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Amebiasis is defined as infection with Entamoeba histolytica, regardless of associated symptomatology. In resource-rich nations, this parasitic protozoan is seen primarily in travelers to and emigrants from endemic areas. Infections range from asymptomatic colonization to amebic colitis and life-threatening abscesses. Importantly, disease may occur months to years after exposure. Although E histolytica was previously thought to infect 10% of the world's population, 2 morphologically identical but genetically distinct and apparently nonpathogenic Entamoeba species are now recognized as causing most asymptomatic cases. To avoid unnecessary and possibly harmful therapies, clinicians should follow the diagnostic and treatment guidelines of the World Health Organization.
Most notably, 2 additional Entamoeba species that are morphologically indistinguishable from E histolytica have been recognized in humans. As our knowledge of the global epidemiology and pathogenicity of Entamoeba spp increases, new clinical algorithms are developed.
The latest nomenclature and recommendations, although unfamiliar and confusing to many, are important for appropriate patient care. Our review discusses what is known about these 3 Entamoeba spp and clarifies the currently accepted recommendations for diagnosis and treatment.
THE “NEW” ENTAMOEBA SPECIES: ENTAMOEBA DISPAR AND ENTAMOEBA MOSHKOVSKII
It is a long-held misconception that 10% of the world's population is infected with E histolytica. In fact, most of these infections should be attributed to the morphologically identical but nonpathogenic E dispar. Emile Brumpt
first proposed the existence of 2 indistinguishable Entamoeba spp, one pathogenic and one nonpathogenic, in 1925. However, not until 1978 was evidence for the existence of 2 separate entities provided by new technology (isoenzyme analysis).
Little is currently known about their epidemiology in resource-rich nations, where the incidence of both is rare, but previous reports of infection with E histolytica based only on morphology likely represent E dispar.
The discovery of a third morphologically identical Entamoeba spp further complicated our understanding of the epidemiology of E histolytica. The new species, named E moshkovskii, was first recognized as a ubiquitous free-living organism in 1941
Although largely nonpathogenic, some recent evidence suggests that it may have a role in human intestinal disease. Much remains unknown regarding the pathogenicity and epidemiology of E moshkovskii.
THE DEFINITIVE PATHOGEN: E HISTOLYTICA
E histolytica is the pathogenic species responsible for amebic colitis throughout the world. It infects people of both sexes and all ages; however, populations at risk may vary with geographic location, host susceptibility, and differences in organism virulence. People in highly endemic areas probably have recurrent asymptomatic infections, thus accounting for the high reported prevalence.
Men who have sex with men were previously thought to have an increased incidence of infection, but this supposition was based on morphologic studies. New evidence suggests that these men were colonized primarily with E dispar rather than E histolytica.
This association with poor sanitation explains why resource-poor nations carry the bulk of the world's disease. After ingestion and passage through the stomach, the organism excysts and emerges in the large intestine as an active trophozoite. Trophozoites multiply by simple division and encyst as they move further down the large bowel. Cysts are then expelled with the feces and may remain viable in a moist environment for weeks to months.
This clinical picture corresponds histologically with trophozoites invading and laterally undermining the intestinal surface to form the so-called flask-shaped ulcers (Figure 1). The right side of the colon is commonly involved.
Severe cases of amebic colitis are characterized by copious bloody diarrhea, diffuse abdominal pain, and (rarely) fever. Extensive fulminant necrotizing colitis, the most severe form of intestinal disease, is often fatal.
Complications of intestinal disease include stricture, rectovaginal fistulas, formation of an annularintraluminal mass (ameboma), bowel obstruction, perianal skin ulceration, toxic megacolon, perforation, peritonitis, shock, and death.
On rare occasions, E histolytica trophozoites enter the bloodstream and disseminate to other body sites, most commonly the liver via spread from the intestine through the portal vein. The right lobe is 4 times more likely to be involved than the left because it receives the bulk of the venous drainage from the right colon.
Perhaps the most serious complication is amebic metastasis from the liver. Rarely, trophozoites end up in other regions of the body, such as the brain, spleen, lungs, and genitourinary tract, through hematogenous or direct spread.
Disseminated disease is not an adaptive mechanism for the parasite because its life cycle cannot be completed outside the intestine.
RADIOLOGIC AND ENDOSCOPIC FEATURES OF INTESTINAL AND EXTRAINTESTINAL DISEASE
When amebiasis is suspected, radiologic and endoscopic examination may lend further support for a diagnosis. Colonoscopy can provide a wide spectrum of findings, from rare large-bowel ulcers in mild disease to diffuse mucosal ulceration, hemorrhage, colonic stricture, and presence of an ameboma.
Radiologic studies may also be helpful in evaluating a patient with possible ALA. Chest and abdominal radiography often reveal a pleural effusion and raised hemi-diaphragm overlying the involved liver lobe.
Computed tomography and magnetic resonance imaging can further characterize an abscess and allow for better detection of smaller lesions. All 3 techniques may facilitate guided needle biopsy and drainage if indicated.
An abscess can usually be distinguished from solid lesions and biliary tract disease, but the differentiation between bacterial and amebic abscesses is less clear. Gallium scans may have a role in this differential diagnosis because amebic abscesses are usually “cold” on scan because of the lack of white blood cells in the abscess, whereas bacterial abscesses are typically “hot.”
The diagnosis of invasive amebiasis is usually suggested by the patient's presenting symptoms, exposure history, and radiologic findings but should be confirmed with microbiological laboratory results. Many laboratory methods exist for identification of E histolytica, E dispar, and/or E moshkovskii, and the clinician should be aware that tests vary considerably in price, sensitivity, specificity, and the ability to definitively differentiate among the 3 species.
Light microscopic examination of fecal specimens (ie, “ova and parasite” examination) is often the first step in diagnosis
; the characteristic trophozoites and cysts can often be identified through direct, concentrated, and/or permanently stained smears (Figure 2). Because organisms may appear intermittently, current recommendations call for submission of 3 stool specimens on different days during a period of 10 days.
Unfortunately, microscopy alone cannot differentiate E histolytica from E dispar and E moshkovskii; additional tests are required for definitive speciation. The rare exception is when trophozoites containing ingested red blood cells are identified; they are strongly (but not definitively) indicative of invasive amebiasis.
Identification methods include biopsy, serology, antigen detection, and molecular assays. Culture may be performed by some large specialty laboratories but is technically challenging and time-consuming. Furthermore, a negative culture result from intestinal samples does not exclude E histolytica
Serologic tests detect the presence of species-specific antibodies in the patient's serum. They are particularly useful in nonendemic countries where prevalence is low and have a good sensitivity and specificity for detecting invasive intestinal disease. They are also the test of choice for ALA because titers are typically high and test sensitivities and specificities exceed 95% with most assays.
The primary disadvantage of serologic tests is that they cannot distinguish between past and current infection unless IgM is detected; IgM antibodies to E histolytica are short-lived and rarely detected. In contrast, IgG antibodies are long-lived but highly prevalent in endemic settings because of past exposure.
Antigen tests are useful for confirming microscopic findings and providing a diagnosis in patients with negative fecal smear results. They are also helpful for interpreting positive results on amebic serology in patients from endemic countries because positive results on an antigen test indicate current rather than past infection.
Not all commercial kits are capable of speciation; some demonstrate cross-reactivity between E histolytica and E dispar. Antigen detection methods are also not as sensitive as polymerase chain reaction assays
Clinicians should be familiar with the specifications of the kits used in their laboratory and confirm a suspected diagnosis if indicated.
The highest sensitivity and specificity for the diagnosis of E histolytica are offered by DNA-based tests. Many assays are available, including conventional and real-time polymerase chain reaction formats
; however, they are currently used primarily by research and reference laboratories. Like most molecular amplification assays, they remain impractical for resource-limited settings because of their equipment, personnel, and facility requirements.
The differential diagnosis of amebic colitis must include bacterial (eg, Salmonella and Shigella spp, Mycobacterium tuberculosis), parasitic (eg, Schistosoma mansoni, Balantidium coli), and noninfectious (eg, inflammatory bowel disease, carcinoma, ischemic colitis, diverticulitis) causes of dysentery.
Biopsy specimens of intestinal ulcers are useful for confirming the presence of trophozoites and for excluding other etiologies.
Given its varied clinical presentation and possible delay of onset, the diagnosis of ALA may not be straightforward. The differential diagnosis includes bacterial abscess, echinococcal cyst, tuberculosis, and primary or metastatic tumor,
all which would have vastly different treatments. Radiology can differentiate between many noninfectious and infectious etiologies; however, bacterial and amebic abscesses may appear remarkably similar. In comparison with bacterial abscesses, ALAs are more likely to be solitary, subcapsular, and located in the right lobe of the liver, but these findings are not always reliable.
Radiologic imaging, clinical history, findings on physical examination, and serologic results are essential for including or excluding the diagnosis of ALA.
The WHO/PAHO recommendations state that, when possible, E histolytica should be differentiated from morphologically similar species and treated appropriately. Given the small but substantial risk of invasive disease and the potential to transmit the infection to others, WHO/PAHO recommends treating all cases of proven E histolytica, regardless of symptoms.
Thus, WHO/PAHO recommends withholding treatment from asymptomatic patients when only a morphologic diagnosis by stool examination is available (ie, E histolytica/E dispar/E moshkovskii), unless another reason to suspect E histolytica infection exists.
Even if patients diagnosed as being infected with E histolytica/E dispar/E moshkovskii have symptoms, other causes of disease, such as bacterial colitis, should not be excluded until further testing is done.
The medications recommended to treat confirmed amebiasis vary with clinical manifestation. Asymptomatic intestinal infection with E histolytica should be treated with luminal amebicides, such as paromomycin and diloxanide furoate.
Abdominal cramps and nausea are the most commonly reported adverse effects. A 10-day course at 30 mg/kg per day (divided into 3 daily doses) is typical. Some recommend follow-up stool examination to confirm eradication of cysts.
Compared with asymptomatic infection, intestinal and extraintestinal invasive disease are aerobic processes and should be treated with tissue amebicides, such as 5-nitroimidazoles (eg, metronidazole), which are readily absorbed into the bloodstream.
Metronidazole may also be given parenterally to critically ill patients and can be supplemented with an antibiotic to cover secondary sepsis with bowel flora. The most common adverse effects of metronidazole are abdominal discomfort and nausea; most patients, however, are able to complete a full 5- to 10-day course. Serious adverse drug reactions include confusion, ataxia, and seizures.
A promising new regimen for invasive amebiasis is a 3-day course of nitazoxanide. This drug is effective against both luminal and invasive forms and has the added benefit of eliminating other intestinal parasites, including helminths.
Surgery may be necessary in cases of perforation, abscess, obstruction, stricture, or toxic megacolon. However, given the friable nature of the inflamed mucosa, bowel repair is risky and should be avoided when possible.
Surgical or percutaneous drainage of ALAs is generally not recommended because of the risk of content spillage and/or bacterial superinfection; exceptions are cases of imminent rupture, failure to respond to treatment after 4 to 5 days, and secondary bacterial infection.
Amebae rarely disseminate beyond the portal circulation. Given the small number of cases, no definitive treatment guidelines are available for management of extraintestinal, extrahepatic disease. As mentioned previously, infections with E dispar do not require treatment. Less is known about E moshkovskii, but it is likely that this infection also would not require treatment in most cases.
Recent discoveries have revolutionized our understanding of the epidemiology of Entamoeba spp infections and have led to important treatment and diagnostic recommendations. To avoid unnecessary and possibly harmful therapies, clinicians should follow the precise guidelines promulgated by the WHO/PAHO in 1997, including definitive differentiation of E histolytica from morphologically identical nonpathogenic species. Such definite differentiation is especially important in countries with adequate sanitation measures, where the predominant organism identified from morphologic stool examination will be E dispar. Because they have the highest sensitivity and specificity, molecular technologies offer the greatest diagnostic potential for laboratories in resource-rich countries at this juncture; however, some antigen detection tests can also provide reliable speciation. When speciation is impossible, we recommend using the phrase “E histolytica/E dispar/E moshkovskii” to describe the morphologically identical species seen on stool examination. Continued use of new technologies will be crucial in elucidating the true epidemiology and pathogenesis of Entamoeba spp, including the less well-studied E moshkovskii. Continued development of affordable, sensitive, and specific diagnostic tools will be required for use in resource-poor settings, where the incidence of disease is highest.
On completion of this article, you should be able to: (1) request the appropriate tests for differentiating the morphologically identical Entamoeba species E histolytica, E dispar, and E moshkovskii on the basis of initial laboratory reports; (2) recognize symptoms and complications of invasive amebiasis; and (3) apply the World Health Organization/Pan American Health Organization guidelines for diagnosis and treatment of amebiasis.