Introduction to Medical Protozoa
The protozoa are unicellular (single celled) animals, which can
complete various physiological functions one their own. All those
parasitizing human body are microscopic in size, from 2~200 μm. About
40 species are relative to human diseases, which are called medical
protozoa.
I. Basic structure
1. The cell membrane is the bio-membrane described by the fluid
mosaic model（液态镶嵌模型）. It has the functions of sensation,
recognition, taking food, material exchange, locomotion and pathogenisis.
There are many accessory structures on it, such as ligand, receptor, carrier,
enzyme, antigen and toxin on it, some of which are the material base
leading to disease.
2. The cytoplasm is differentiated into ectoplasm and endoplasm. The
ectoplasm is a hyaline outer layer that is protective in function and also
gives rise to the locomotive organ, such as pseudopodium, flagella and
cilia.
The endoplasm is the inner position where there are various
organelles and food vacuoles. It manages the metabolism.
3. The nucleus is the most important organelle, which controls the
metabolism, heredity and reproduction. If nucleus is injured the cell will
soon die. There are two types of nuclei, vesicular and compact. The
morphological feature of the nucleus is used to identify the kind of
protozoa.
II. Life cycle
1. Trophozoite is a living stage of protozoa when they can move, take
food and reproduce. (It is usually the pathogenic stage.)
2. Cyst is the resting stage of a protozoa with a protective wall. It is
usually the infective stage. Its functions are protection, transmission and
multiplication.
Encystation
Trophozoite
Cyst
Excystation
3. Site of inhabitation: digestive tract; urogenital canals; blood and tissues
4. Infective route: mouth; direct or indirect contact; sexual transmission;
placenta; insect sucking blood, blood transfusion and breath.
III. Reproduction
1. Asexual reproduction
(1) Binary fission is the simplest form of division. The organism is
transversely or longitudinally divided into two daughter parasites.
(2) Multiple fission (schizogony): First, multiple division of the nucleus
takes place and then each nucleus is surrounded by a portion of cytoplasm.
Finally, many daughter cells will be formed
(3) Endodyogeny: a cell undergoes a single internal budding and then two
daughter cells are produced, such as Toxoplasma gondii.
2. Sexual reproduction
(1) Conjugation: two cells temporarily attach to each other, exchange their
nuclear material and then separate, such as Balantidium coli.
(2) Gametogony (syngamy): two sexually differentiated cells unite to form
the zygote and then produce many daughter cells, such as the gametogony
of Plasmodium vivax.
3. Alternation of generation: In life cycles of some protozoa, there is the
regular alternation of sexual
and
asexual reproductions , this
phenomenon is called alternation of generation, such as it in the life cycle
of Plasmodium vivax.
IV. Pathogenic mechanism
1. Parasites massively multiply and cooperate with bacteria in pathogenesis,
such as Trichomonas vaginalis.
2. Parasites massively multiply and destroy the cells and tissues of the host,
such as Plasmidium vivax.
3. Parasites massively multiply and invade the adjacent tissues, such as
Entamoeba histolytica.
4. Intracellular parasites are carried to all parts of the body by blood
stream.
5.Opertunistic protozoa: the protozoan living in the human body in
commensalisms make the host attack when his immunity is lower or
restrained,such as Pneumocystis carinii and Toxoplasma gondii.
V. Classification:
The classification of protozoa mainly depends on their locomotive mode.
1.Class Zoomastigophora: Leishmania Donovani moves by flagellum.
2.Class Lobosea: Entamoeba histolytica moves by pseudopodium.
3.Class Sporozoa: Plasmodium vivax
4.Class Kinetofragminophorea: Balantidium coli move by cilia
Pathogenic and non-pathogenic amoebae
Entamoeba histolytica
E. histolytica is the only pathogenic amoeba among all intestinal
amoebae, infecting perhaps 10% of world’s population
I. Morphology
1. Trophozoite, active form
The size averages 20 ~40 μm. It can actively move by the Pseudopodium
when living. The difference between endoplasm and ectoplasm is distinct.
RBC can be found in the endoplasm. The nucleus, vesicular type, can be
clearly seen in the specimen stained with hematoxylin; nucleus membrane
is delicate but distinct line; peripheral chromatin granules are fine and
well-distributed on the inner surface of the nuclear membrane. The
karyosome is small and centrally located. *The characteristics of the
nucleus of E. histolytica are useful in differentiation of the pathogenic
amoeba from other non-pathogenic amoebae.
2. Cyst, non-motile form
(1) Immature cyst is spherical in shape, about 10 ~20 μm, and has one or
two nuclei.
(2) Mature cyst: the shape and size is same as the immature cyst, but it has
4 nuclei.
The characteristics of the cyst nucleus are similar to that of the
trophozoite. The other two morphological features are the glycogen
vacuole and the chromatoid body (or bar). The chromatoid bar has two
round and smooth ends. the glycogen vacuole is the food reseroir. Both
glycogen vacuole and the chromatoid bar become smaller and smalleeras
the cyst ages.
In the iron-hematoxylin stained specimens the chromatoid bar and
nucleus are dark blue in color. The glycogen vacuole has been dissolved
during the process of staining, so it appears as an empty vacuole. When the
cyst is stained with iodine, the glycogen appears brown in color, but the
chromatoid bar can not be stained and has are refractory appearance.
II. Life cycle
1. definitive host: man; 2. infective stage: cyst with 4 nuclei; 3.
infective route: by mouth; 4. site of inhabitation: intestine, excystation in
duodenum, encystation in colon; 5. multiplication: binary fission; 6.
normal life cycle: cyst--------trophozoite--------cyst; 7. the action of
trophozoite in the intestinal lumen is different from that in the tissue.
ingested by man
excystation
Cyst with 4 nuclei---------------------duodenal--------------- 8 trophozoites
Brain abscess
Lung abscess
liver abscess
Tissue trophozoite
Immature cyst
blood stream
Fistula on body wall
Intestinal ulcer
Prianal ulcer
invade adjacent
and Fistula
tissue & organs
Mature cyst
amoebic vaginitis
d ischarged
amoebic urethritis
in feces
Outside of the body
III. Pathology and symptomatology
1. Pathogenic mechanism
The pathogenic materials of E. histolytica include lectin（凝集素）
，
amoebic pore forming （阿米巴穿孔素）and protein dissolving enzyme（蛋
白溶解酶）.The lectin guides the trophozoite to adhere to the intestinal
epithelia, neutrophils and red blood cells. These pathogenic materials
destroy the host’s tissues. The trophozoite invade the mucosa by producing
tiny pinpoit lesionsat the site of entry, spread into the submucosa and
produce typical flask-shaped ulcer. The open of the ulcer toward intestinal
lumen looks like a crater.
2. Clinical Manifestation
(1) Intestinal amoebiasis
a. Amoebic dysentery is the most common form of amoebiasis. The acute
case discharges unformed feces several times per day with the pain in right
inferior part of the abdomen, nausea, low fever around 38℃, and fatigue.
The jam-like stool with foul smell, in which there are great number of
trophozoites and Charcot-Leyden crystals.
b. Chronic amibic colitis manifests vague abdomen discomfort, alternation
of constipation and dearrhea, stool with nucus and foul smell, nausea,
anorexia, fatigues, weight loss.
(2) Extra-intestinal amoebiasis or complications of intestinal amoebiasis
a. Acute non-suppurative hepatitis
b. Amoebic liver abscess: The patient has suffered from amoebic dysentery
or colitis. The clinical manifestation includes hepatomegaly and tenderness,
hepatic pain becomes more severe and continuous, local skin red and
swollen, high fever, nausea, vomiting, jaundice (icterus), delirum（谵妄）,
septic shock, coma, fatalness and mortality (death rate) of 50% .
* The most common extra-intestinal amoebiasis is the liver abscess due to
the parasite getting into the liver through the portal vein system.
c. Amoebic abscess of pulmonary: chest pain, cough, jam-like sputum, the
shadow of the abscess can be seen on X-ray.
d. Intestinal perforation: acute abdominal pain, high fever, septic shock.
e. Amoebic abscess of brain: headache, nausea, vomiting, delirum（谵妄）,
coma.
f. Amoebic vaginitis; burning sensation, foul leucorrhoea
g. Amoebic urithritis: irritability of the bladder--frequent micturition,
urodynia, cipitant urination
h. Skin fistulas of anus or liver portion.
IV. Diagnosis
1. Stool examination
a. Living trophozoite in unformed feces: direct fecal smear with normal
saline
One must pay attention to :
(1) Before the patient taking medicine the stool specimen should be
collected.
(2) The container must be clean and free of salt, acid and alkaline.
(3) Trophozoites should be examined soon after they have been passed.
(4) keep the specimen warm in order to keep the trophozoite’s activity.
(5) Select the bloody and mucous portion for examination.
(6) If Charcot-leyden crystals are found the stool must be carefully examined
for the trophozoite.
b. Cyst in formed feces: iodine stain for the chronic case or carrier
(haemotoxylin stain for teaching).
2. Immunological test: for reference
3. X-ray for lung amoebic abscess
4. Biopsy of rectum and sigmoid by rectoscope or sigmoidoscope, for
diagnosis of intestinal amoebiasis and differential diagnosis from other
intestinal diseases such as rectal cancer or sigmoid cancer.
5. B ultrasonography and Liver puncture for the liver abscess.
6. Computed tomograph ( CT ) and nuclear magnetic resonance (NMR) for
the brain abscess.
V. Epidemilogy
E.histolitica infection are worldwide distribution but more prevalent in the
tropics and subtropics. The high incidence of amoebiasis is due to both
natural factors and poor sanitation. This disease is transmitted by the food,
vegetable, fruit and drinking water contaminated by the stool with 4 nuclei
cysts. The contamination of source of drinking water can result in the
outbreak of amoebiasis, such as an outbreak at the Chicago World Fair in
1933 leaded to approximately 100 deaths. The routine chlorination as
employed in modern city water works does not destroy the cysts of
E.histolytica.
The acute case of amoebic dysentery os of no significance in
transmission of the disease as trophzoites cannot survive long outside the
body of the host. The cyst may remain viable in a moist, cold condition for
over 12 days, but be killed by drying, by temperature over 55.
VI. Treatment and Prevention
1. The principle of radically cure of amoebiasis is to destroy intra and
extra-intestinal pathogens. *Metronidazole is recommended for acute
amoebic dysentery. The patient should be warned to abstain from alcohol
during treatment with metronidazole. Chloroquine is used for treating and
preventing liver amoebic abscess. *The compatibility of metronidazole and
chloroquine is used for the radical cure of amoebiasis.
2. Prevention
sanitary disposal of stool; preventing food, vegetable, fruit and drinking
water from the contamination of cyst.
Drinking water must be boiled
Food and drinks must be protected from flies
Pay attention to personal hygienic and health check of food handler and
waiter in the restaurant.
Non-pathogenic amoeba
Entamoeba dispar（迪斯帕内阿米巴）
Entamoeba coli （结肠内阿米巴）
Entamoeba hartmanni（哈氏内阿米巴）
Entamoeba gingivalis（齿龈内阿米巴）
Endolimax nana （微小内蜒阿米巴）
Iodamoeba butshlii（布氏嗜碘阿米巴）
Dientamoeba fragilis（脆弱双核阿米巴）
Naegleria fowleri（福氏耐格里阿米巴）
Morphological differences between E.histolytica an non-pathogenic amoebae
_______________________________________________________________________
E. histolytica
E. dispar
E. coli
E. hartmanni
I. butschlli
_______________________________________________________________________
Living troph.
Active
active
sluggish
sluggish
sluggish
movement
_______________________________________________________________________
inclusion of troph.
RBC
(-)
(-)
(-)
(-)
Haematoxylin stain
_______________________________________________________________________
Karysome
central, small
central, small
central small
eccentric large large, irragular
_______________________________________________________________________
Peripheral
incospicuous
symmetrical
chromatin
fine
fine
symmetrical
asymmetrical
coarse
_______________________________________________________________________
Mature cyst
4
4
8
4
1
No. of nuclei
_______________________________________________________________________
Shape & size
circular
circular
circular
circular
irregular ellipse
of cyst
_______________________________________________________________________
Chromatoid
round end
round end
splintered ends
round end
(-)
body round
_______________________________________________________________________
Glycogen
in immature
cyst
in immature
cyst
in immature in immature large, sharply
cyst
cyst
demarcated
_______________________________________________________________________
E.dispar and E. coli are the most common non-pathogenic amoebae