Although clinical evidence is limited, studies have been done on the in vitro susceptibility of B. hominis to

numerousdrugs. Currently, metronidazole (Flagyl) appears to be the most appropriate drug. Diiodohydroxyquin

(Yodoxin) also has been effective, and dosage schedules for these two drugs are as recommended for other intestinal

protozoa. The development of new drug sensitivity assays may improve researchers’ ability to evaluate the activities of

various drugs against this organism.

Prevention:

 Prevention requires improved personal hygiene and sanitary conditions, in addition to proper disposal of fecal material.

FLAGELLATES:

The Mastigophora, or flagellates, have specialized locomotor organelles called flagella; these are long, thin, cytoplasmic

extensions that may vary in number and position, depending on the species. Different genera of flagellates may live in the

intestinal tract, the bloodstream, or various tissues.

 Four common species of flagellates are found in the intestinal tract: Giardia lamblia, Dientamoeba fragilis, Chilomastix

mesnili, and Pentatrichomonas hominis (Figures 48-16 to 48-23). Several other smaller, nonpathogenic flagellates, such

as Enteromonas hominis and Retortamonas intestinalis (see Figure 48-16) G. lamblia and D. fragilis are the flagellates

considered pathogenic. D. fragilis has been associated with diarrhea, nausea, vomiting, and other nonspecific intestinal

complaints. Trichomonas vaginalis is pathogenic but occurs in the urogenital tract. Trichomonas tenax is occasionally

found in the mouth and may be associated with poor oral hygiene.

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Giardia lamblia:

General Characteristics:

 G. lamblia is the most common cause of intestinal infection worldwide.

Other than B. hominis, G. lamblia (also called G. duodenalis and G. intestinalis) is probably the most common protozoan

organism identified in individuals in the United States. It causes symptoms ranging from mild diarrhea, flatulence, and

vague abdominal pains to acute, severe diarrhea to steatorrhea and a typical malabsorption syndrome. Various

documented waterborne and food-borne outbreaks have occurred during the past several years. A number of animals may

serve as reservoir hosts for G. lamblia. Differentiation of flagellates is based on overall shape, numbers, an arrangements

of flagella. Both the trophozoite and cyst stages are included in the life cycle of G. lamblia. Trophozoites divide by means

of longitudinal binary fission, producing two daughter trophozoites. The organism is found most commonly in the crypts

in the duodenum. Trophozoites are the intestinal dwelling stage and attach to the epithelium of the host villi by means of

the ventral disk. The attachment is substantial and results in disk “impression prints” when the organism detaches from

the surface of the epithelium.Trophozoites may remain attached to or may detach from the mucosal surface. Because the

epithelial surface sloughs off the tip of the villus every 72 hours, the trophozoites apparently detach at that time. G.

lamblia trophozoites are teardrop shaped and have been described as “someone looking at you”. Cyst formation takes

place as the organisms move down through the jejunum after exposure to biliary secretions. The trophozoites retract the

flagella into the axonemes, the cytoplasm becomes condensed, and the cyst wall is secreted (see Figures 13 to 14).

Figure 13 A

to C,

Trophozoites

of Giardia

lamblia. D to

F, Cysts of

G. lamblia.

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the cyst matures, the internal structures are doubled, so that when excystation occurs, the cytoplasm divides, producing

two trophozoites. Excystation occurs in the duodenum or appropriate culture medium.

Epidemiology:

 Transmission of G. lamblia occurs by ingestion of viable cysts. Although contaminated food or drink may be the

source, intimate contact with an infected individual may also result in transmission of the organism. This organism is

found more frequently in children or in groups living in close quarters. Outbreaks have been associated with poor

sanitation facilities or sanitation breakdowns, as evidenced by infections of travelers and campers. Limited information is

available on seasonal variations in giardiasis. Some data suggest an association with the cooler, wetter months of the year,

which may implicate environmental conditions as advantageous to cyst survival.

Certain occupations may place an individual at risk for infection, such as sewage and irrigation workers, who may be

exposed to infective cysts. In situations in which young children are grouped together, such as in nursery schools, an

increased incidence of exposure and subsequent infection of both children and staff members may be seen. A high

incidence of giardiasis occurs in patients with immunodeficiency syndromes, particularly in those with common variable

hypogammaglobulinemia.

Giardiasis is the most common cause of diarrhea in these patients and may be associated with mild to severe villus

atrophy An estimated 200 million people in Asia, Africa, and Latin America have symptomatic infections. In the United

States, approximately 20,000 cases are reported yearly. However, an estimated 2 million cases may occur annually.

Figure 14 A, Giardia lamblia trophozoite. B, G. lamblia trophozoite, iodine stain. C, G. lamblia cysts

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Pathogenesis and Spectrum of Disease:

 The incubation period for giardiasis ranges from approximately 12 to 20 days. Giardiasis may not be recognized as the

cause, because the infection mimics acute viral. enteritis, bacillary dysentery, bacterial or other food poisonings, acute

intestinal amebiasis, or “traveler’s diarrhea” (toxigenic Escherichia coli). However, the type of diarrhea plus the lack of

blood, mucus, and cellular exudate is consistent with giardiasis.

Asymptomatic Infection. Although the parasites in the crypts of the duodenal mucosa may reach very highnumbers, they

may not cause a pathologic condition. The organisms feed on the mucous secretions and do not penetrate the mucosa.

Although organisms have been seen in biopsy material obtained from inside the intestinal mucosa, others have been seen

attached to the epithelium.

Intestinal Disease. For unknown reasons, symptomatic patients may have irritation of the mucosal lining, increased

mucus secretion, and dehydration. The onset may be accompanied by nausea, anorexia, malaise, lowgrade fever, and

chills, in addition to a sudden onset of explosive, watery, foul-smelling diarrhea. Other symptoms include epigastric pain,

flatulence, and diarrhea with increased amounts of fat and mucus in the stool but no blood. Weight loss often

accompanies these symptoms. Although some speculate that the organisms coating the mucosal lining may act to prevent

fat absorption, this does not completely explain the prevention of the uptake of other substances normally absorbed at

other intestinal levels. Severe malabsorption has also been linked with isolated levothyroxine malabsorption, leading to

severe hypothyroidism and secondary impairmen of pancreatic function. In both cases, treatment with metronidazole led

to complete remission of symptoms. Occasionally the gallbladder is involved, resulting in gallbladder colic and jaundice.

G. lamblia also has been identified in bronchoalveolar lavage fluid.

Chronic Disease. The acute phase often is followed by a subacute or chronic phase. Symptoms include recurrent, brief

episodes of loose, foul-smelling stools and possibly increased distention and foul flatus. Between episodes of mushy

stools, the patient may have normal stools or may be constipated. Abdominal discomfort includes marked distention and

belching with a rotten-egg taste. Chronic disease must be differentiated from amebiasis; disease caused by other intestinal

parasites (e.g., D. fragilis, Cryptosporidium spp., Cyclospora cayetanensis, Isospora belli, Strongyloides stercoralis);

inflammatory bowel disease; and irritable colon. On the basis of symptoms such as upper intestinal discomfort, heartburn,

and belching, giardiasis must also be differentiated from duodenal ulcer, hiatal hernia, and gallbladder and pancreatic

disease.

Laboratory Diagnosis:

Routine Methods. Routine stool examinations are normally recommended for the recovery and identification of

intestinal protozoa. However, in the case of G. lamblia, because the organisms are attached securely to the mucosa by

means of the sucking disk, a series of five or six stool samples may be examined without recovering the organism. The

organisms also tend to be passed in the stool on a cyclic basis. The Entero-Test capsule can be helpful for recovering the

organisms, as can the duodenal aspirate. Although cysts often can be identified on the wet stool preparation, many

infections may be missed without examination of a permanent stained smear. If material from the string test or mucus

from a duodenal aspirate is submitted, it should be examined as a wet preparation for motility; however, motility may be

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represented by nothing more than a slight flutter of the flagella, because the organism is caught up in the mucus. After

diagnosis, the positive specimen can be preserved as a permanent stain.

Antigen Detection. The development of fecal immunoassays to detect Giardia antigen in stool has dramatically

improved the sensitivity seen with the routine O&P examination. The ELISA has been used to detect Giardia antigen in

feces. Fluorescent methods with monoclonal antibodies have also proven extremely sensitive and specific in detecting G.

lamblia in fecal specimens. Other products are available as a cartridge format that uses an immunochromatographic strip–

based detection system for G. lamblia and/or Cryptosporidium spp. Any antigen detection system should always be

reviewed for compatibility with stools submitted in preservatives rather than fresh specimens. Some limitations exist on

the use of kits for organisms in the genus Entamoeba. However, commercial reagent kits for detecting Giardia and

Cryptosporidium spp. can be used with formalin-based stool preservatives or with fresh or frozen specimens. Many of

these cartridge format tests provide an answer within 10 minutes and are equal to or better than other immunoassays with

regard to sensitivity and specificity. Many of these newer methods are being used to test patients suspected of having

giardiasis or those who may be involved in an outbreak. The detection of antigen in stool or visual identification of

organisms by using monoclonal antibody reagents indicates current infection. The value of these detection assays as

rapid, reliable immunodiagnostic procedures has been emphasized by the increase in Giardia infections and the greater

awareness of particular incidences (e.g., nursery school settings). Because the organisms are shed so sporadically, use of a

fecal immunoassay does not eliminate the need to analyze multiple stool specimens for sensitive detection of G. lamblia;

a minimum of two stools should be tested. If the first specimen is negative, it may represent a false negative.

Histology. Trophozoites are detectable in the duodenum and proximal jejunum; however, mucosal invasion generally has

been found in areas where necrosis or mechanical trauma was present.

Apparently, patients with giardiasis also have reduced mucosal surface areas compared with control patients.

Nucleic Acid-Based Techniques: Currently, there are no molecular-based assays commercially available for the

detection of G. lamblia.

Prevention:

 The most effective practice for preventing the spread of infection in the child care setting is thorough hand washing by

the children, staff members, and visitors. Filtration systems have also been recommended, although they have certain

drawbacks, such as clogging.

Chilomastix mesnili:

General Characteristics

The C. mesnili trophozoite is pear shaped, measuring 6 to 24 μm long and 4 to 8 μm wide. It has a single nucleus and a

distinct oral groove, or cytostome (mouth), close to the nucleus. Flagella are difficult to see without obvious motility in a

wet preparation. The morphology can be

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seen on the permanent stained smear; the cytostome may be visible in some trophozoites. The cysts are pear or lemon

shaped and range from 6 to 10 μm long and 4 to 6 μm wide . They have a single nucleus and a typical curved cytostomal

fibril, called the shepherd’s crook. The cyst’s definitive morphology can be seen on a permanent stain.

Epidemiology:

C. mesnili tends to have a cosmopolitan distribution, although it is found more frequently in warm

climates.Transmission occurs through ingestion of infective cysts.

Pathogeneis and Spectrum of Disease:

C. mesnili is considered nonpathogenic and does not cause disease.

Laboratory Diagnosis

 Although cysts sometimes can be seen in a wet preparation, definitive identification of C. mesnili relies on

examination of permanent stained smears.

Therapy

 Specific treatment is not recommended for C. mesnili. Because these nonpathogenic organisms are acquired

through fecal-oral contamination.

Prevention:

Prevention depends on adequate disposal of human excreta and improved personal hygiene, preventive

measures that apply to most of the intestinal protozoa.

Dientamoeba fragilis

General Characteristics:

 D. fragilis was described in 1918. D. fragilis tends to be common in some pediatric populations, and the incidence is

higher for patients under 20 years of age in some studies. Some speculate that D. fragilis may be infrequently recovered

and identified; a low incidence or absence from survey studies may be due to poor laboratory techniques and a general

lack of knowledge about the organism. The D. fragilis trophozoite is characterized as having one nucleus (20% to 40%) or

two nuclei (60% to 80%). The nuclear chromatin usually is fragmented into three to five granules, and normally no

peripheral chromatin is seen on the nuclear membrane. The cytoplasm is usually vacuolated and may contain ingested

debris and some large, uniform granules. The cytoplasm can also appear uniform and clean with few inclusions. Size and

shape vary considerably among organisms, even on a single smear(Figure 15)

Pathogenesis and Spectrum of Disease:

 D. fragilis has been associated with a wide range of symptoms. Case reports of children infected with D.

fragilis reveal a number of symptoms, including intermittent diarrhea, abdominal pain, nausea, anorexia,

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malaise, fatigue, poor weight gain, and unexplained eosinophilia. The most common symptoms in patients

infected with this parasite appear to be intermittent diarrhea and fatigue. In some patients, both the organism

and the symptoms persist or reappear until appropriate treatment is initiated.

Laboratory Diagnosis:

Routine Methods. Diagnosis of D. fragilis infections depends on proper collection and processing techniques

(a minimum of three fecal specimens). Although the survival time for this parasite has been reported as 24 to 48

hours in the trophozoite form, the survival time in terms of morphology is limited, and stool specimens must be

examined immediately or preserved in a suitable fixative soon after defecation. It is particularly important to

examine permanent stained smears of stool with an oil immersion objective (×100).These trophozoites have

been recovered in formed stool; therefore, a permanent stained smear must be prepared for every stool sample

submitted for examination. Organisms seen in direct wet mounts may appear as refractile, round forms; the

nuclear structure cannot be seen without examination of the permanent stained smear.

Antibody Detection. On indirect immunofluorescence assay, serum samples from patients with confirmed D.

fragilis infections showed positive titers, and all matched controls had positive titers ranging from 20 to 160.