Daniel P. Beiting1 and Audrey R. Odom John2 1 University of Pennsylvania, School of Veterinary Medicine, Philadelphia, PA, USA 2 Children’s Hospital of Philadelphia, Division of Pediatric Infectious Diseases, Philadelphia, PA, USA Intestinal protozoan parasites are important causes of diarrheal illness worldwide. Infection with Entamoeba histolytica, the causative agent of amebic dysentery and amebic liver abscess, is primarily a disease of lower‐ and middle‐income countries (LMICs), whereas infections caused by Giardia lamblia, Cryptosporidium parvum, and Cyclospora cayetanensis pose serious threats to public health around the world. Physicians must consider these pathogens in the differential diagnosis of acute and chronic diarrhea and should be familiar with the optimal diagnostic (Table 69.1) and therapeutic approaches to these diseases. Because conventional microscopic examination of the stool for ova and parasites is time‐consuming and expensive, special requests for microscopic examinations may need to be made to the laboratory. Enzyme immunoassay methods are now routinely used in clinical microbiology laboratories to screen stools for Giardia and Cryptosporidium and a microscopic exam is not performed unless there is a specific clinical request. Highly sensitive and specific multiplex PCR panels for diagnostic evaluation of infectious diarrhea are also increasingly available. Improved sanitation conditions have greatly reduced the number of cases of amebiasis in the United States. Disease in the United States is most commonly detected among individuals with a recent travel history but should be considered for all persons with dysentery and an appropriate exposure history. E. histolytica trophozoites can be seen in wet mounts of stool, ulcer scrapings, or intestinal aspirates obtained during endoscopy and in fixed specimens stained with trichrome (Figure 69.1). E. histolytica is morphologically identical to the genetically distinct and nonpathogenic Entamoeba dispar species, thus creating significant challenges to using microscopy for diagnosis. Antigen detection enzyme‐linked immunosorbent assays (ELISA) and PCR‐based diagnostics that specifically distinguish between E. histolytica and E. dispar in stool have become the tests of choice for the diagnosis of intestinal amebiasis. E. histolytica trophozoites invade the colonic mucosa and cause discrete ulcers covered with yellowish‐white exudate (Figure 69.2a), multiple well‐defined ulcers (Figure 69.2c), diffuse erythema and ulceration (Figure 69.2d), pseudomembrane formation, and, rarely, heaped‐up inflammatory and granulation tissue that forms an ameboma (Figure 69.2b). The most frequent extraintestinal manifestation of E. histolytica infestation is amebic liver abscess. Patients usually have the triad of fever, right upper quadrant pain, and tenderness, and a space‐occupying lesion in the liver. As illustrated in Figure 69.3, an initial clue to the presence of amebic liver abscess may be an abnormal chest radiograph showing elevation of the right hemidiaphragm, right pleural effusion, and possibly a right basilar infiltrate. Amebic liver abscesses are often visible on computed tomographic (CT) scans as large, generally homogenous, low‐attenuation lesions (Figure 69.4). However, multiple abscesses can develop. Blastocystis hominis is one of the protozoan organisms most frequently detected in stools (Figure 69.5). The pathogenicity of this organism continues to be controversial. Giardia lamblia has been the most common intestinal parasitic cause of diarrhea in the United States in recent years. Groups at high risk for giardiasis include children in day‐care facilities and their adult contacts, travelers, and those who consume contaminated water. Abundant Giardia trophozoites may be seen in biopsy samples from the small intestine of these individuals (Figure 69.6). Historically, the diagnosis of GI parasite infections was based on microscopic examination of stool for defined stages of parasites (Figure 69.8). However, more specific and sensitive stool immunoassays are now routinely available for the detection of G. lamblia, E. histolytica, and C. parvum. Giardia trophozoites attach to the villi of the small intestine where they replicate through longitudinal binary fission to form clusters of parasites attached in the same region (Figures 69.7 and 69.8). Each trophozoite uses its ventral disk to attach to the brush border microvilli of enterocytes, which may play a role in maintenance of infection and disruption of enterocyte function. Table 69.1 Morphology of human gastrointestinal protozoan parasites.
CHAPTER 69
Parasitic diseases: protozoa
Amebiasis
Blastocystis hominis
Giardiasis
Stool
Intestinal biopsy
Extracellular
Ameboid
Entamoeba histolytica
Trophozoite 10–20 μm; pale, round nucleus with small central karyosome; cyst 9–25 μm, four nuclei; morphologically indistinguishable from E. dispar; with ELISA, fecal antigen detection of trophozoite antigen can distinguish E. histolytica from E. dispar; stool PCR is available and serology may be useful adjunct
Trophozoites but not cysts seen invading colonic mucosa, causing colonic ulcerations
Blastocystis hominis (pathogenicity is controversial)
Organisms 6–40 μm, round with large central body or vacuole
Flagellates
Giardia lamblia
Trophozoite pear‐shaped, 10–20 μm long, characteristic face‐like image secondary to two nuclei, each with prominent karyosome; cyst oval, 7–10 μm long; direct fluorescence antibody test available in many laboratories for detection of cysts. Fecal immunoassays are routinely used to detect Giardia in many US hospitals
Trophozoites seen most commonly on duodenal mucosal surface but also on jejunal and ileal biopsy; histology usually normal, but villous atrophy seen in severe infections
Dientamoeba fragilis
Ameboid in shape, flagella not visible, 5–15 μm, 1–2 nuclei; no known cyst form
Ciliates
Balantidium coli (rare)
Trophozoite 50–200 μm in length, motile; cysts are rarely seen in stool and are 50–75 μm in diameter
Trophozoites can invade colonic mucosa, causing ulceration
Intracellular
Cryptosporidium parvum
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
Get Clinical Tree app for offline access
