Stool analysis for protozoa

Detection and differentiation of protozoa in stool analysis (distinguishing pathogenic forms from non-pathogenic) is a rather complex task. Most unicellular organisms in stool are found in two forms: vegetative (trophozoite stage) - active, mobile, vital, easily susceptible to harmful effects (in particular, cooling) and therefore quickly dying after being excreted from the intestine, and in the form of cysts (oocysts) resistant to external influences. In formed stool, protozoa are usually found only in the encysted state; to detect vegetative forms, it is necessary to examine the stool while it is still warm. This is due to the fact that in cooled stool, vegetative forms of protozoa quickly die and, while dead, are quickly subject to the action of proteolytic enzymes, as a result of which they lose the characteristic features of their structure. In addition, when cooling, the mobility of protozoa decreases and then disappears - an important auxiliary factor in their differentiation.

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Types of parasites in feces

The main types of parasites and the best methods for detecting them

Type of parasite	Commonly affected organs	Diagnostic method
Leishmania donovani	Spleen, liver, lymph nodes	Detection of parasites in punctures of the spleen, liver, bone marrow, affected skin, detection of specific antibodies in the blood
Trypanosoma rhodesiense and T. gambiense	Lymph nodes and brain	Detection of parasites in affected skin, lymph nodes, cerebrospinal fluid
Echinococcus granulosus or Echinococcus multilocularis	Liver, lungs, brain	Detection of specific antibodies in the blood
Schistosoma haematobium, S. mansoni, S. japonicum, etc.	Urinary tract, liver, portal vein	Detection of parasite eggs in urine, feces, rectal biopsy, liver tissue or specific antibodies in the blood
Fasciola hepatica	Liver	Detection of parasite eggs in feces or specific antibodies in blood
Clonorchis sinensis, Opisthorchis felineus, Opisthorchis viverrini	Biliary system	Detection of parasite eggs in feces or bile
Trichostrongylus capricola, T. vitrinus	Small intestine	Detection of parasite eggs in feces
Echinostoma ilocanum	Small intestine	Detection of parasite eggs in feces
Angiostrongylus costaricensis	Small and large intestine	Detecting eggs In intestinal biopsy
Dipylidum caninum	Small and large intestine	Detection of parasite eggs or ingested stool

There are 20 species of protozoa (8 pathogenic and opportunistic and 12 commensals) that can be detected in feces. Intestinal protozoa live in the small or large intestines in the trophozoite and/or cyst stage. They belong to one of 4 groups: amoebae, flagellates, ciliates and coccidia.

Dysenteric amoeba in feces

Entamoeba histolytica (dysentery amoeba) causes amoebiasis in humans. It is localized in the large intestine and excreted in the form of trophozoites (with liquid feces) and/or cysts (in formed stool). Since most types of amoebas (intestinal, Hartmann, Bütschli) are not pathogenic for humans, great caution should be exercised when evaluating the results of a fecal examination. Only detection of hematophagous trophozoites (tissue form of E. histolytica forma magna) can serve as a reliable sign of amoebic dysentery and/or amoebic ulcerative colitis in a patient. The presence of erythrocytes in the protoplasm of amoebas is a very important diagnostic sign, since nonpathogenic forms of amoebas never contain them. In all other cases, detection of E. histolytica-like forms of trophozoites that do not contain erythrocytes is not a basis for diagnosing amoebiasis as a disease. The results of detection of only E. histolytica cysts (luminal form), which can be detected in individuals recovering from acute amoebiasis, in those suffering from the chronic form of amoebiasis, and in carriers, are assessed in a similar manner.

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Giardia in feces

Lamblia intestinalis (Giardia) belongs to the class of flagellates. Giardia parasitize in the small intestine, mainly in the duodenum, and also in the gall bladder. The existence of trophozoites (the vegetative form of Giardia) requires a liquid environment, therefore, when entering the large intestine, Giardia encyst, and only cysts are found in the feces. Only with profuse diarrhea or after the action of laxatives can vegetative forms be detected in the feces.

Balantidia in feces

Balantidium coli is the only ciliate parasite in the human intestine that causes diseases of varying severity, from mild colitis to severe ulcerative lesions. The pathogen is found in feces in the form of trophozoites or cysts. Carriage is possible in healthy people.

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Cryptosporidium in feces

Representatives of the genus Cryptosporidium are currently considered the most important causative agents of diarrhea. Cryptosporidium (from the Greek "hidden spore") are obligate parasites that affect the microvilli of the mucous membranes of the gastrointestinal tract and respiratory tract of humans and animals. Gastrointestinal infections caused by cryptosporidia have been registered in all countries of the world. Such a wide distribution of cryptosporidiosis is associated with a large number of natural reservoirs of infection, a low infectious dose and high resistance of the pathogen to disinfectants and antiparasitic drugs.

Among cryptosporidia, the species potentially pathogenic for humans are Cryptosporidium parvum and Cryptosporidium felis (identified in HIV-infected individuals). The most typical localization of infection in humans is the distal parts of the small intestine. In patients with severe immunodeficiencies, the entire gastrointestinal tract can be infected - from the oropharynx to the mucous membrane of the rectum.

Diagnosis of cryptosporidiosis is based in most cases on the detection of cryptosporidia oocysts in feces and/or (much less frequently) in a biopsy of the small intestinal mucosa in cases of watery diarrhea syndrome. Microscopy of prepared preparations stained by Gram is used. In most cases, this staining method does not allow the detection of oocysts due to their weak ability to retain dye and the inability to distinguish them from yeast-like fungi. Therefore, acid-fast staining is used. With this staining method, cryptosporidia oocysts are stained red or pink and are clearly visible against the blue-violet background in which other microorganisms and intestinal contents are stained.

In acute cryptosporidiosis, the number of oocysts in the feces is high, which makes them easy to detect by microscopy of stained preparations. However, in chronic cryptosporidiosis with a mild course, when the number of oocysts in the feces is small, enrichment methods must be used to increase the probability of their detection. In recent years, serological methods have become common for the diagnosis of cryptosporidiosis.

Cryptosporidiosis of the biliary tract may manifest itself as cholecystitis, much less often as hepatitis (with an increase in the concentration of bilirubin, AST, ALT, alkaline phosphatase activity in the blood) and sclerosing cholangitis. To diagnose biliary cryptosporidiosis, liver biopsies and bile are examined, where cryptosporidia can be detected at various stages of development.

To monitor the effectiveness of treatment of protozoan intestinal lesions, feces are examined depending on the disease identified: in case of amebiasis, balantidiasis - immediately after treatment, in case of giardiasis - after 1 week. After treatment of biliary tract invasions, effectiveness can be monitored both by examining feces and bile.

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Scraping from perianal folds for enterobiasis

Perianal fold scraping for enterobiasis is a targeted test to detect pinworm eggs (Enterobius vermicularis). Since mature female pinworms crawl out to lay eggs in the folds around the anus, pinworm eggs are rarely found in feces; they are easier to detect in scrapings from the folds around the anus or in rectal mucus.