trout) causing whirling disease

high mortality in young fish, permanent deformities if fish survives

no effective treatment exists

spore (called triactinomyxon) exsporulates in fish epidermis - sporoplasm migrates to head and

spine cartilage - growth, division, spore production - spores released when fish is eaten or

dies, and enter tubifex oligochaetes - duplication and development in tubifex worms -

production and release of triactinomyxon spores

Phylum Microspora - less than 1000 species

spores of unicellular origin without polar capsules

Class Microsporea - spores with polar filaments

spore ingested - polar filament discharges penetrating host cell membrane - sporoplasm enters

cell through hollow polar filament - schizogony and sporogony - spores

Nosema apis - nosema disease of bees

midgut epithelium; spores in feces; parasitic castration of queen bee

N. bombycis - disease of silkworm larvae

in all host tissues; transovarial transmission

four genera reported in AIDS patients - no effective treatment
Phylum Haplosporidia - parasites of invertebrates

spores of multicellular origin, no polar capsules, no polar filaments

Haplosporidium nelsoni (named after Julius Nelson and son, Thurlow)

Captain John Smith enters Chesapeake Bay in 1607 - amazed at oyster abundance

at peak of oyster industry - 15 million bushels worth $112 million per year

disaster starts in Delaware Bay in 1957 - east coast oyster industry ruined

MSX (multinucleate sphere X) - eventually named H. nelsoni

method by which oysters become infected is still unknown

30 years of oyster breeding has produced strains with some resistance

Phylum Ciliophora - cilia; two types of nuclei (macronucleus and micronucleus)

Class Litostomatea - body cilia similar to mouth cilia

Balantidium coli - largest protozoan parasite of humans; causes balantidiasis

cosmopolitan in large intestine of humans (parasitic) and pigs (commensal)

trophozoite and cyst stages; cytostome and cytopyge (cellular anus)

invades, digests, and spreads like Entamoeba histolytica in humans

intestinal perforation (in humans) is usually fatal

Class Oligohymenophorea - body cilia different than mouth cilia

Ichthyophthirius multifiliis - “ick” pustules form in skin of freshwater fishes’ skin

causes excessive mucus production from skin of host

pustules rupture releasing trophozoites - settle to bottom - encyst - many internal divisions -

infective trophozoites released - bore into skin or attack gill filaments


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