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Cryptosporidium parvum - Coggle Diagram
Cryptosporidium parvum
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MORPHOLOGY
Oocyst is the diagnostic form excreted in human faeces. Cryptosporidium oocyst is the smallest coccidian known to cause infection in man. It is colorless, spherical to oval, and measures 4.5 µm to 6 µm in diameter. It doesn’t stain with iodine and is acid-fast. The cyst is surrounded by a 50 nm thin cyst wall. The latter consists of an electroluscent middle zone surrounded by two electron dense layers.
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Each oocyst contains upto four slender and fusiform sporozoites. These four slender bow-shaped sporozoites always remain parallel to each other within an oocyst and are released only after partial digestion of the oocyst.
The sporozoites are slender, crescent-shaped and measures 1.5 µm to 1.75 µm in diameter. The anterior end containing a prominent nucleus is rounded. These sporozites invade enterocytes in which they parasitise.
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In the enterocytes, the sporozoites subsequently differentiate into intracellular trophozoites.
Trophozoites are the intracellular transitional form of the parasite. They are round or oval and measure 2 µm to 2.5 µm in diameter. Each trophozoite consists of a large nucleus with or without a conspicuous nucleolus.
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life cycle
Cryptosporidium completes its life cycle through the stages of asexual generation (schizogony) and sexual generation (gametogony) in a single host.
All the morphological stages of the parasite are truly intracellular and are being surrounded by a host cell membrane, which is extra-cytoplasmic.
Man acquires infection on ingestion of food or drinks contaminated with the faeces, containing sporulated thick-walled oocysts of Cryptosporidium.
On ingestion, the sporozoites are released from the oocysts in the small intestine. These sporozites invade enterocytes in which they parasitise.
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In the enterocytes, the sporozoites subsequently differentiate into intracellular trophozoites.
These trophozoites multiply asexually by nuclear division to produce two types of meronts; type I and type II.
These meronts in turn produce type I and type II merozoites that resemble sporozoites and amplify asexual infectious cycles.
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Some of the type II merozoites invade new host cells and initiate sexual replication. Inside the host cells, they differentiate either into female (macrogamont) or male (microgamont) forms. Each microgametocyte produces 16 sperm-like microgametes, which fertilize the maccrogamonts resulting in the formation of oocysts (zygote). Four sporozoites are formed inside each sporulating oocyst in situ.
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The thin-walled oocysts release the sporozoites inside the lumen of the intestine and cause auto-infection in the same host by repeating the cycle of schizogony and gametogony.
The thick-walled oocysts excreted in the faeces are infective to other human hosts. The cysts under favorable conditions remain viable and infectious relatively for a long time. These cysts when taken up by other susceptible human hosts, cause infection and the cycle are repeated.
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Lab diagnosis
Specimens: Faeces is the specimen of choice. Sputum, bronchial washings and duodenal or jejunal aspirations are less frequent specimens collected from immunocompromised patients like AIDS.
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Wet mount examination: Direct wet mount of stool in iodine is used for screening of stool specimens for oocysts.
Stained smear examination: Kinyoun’s modified acid fast technique, hot safranine-methylene blue stain, modified Kohn’s stain, modified Koster stain, fluorescent stains etc. are used to stain Cryptosporidium oocysts in faecal smears. Red-stained acid-fast oocysts against blue background are seen after Kinyoun’s modified acid-fast staining technique.
Fluorescent microscopy: Direct immunofluorescence microscopy using auramineO, auramine-rhodamine, auramine-carbol fuchsin, acridine orange etc. is the method of choice to detect oocysts in stool smears.
Antigen detection in stool: ELISA is employed to detect Cryptosporidium copro-antigen in the faeces.
Serodiagnosis: The indirect fluorescent antibody (IFA), or ELISA using purified oocysts as antigens have been used to detect circulating antibodies specific to Cryptosporidium in the serum which appear in about 6 to 8 weeks after the onset of infection.
Molecular diagnosis: Detection of Cryptosporidium by PCR is still a research tool and is yet to be widely used in clinical diagnosis.
Histopathological diagnosis: This is based on the demonstration of the developmental stages of parasite in the biopsy specimen from the jejunum and occasionally from the rectum.
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Treatment
Limited and transient benefit has been reported in patients treated orally with spiramycin (a macrolide antibiotic) in a dosage of 1 gm three to four times daily. Nitazoxanide has also been reported to be effective against cryptosporidial diarrhea in AIDS patients.
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