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Introduction to Medical Protozoa

 

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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
۱٫ 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.
۲٫ 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.
۳٫ 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
۱٫ Trophozoite is a living stage of protozoa when they can move, take food and reproduce. (It is usually the pathogenic stage.)
۲٫ 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

۳٫ Site of inhabitation: digestive tract; urogenital canals; blood and tissues
۴٫ Infective route: mouth; direct or indirect contact; sexual transmission; placenta; insect sucking blood, blood transfusion and breath.
III. Reproduction
۱٫ Asexual reproduction
(۱) Binary fission is the simplest form of division. The organism is transversely or longitudinally divided into two daughter parasites.
(۲) 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
(۳) Endodyogeny: a cell undergoes a single internal budding and then two daughter cells are produced, such as Toxoplasma gondii.
۲٫ Sexual reproduction
(۱) Conjugation: two cells temporarily attach to each other, exchange their nuclear material and then separate, such as Balantidium coli.
(۲) Gametogony (syngamy): two sexually differentiated cells unite to form the zygote and then produce many daughter cells, such as the gametogony of Plasmodium vivax.
۳٫ 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
۱٫ Parasites massively multiply and cooperate with bacteria in pathogenesis, such as Trichomonas vaginalis.
۲٫ Parasites massively multiply and destroy the cells and tissues of the host, such as Plasmidium vivax.
۳٫ Parasites massively multiply and invade the adjacent tissues, such as Entamoeba histolytica.
۴٫ Intracellular parasites are carried to all parts of the body by blood stream.
۵٫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.
۱٫Class Zoomastigophora: Leishmania Donovani moves by flagellum.
۲٫Class Lobosea: Entamoeba histolytica moves by pseudopodium.
۳٫Class Sporozoa: Plasmodium vivax
۴٫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
۱٫ 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.
۲٫ Cyst, non-motile form
(۱) Immature cyst is spherical in shape, about 10 ~20 μm, and has one or two nuclei.
(۲) 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
۱٫ 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 Tissue trophozoite Immature cyst
liver abscess blood stream
Fistula on body wall Intestinal ulcer
Prianal ulcer invade adjacent
and Fistula tissue & organs Mature cyst
amoebic vaginitis discharged
amoebic urethritis in feces
Outside of the body

III. Pathology and symptomatology
۱٫ 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.
۲٫ Clinical Manifestation
(۱) 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.
(۲) 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
۱٫ Stool examination
a. Living trophozoite in unformed feces: direct fecal smear with normal saline
One must pay attention to :
(۱) Before the patient taking medicine the stool specimen should be collected.
(۲) The container must be clean and free of salt, acid and alkaline.
(۳) Trophozoites should be examined soon after they have been passed.
(۴) keep the specimen warm in order to keep the trophozoite’s activity.
(۵) Select the bloody and mucous portion for examination.
(۶) 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).
۲٫ Immunological test: for reference
۳٫ X-ray for lung amoebic abscess
۴٫ 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.
۵٫ B ultrasonography and Liver puncture for the liver abscess.
۶٫ 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
۱٫ 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.
۲٫ 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
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E. histolytica E. dispar E. coli E. hartmanni I. butschlli
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Living troph. Active active sluggish sluggish sluggish
movement
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inclusion of troph. RBC (-) (-) (-) (-)
Haematoxylin stain
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Karysome central, small central, small central small eccentric large large, irragular
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Peripheral symmetrical symmetrical asymmetrical incospicuous
chromatin fine fine coarse
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Mature cyst 4 4 8 4 1
No. of nuclei
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Shape & size circular circular circular circular irregular ellipse
of cyst
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Chromatoid round end round end splintered ends round end (-)
body round
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Glycogen in immature in immature in immature in immature large, sharply
cyst cyst cyst cyst demarcated
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E.dispar and E. coli are the most common non-pathogenic amoebae

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