Intestinal Protozoa

PARASITES

Intestinal Protozoa
Entamoeba histolytica (Intestinal Amoeba)
Diseases: Entamoeba histolytica causes amebic dysentery and liver abscess.
Important Properties:
The life cycle has two stages: the motile ameba (trophozoite) and the nonmotile cyst. The trophozoite is found within the intestinal and extraintestinal lesions and in diarrheal stools. The cyst predominates in nondiarrheal stools. These cysts are not highly resistant and are readily killed by boiling but not by chlorination of water supplies. They are removed by filtration of water.
The cyst has four nuclei, an important diagnostic criterion. The mature trophozoite has a single nucleus with an even lining of peripheral chromatin and a prominent central nucleolus (karyosome).
Antibodies are formed against trophozoite antigens in invasive amebiasis, but they are not protective; previous infection does not prevent reinfection. The antibodies are useful, however, for serologic diagnosis.












Pathogenesis & Epidemiology:
1. The organism is acquired by ingestion of cysts that are transmitted primarily by the fecal–oral route in contaminated food and water. Anal–oral transmission, e.g., among male homosexuals. There is no animal reservoir.
2. The ingested cysts differentiate into trophozoites in the ileum but tend to colonize the cecum and colon.
3-The trophozoites invade the colonic epithelium and secrete enzymes that cause localized necrosis. Little inflammation occurs at the site. As the lesion reaches the muscularis layer, a typical "flask-shaped" ulcer forms.
4-Progression into the submucosa leads to invasion of the portal circulation by the trophozoites. By far the most frequent site of systemic disease is the liver, where abscesses containing trophozoites form.
Clinical Findings:
-Acute intestinal amebiasis presents as dysentery (i.e., bloody, mucus-containing diarrhea) accompanied by lower abdominal discomfort, flatulence, and tenesmus.
-Chronic amebiasis: diarrhea, weight loss, and fatigue also occur. Roughly 90% of those infected have asymptomatic infections, but they may be carriers, whose feces contain cysts that can be transmitted to others. In some patients, a granulomatous lesion called an ameboma may form in the cecal or rectosigmoid areas of the colon. These lesions can resemble an adenocarcinoma of the colon.
-Amebic abscess of the liver is characterized by right-upper-quadrant pain, weight loss, fever, and a tender, enlarged liver. Right-lobe abscesses can penetrate the diaphragm and cause lung disease. Aspiration of the liver abscess yields brownish-yellow pus with the consistency of anchovy-paste.
Laboratory Diagnosis:
-intestinal amebiasis: finding either trophozoites in diarrheal stools or cysts in formed stools. Diarrheal stools should be examined within 1 hour of collection to see the ameboid motility of the trophozoite. Trophozoites contain ingested red blood cells. Because cysts are passed intermittently, at least three specimens should be examined.
-E. histolytica can be distinguished by two criteria. (1) The nature of the nucleus of the trophozoite (has a small central nucleolus and fine chromatin granules along the border of the nuclear membrane). (2) The second is cyst size and number of its nuclei.
-Two tests are highly specific for E. histolytica in the stool: one detects E. histolytica antigen and the other detects nucleic acids of the organism in a PCR-based assay.
-A complete examination for cysts includes a wet mount in saline, an iodine-stained wet mount, and a fixed, trichrome-stained preparation. These preparations are also helpful in distinguishing amebic from bacillary dysentery. In the latter, many inflammatory cells such as polymorphonuclear leukocytes are seen, whereas in amebic dysentery they are not.
-Serologic testing is useful for the diagnosis of invasive amebiasis. The indirect hemagglutination test is usually positive in patients with invasive disease but is frequently negative in asymptomatic individuals who are passing cysts.
Treatment:
-The treatment of choice for symptomatic intestinal amebiasis or hepatic abscesses is metronidazole (Flagyl) or tinidazole.
-Asymptomatic cyst carriers should be treated with iodoquinol or paromomycin.
Prevention:
-avoiding fecal contamination of food and water.
-good personal hygiene such as hand washing.
-Purification of municipal water supplies is usually effective.
-In areas of endemic infection, vegetables should be cooked.
Intestinal Flagellate:
1. GIARDIA
Disease Giardia lamblia causes giardiasis.
Important Properties:
The life cycle consists of two stages, the trophozoite and the cyst. The trophozoite is pear-shaped with two nuclei, four pairs of flagella, and a suction disk with which it attaches to the intestinal wall. The oval cyst is thick-walled with four nuclei and several internal fibers. Each cyst gives rise to two trophozoites during excystation in the intestinal tract.





Pathogenesis & Epidemiology:
The organism is found worldwide. In addition to being endemic, giardiasis occurs in outbreaks related to contaminated water supplies. Chlorination does not kill the cysts, but filtration removes them.
Many species of mammals as well as humans act as the reservoirs. They pass cysts in the stool, which then contaminates water sources. Giardiasis is common in male homosexuals as a result of oral-anal contact. The incidence is high among children in day-care centers and among patients in mental hospitals.
Transmission occurs by ingestion of the cyst in fecally contaminated food and water.
Excystation takes place in the duodenum, where the trophozoite attaches to the gut wall but does not invade. The trophozoite causes inflammation of the duodenal mucosa, leading to malabsorption of protein and fat.
Clinical Findings:
Watery (nonbloody), foul-smelling diarrhea is accompanied by nausea, anorexia, flatulence, and abdominal cramps persisting for weeks or months. There is no fever.
Laboratory Diagnosis:
-Finding trophozoites or cysts or both in diarrheal stools.
-In formed stools, e.g., in asymptomatic carriers, only cysts are seen.
-An ELISA test that detects a Giardia cyst wall antigen in the stool is also very useful.
-If microscopic examination of the stool is negative, the string test, which consists of swallowing a weighted piece of string until it reaches the duodenum, should be performed. The trophozoites adhere to the string and can be visualized after withdrawal of the string.
Treatment:
The treatment of choice is metronidazole (Flagyl) or quinacrine hydrochloride.
Prevention:
Prevention involves drinking boiled, filtered, or iodine-treated water in endemic areas.
2. Chilomastix mesnili
A non-pathogen - must be differentiated from Giardia. Found in cecum and colon. Transmissio- by ingestion of mature cysts.
Morphology
? Trophozoite - 4 flagella (3 anterior, 1 associated with the cytostome; one nucleus, always located anteriorly.
? Cyst - lemon shape; 1 nucleus; cytostome may be seen.

Balantidium coli (ciliates)
Balantidium coli, the cause of balantidiasis or balantidial dysentery, is the largest intestinal protozoan of humans.
Morphology & Identification:
The trophozoite is a ciliated, oval organism. Its motion is a characteristic combination of steady progression and rotation around the long axis. The cell wall is lined with spiral rows of cilia. The cytoplasm surrounds two contractile vacuoles, food particles and vacuoles, and two nuclei—a large, kidney-shaped macronucleus and a much smaller, spherical genetic micronucleus. The macronucleus, contractile vacuoles, and portions of the ciliated cell wall may be visible in the cyst.
Culture: These organisms may be cultivated in many media used for cultivation of intestinal amebas.










Pathogenesis, Pathology, & Clinical Findings:
When cysts are ingested by the new host, the cyst walls dissolve and the released trophozoites descend to the colon, where they feed on bacteria and fecal debris, multiply both sexually and asexually, and form cysts that pass in the feces.
However, rarely, the trophozoites invade the mucosa and submucosa of the large bowel and terminal ileum. As they multiply, abscesses and irregular ulcerations with overhanging margins are formed. The number of lesions formed depends upon intensity of infection and degree of individual host susceptibility.
Chronic recurrent diarrhea, alternating with constipation, is the most common clinical manifestation, but there may be bloody mucoid stools, tenesmus, and colic. Extreme cases may mimic severe intestinal amebiasis, and some have been fatal.
Diagnostic Laboratory Tests:
-laboratory detection of trophozoites in liquid stools or, more rarely, of cysts in formed stools.
-Sigmoidoscopy may be useful for obtaining material directly from ulcerations for examination. Culturing is rarely necessary.
Immunity:
Humans appear to have a high natural resistance to balantidial infection. Factors underlying individual susceptibility are not known.
Treatment:
A course of oxytetracycline may be followed by iodoquinol or metronidazole if necessary.

Epidemiology:
-B coli is found in humans throughout the world, particularly in the tropics.
-Infection results from ingestion of viable cysts previously passed in the stools by humans and possibly by swine. Pig farmers and slaughterhouse workers are particularly at risk, though poor sanitation and crowding in jails, mental institutions, or encampments are associated with infection.