Intestinal and urogenital protozoa, Biology tutorial


Intestinal and luminal protozoa significant to human health include

  • Entamoeba histolytica (Amebae)
  • Balantidium coli (Ciliates)
  • Giardia lamblia and Trichomonas vaginalis (Flagellates)
  • Cryptosporidium parvum and Isospora belli (Sporozoa)

1) Amebae:

Species of Entamoeba have been found in the number of invertebrate and vertebrate hosts. 5 species of genus Entamoeba infect man: E. histolytica, a species that is at times pathogenic in caecum and colon; E. hartmanni, harmless species closely connected to E. histolytica and not considered as separate species by some workers; E. coli, a harmless species; E. gingivalis, in mouth; and E. polecki, a species uncommon in man and possibly usually occurring in pigs.

Infections of Entamoeba histolytica usually result in amoebiasis, term histolytica literally meaning tissue-dissolving, referring to potential carnivorous habits of organism. Where clinical symptoms result, disease is referred to as invasive amoebiasis; the non-invasive infection is at times known as luminal amoebiasis.

Epidemiology: 0.5 to 50% of population worldwide harbors E. histolytica parasites with higher rates of infection being in underdeveloped countries. 1 to 3% of populations of USA are infected. Infection is related with poor hygiene. Humans are principal host, though dogs, cats and rodents may be infected.

Morphology: Trophozoite: This form has the ameboid appearance and is generally 15-30 micrometers in diameter, though more invasive strains tend to be larger. Organism has single nucleus with the distinctive small central karyosome. Fine granular endoplasm may have ingested erythrocytes. Nuclear chromatin is equally distributed along periphery of nucleus

Cyst: Entameba histolytica cysts are spherical, with the refractile wall; cytoplasm has dark staining chromatoidal bodies and 1 to 4 nuclei with the central karyosome and equally distributed peripheral chromatin.

Lifecycle: Infection takes place by ingestion of cysts on fecally contaminated food or hands. Cyst is resistant to gastric environment and passes in small intestine where it decysts. Metacyst splits in four and then eight amoebae that move to large intestine. Most of the organisms are passed out of body with feces but, with larger bolus of infection, few amebae join to and invade mucosal tissue forming flask-shaped lesions (bomb craters). Organisms encyst for mitosis and are passed through with feces. There are no transitional or reservoir hosts.

Amebiasis (amebic dysentery, amebic hepatitis) is a disease caused by Entamoeba parasite

Etiology: E. histolytica is main cause of amebic dysentery.

Symptoms: Frequent dysentery with necrotic mucosa and abdominal pain. Repeated episodes of dysentery with blood and mucus in feces. There are prevailing gastrointestinal disturbances and constipation. Cysts are found in stool. Organism may invade liver, lung and brain where it generates abscesses which result in liver dysfunction, pneumonitis, and encephalitis.

Pathology: Intestinal ulcers (craters/flasks) are because of enzymatic degradation of tissue. Infection may result in perforation, appendicitis, pseudo-polyps, stricture granuloma, liver abscess; at times brain, lung and spleen abscesses can also take place. Strictures and pseudo-polyps result from host inflammatory response.

Immunology: There is antibody response after invasive infection (liver abscess or colitis) but it is of uncertain consequence in immunity, as there is return of enteric episodes in the patients.

Diagnosis: In laboratory, infection is established by finding cysts in stool. E. histolytica infection is distinguished from bacillary dysentery by short of high fever and absence PMN leukocytosis. Distinction should be made from other non-pathogenic intestinal protozoa (like Dientamoeba fragilis, Entamoeba coli, Endolimax nana, Entamoeba hartmanni, Iodamoeba buetschlii, etc.).

Treatment: Iodoquinol is utilized to treat asymptomatic infections and metronidazole is employed for symptomatic and chronic amebiasis, comprising extra-intestinal disease.

Other species of Entamoeba:

a) Entamoeba coli: This is non-pathogenic species whose distribution is global, occurring in some 30% of world's population. It is found in large intestine and distinguished from E. histolytica by the numerous characteristics, like: (i) it has slower movement; (ii) its pseudopodium is mostly coarser and not clear like that of E. histolytica; (ii) its nucleus is coarser and with the eccentric karyosome (iv) it contains larger number of food vacuoles.

b) Entamoeba gingivalis: The common parasite of human mouth; linked species occur in mouths of dogs, horses and donkeys. Spaces between teeth and soft pits of gums provide best surfaces for amoebae. Its food vacuoles are generally frequent and have bacteria, leucocytes and occasionally red cells.

c) Entamoeba poleck: This is a species that usually takes place in pigs. It forms uninucleate cysts and illustrates some morphological differences from cyst of E. histolytica.

d) Entamoeba moshkovskii: This species is not animal parasite, but it may suitably be discussed with other forms. Large number of strains has been recognized. Morphologically it strongly looks like E. histolytica throughout life cycle, comprising encystment and metacystic development, differing from it only in details.

2) Giardia:

The flagellate of genus is fairly unlike any of other species in shape or habits. It has been explained in front view as looking like the tennis racquet without a handle and it has comical, face-like appearance. Body is tear-drop shaped with the convex dorsal surface and concave ventral one. Second have two depressions, at times termed adhesive discs (suckers) that make contact with intestinal cells of host. The single or double median body, unique to the genus, is found just below adhesive discs. Giardia shows ideal bilateral symmetry, and there are double sets of nuclei, flagella and kinetosomes. Every nucleus has been illustrated to have haploid number of chromosomes. The median body superficially looks like axostyle of trichomonads, but organization of microtubules that compose it, and fact that body is not always present, make it distinguishing structure. No structures recognizable as mitochondria, smooth endoplasmic reticulum or Golgi complex have been recognized in Giardia. Species of Giardia are confined in their distribution to small intestine, mainly duodenum, rarely invading bile ducts. In severe infections they may carpet large areas of mucosa.

Epidemiology: Giardia has universal distribution and isn't uncommon in South Carolina. It is the most common protozoan intestinal disease in US and most common identified cause of waterborne disease linked with breakdown of water purification systems, drinking from impure streams, travel to endemic areas (Russia, India, Rocky Mountains, etc.) and day care centers.

Morphology: Trophozoite: Giardia is 12 to 15 micrometer, half pear-shaped organism with eight flagella and two axostyles arranged in the bilateral symmetry. There are 2 interiorly located large suction discs. Cytoplasm has two nuclei and two parabasal bodies.

Cyst: Giardia cysts are 9 to 12 micrometer ellipsoidal cells with the smooth well-defined wall. Cytoplasm has four nuclei and several structures seen in trophozoite.

Life cycle: Infection takes place by ingestion of cysts, generally in contaminated water. Decystation takes place in duodenum and trophozoites (trophs) colonize upper small intestine where they may swim freely or join to sub-mucosal epithelium via ventral suction disc. Free trophozoites encyst as they move down stream and mitosis occurs during encystment. Cysts are passed in stool. Man is main host though beavers, pigs and monkeys are infected and act as reservoirs.

Giardiasis (lambliasis): It is a disease caused by Giardi parasite.


Giardia lamblia (a flagellate)

Symptoms: Early symptoms comprise flatulence, abdominal distension, nausea and foul-smelling bulky, explosive, frequently watery, diarrhea. Stool has extreme lipids but seldom any blood or necrotic tissue. More chronic stage is related with vitamin B12 malabsorption, disaccharidase deficiency and lactose intolerance.

Pathology: Covering of intestinal epithelium by trophozoite and flattening of mucosal surface results in malabsorption of nutrients.

Immunology: There is some role for IgA and IgM and there is increased occurrence of infection in immunodeficient patients (like AIDS).

Diagnosis: Symptoms, history, epidemiology are utilized in diagnosis. Giardia caused dysentery is distinct from other dysenteries because of lack of mucus and blood in stool, lack of increased PMN leukocytes in stool and short of high fever. Cysts in stool and trophs in duodenum can be reconized microscopically after content has been attained using the string device.

Treatment: Metronidazole is drug of choice.

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