TOC | GI
Diarrheal diseases continue to be major causes of morbidity and mortality in children in developing countries. For example, in Bangladesh 1 in 30 children dies of diarrhea or dysentery by his or her fifth birthday.1 In developed countries the microorganisms that cause diarrheal disease remain of concern because of their potential use as bioterrorist agents. Bacillary dysentery is most commonly caused by microorganisms belonging to the genus shigella, whereas amebic dysentery is caused by the protozoan parasite Entamoeba histolytica. The annual number of shigella infections throughout the world is believed to be approximately 164 million.2 Estimates of E. histolytica infections have primarily been based on examinations of stool for ova and parasites, but these tests are insensitive and cannot differentiate E. histolytica from morphologically identical species that are nonpathogenic, such as E. dispar and E. moshkovskii.3,4
Specific and sensitive means to detect E. histolytica in stool are now available and include antigen detection and the polymerase chain reaction (PCR).5 Two recent studies in developing countries used these modern diagnostic tests. A three-year study in Dhaka, Bangladesh, showed that preschool children had a 2.2 percent frequency of amebic dysentery, as compared with a 5.3 percent frequency of shigella dysentery6 (and unpublished data). The annual incidence of amebic liver abscess was reported to be 21 cases per 100,000 inhabitants in Hue City, Vietnam.7 In the United States, where fecaloral transmission is unusual, amebiasis is most commonly seen in immigrants from and travelers to developing countries. The disease is more severe in the very young and old and in patients receiving corticosteroids.
Molecular phylogeny places entamoeba on one of the lowermost branches of the eukaryotic tree, closest to dictyostelium. Although the organism was originally thought to lack mitochondria, nuclear-encoded mitochondrial genes and a remnant organelle have now been identified.8,9 Unusual features of entamoeba include polyploid chromosomes that vary in length; multiple origins of DNA replication; abundant, repetitive DNA; closely spaced genes that largely lack introns; a novel GAAC element controlling the expression of messenger RNA; and unique endocytic pathways.10,11,12,13
Ingestion of the quadrinucleate cyst of E. histolytica from fecally contaminated food or water initiates infection (Figure 1). This is a daily occurrence among the poor in developing countries and is a threat to inhabitants of developed countries, as the epidemic linked to contaminated municipal water supplies in Tbilisi, Republic of Georgia, demonstrates.14 Excystation in the intestinal lumen produces trophozoites that use the galactose and N-acetyl-D-galactosamine (Gal/GalNAc)specific lectin to adhere to colonic mucins and thereby colonize the large intestine.15 The reproduction of trophozoites has no sexual cycle, and the overall population of E. histolytica appears to be clonal.16 Aggregation of amebae in the mucin layer most likely triggers encystation by means of the Gal/GalNAc-specific lectin.17 Cysts excreted in stool perpetuate the life cycle by further fecaloral spread.
Colitis results when the trophozoite penetrates the intestinal mucous layer, which otherwise acts as a barrier to invasion by inhibiting amebic adherence to the underlying epithelium and by slowing trophozoite motility.18 Invasion is mediated by the killing of epithelial cells, neutrophils, and lymphocytes by trophozoites, which occurs only after the parasite lectin engages host N-acetyl-D-galactosamine on O-linked cell-surface oligosaccharides.15 The interaction of the lectin with glycoconjugates is stereospecific and multivalent.19 The identity of the high-affinity intestinal epithelial-cell receptor is unknown. Secretion by the ameba of amoebapore, a 5-kD pore-forming protein, may contribute to killing.20 Activation of human caspase 3, a distal effector molecule in the apoptotic pathway, occurs rapidly after amebic contact, and caspases are required for cell killing in vitro and for the formation of amebic liver abscesses in vivo.21,22
Interaction of the parasite with the intestinal epithelium causes an inflammatory response marked by the activation of nuclear factor B and the secretion of lymphokines.23,24 The development of this epithelial response may depend on trophozoite virulence factors such as cysteine proteinase and leads to intestinal abnormalities through neutrophil-mediated damage. Neutrophils can also be protective, however, in that activation of neutrophils or macrophages by tumor necrosis factor or interferon kills amebae in vitro and limits the size of amebic liver abscesses.25 In contrast to the intense inflammatory response typical of early invasive amebiasis, inflammation surrounding well-established colonic ulcers and liver abscesses is minimal, given the degree of tissue damage.26
During chronic infection, E. histolytica evades the host immune response in several ways. The Gal/GalNAc-specific lectin has sequence similarity and antigenic cross-reactivity to CD59, a human leukocyte antigen that prevents the assembly of the complement C5bC9 membrane attack complex.27 Amebic cysteine proteinases rapidly degrade the complement anaphylatoxins C3a and C5a.28 The cysteine proteinases also degrade secretory IgA and serum IgG, possibly protecting amebae from opsonization. Finally, amebae appear to suppress both the macrophage respiratory burst and antigen presentation by class II major-histocompatibility-complex (MHC) molecules.
Immunity to infection with E. histolytica is associated with a mucosal IgA response against the carbohydrate-recognition domain of the Gal/GalNAc lectin. Over a one-year period, children with this response had 86 percent fewer new infections than children without this response.6 Cell-mediated responses have been described in patients with amebic liver abscess, characterized by lymphocyte proliferation and lymphokine secretion that is amebi-cidal in vitro.29 One study found that in patients with liver abscess, the prevalence of the class II MHC haplotype HLA-DR3 is increased by a factor of more than three, suggesting a role of CD4+ T-cell function in the outcome of the disease.30 It is noteworthy, however, that the acquired immunodeficiency syndrome pandemic has not led to increases in invasive amebiasis, although asymptomatic intestinal colonization is undoubtedly common.31 In fact, in the murine model of amebic colitis, the depletion of CD4+ T cells decreases the severity of the disease.32
Infection with E. histolytica may be asymptomatic or may cause dysentery or extraintestinal disease (Figure 2 and Figure 3). Asymptomatic infection should be treated because of its potential to progress to invasive disease. Patients with amebic colitis typically present with a several-week history of cramping abdominal pain, weight loss, and watery or bloody diarrhea. The insidious onset and variable signs and symptoms make diagnosis difficult, with fever and grossly bloody stool absent in most cases.33,34,35 The differential diagnosis of a diarrheal illness with occult or grossly bloody stools should include infection with shigella, salmonella, campylobacter, and enteroinvasive and enterohemorrhagic Escherichia coli. Noninfectious causes include inflammatory bowel disease, ischemic colitis, diverticulitis, and arteriovenous malformation.
Unusual manifestations of amebic colitis include acute necrotizing colitis, toxic megacolon, ameboma (Figure 3B), and perianal ulceration with potential formation of a fistula. Acute necrotizing colitis is rare (occurring in less than 0.5 percent of cases) and is associated with a mortality rate of more than 40 percent.35 Patients with acute necrotizing colitis typically appear very ill, with fever, bloody mucoid diarrhea, abdominal pain with rebound tenderness, and signs of peritoneal irritation. Surgical intervention is indicated if there is bowel perforation or if the patient has no response to antiamebic therapy. Toxic megacolon is rare (occurring in approximately 0.5 percent of cases) and is typically associated with the use of corticosteroids. Early recognition and surgical intervention are important, since patients with toxic megacolon usually have no response to antiamebic therapy alone. Ameboma results from the formation of annular colonic granulation tissue at a single site or multiple sites, usually in the cecum or ascending colon. An ameboma may mimic carcinoma of the colon (Figure 3B).
In developing countries, intestinal amebiasis is most commonly diagnosed by identifying cysts or motile trophozoites on a saline wet mount of a stool specimen (Figure 2E, Figure 2F, and Figure 2H). The drawbacks of this method include its low sensitivity and false positive results owing to the presence of E. dispar or E. moshkovskii infection. The diagnosis should ideally be based on the detection in stool of E. histolyticaspecific antigen or DNA and by the presence of antiamebic antibodies in serum (Table 1). Field studies that directly compared PCR with stool culture or antigen-detection tests for the diagnosis of E. histolytica infection suggest that these three methods perform equally well.5,36 An important aid to antigen-detection and PCR-based tests is the detection of serum antibodies against amebae, which are present in 70 to more than 90 percent of patients with symptomatic E. histolytica infection.37,38,39,40 A drawback of current serologic tests is that patients remain positive for years after infection, making it difficult to distinguish new from past infection in regions of the world where the seroprevalence is high. Examination of colonic mucosal biopsy specimens and exudates can reveal a wide range of histopathological findings associated with amebic colitis, including diffuse, nonspecific mucosal thickening with or without ulceration and, in rare cases, the presence of amebae in the mucinous exudate; focal ulcerations (Figure 2B) with or without amebae in a diffusely inflamed mucosal layer; classic flask-shaped lesions (Figure 2C) with ulceration extending through the mucosa and muscularis mucosa into the submucosa; and necrosis and perforation of the intestinal wall.41 Staining with periodic acidSchiff or immunoperoxidase and antilectin antibodies aids in the visualization of amebae.42
Amebic Liver Abscess
Amebic liver abscess is 10 times as common in men as in women and is a rare disease in children.37,43,44 Approximately 80 percent of patients with amebic liver abscess present with symptoms that develop relatively quickly (typically within two to four weeks), including fever, cough, and a constant, dull, aching abdominal pain in the right upper quadrant or epigastrium. Involvement of the diaphragmatic surface of the liver may lead to right-sided pleural pain or referred shoulder pain. Associated gastrointestinal symptoms, which occur in 10 to 35 percent of patients, include nausea, vomiting, abdominal cramping, abdominal distention, diarrhea, and constipation. Hepatomegaly with point tenderness over the liver, below the ribs, or in the intercostal spaces is a typical finding.43,44
Laboratory studies may reveal a mild-to-moderate leukocytosis and anemia. Patients with acute amebic liver abscess tend to have a normal alkaline phosphatase level and an elevated alanine aminotransferase level; the opposite is true of patients with chronic disease.44 Ultrasonography, abdominal computed tomography, and magnetic resonance imaging are all excellent for detecting liver lesions (usually single lesions in the right lobe) but are not specific for amebic liver abscess (Figure 3).
The differential diagnosis of a liver mass should include pyogenic liver abscess, necrotic hepatoma, and echinococcal cyst (usually an incidental finding that is not the cause of fever and abdominal pain). Patients with amebic liver abscess are more likely than patients with pyogenic liver abscess to be male, to be younger than 50 years of age, to have immigrated from or traveled to a country where the disease is endemic, and not to have jaundice, biliary disease, or diabetes mellitus. Less than half of patients with amebic liver abscess have parasites detected in their stool by antigen detection.
Helpful clues to the diagnosis include the presence of epidemiologic risk factors for amebiasis and the presence of serum antiamebic antibodies (present in 70 to 80 percent of patients at the time of presentation) (Table 1). Preliminary studies indicate that the detection of serum amebic antigens is a sensitive, noninvasive means of diagnosis.5 Occasionally, aspiration of the abscess is required to rule out a pyogenic abscess. Amebae are visualized in the abscess fluid in a minority of patients with amebic liver abscess.
Complications of amebic liver abscess may arise from rupture of the abscess with extension into the peritoneum, pleural cavity, or pericardium. Extrahepatic amebic abscesses have occasionally been described in the lung, brain, and skin and presumably result from hematogenous spread.
Therapy for invasive infection differs from therapy for noninvasive infection. Noninvasive infections may be treated with paromomycin. Nitroimidazoles, particularly metronidazole, are the mainstay of therapy for invasive amebiasis45 (Table 2). Nitroimidazoles with longer half-lives (namely, tinidazole, secnidazole, and ornidazole) are better tolerated and allow shorter periods of treatment but are not available in the United States. Approximately 90 percent of patients who present with mild-to-moderate amebic dysentery have a response to nitroimidazole therapy. In the rare case of fulminant amebic colitis, it is prudent to add broad-spectrum antibiotics to treat intestinal bacteria that may spill into the peritoneum; surgical intervention is occasionally required for acute abdomen, gastrointestinal bleeding, or toxic megacolon. Parasites persist in the intestine in as many as 40 to 60 percent of patients who receive nitroimidazole. Therefore, nitroimidazole treatment should be followed with paromomycin or the second-line agent diloxanide furoate to cure luminal infection. Metronidazole and paromomycin should not be given at the same time, since the diarrhea that is a common side effect of paromomycin may make it difficult to assess the patient's response to therapy.46,47,48
Therapeutic aspiration of an amebic liver abscess is occasionally required as an adjunct to antiparasitic therapy. Drainage of the abscess should be considered in patients who have no clinical response to drug therapy within five to seven days or those with a high risk of abscess rupture, as defined by a cavity with a diameter of more than 5 cm or by the presence of lesions in the left lobe.49 Bacterial coinfection of amebic liver abscess has occasionally been observed (both before and as a complication of drainage), and it is reasonable to add antibiotics, drainage, or both to the treatment regimen in the absence of a prompt response to nitroimidazole therapy. Imaging-guided percutaneous treatment (needle aspiration or catheter drainage) has replaced surgical intervention as the procedure of choice for reducing the size of an abscess.49
The Need for a Vaccine
In a perfect world amebiasis would be prevented by eradicating fecal contamination of food and water. However, providing safe food and water for all children in developing countries would require massive societal changes and monetary investments. An effective vaccine would be much less costly, and there are several reasons to indicate that a vaccine is a desirable and feasible goal. The high incidence of amebiasis in recent community-based studies suggests that an effective vaccine would improve child health in developing countries. That humans naturally acquire partial immunity against intestinal infection indicates that there should not be insurmountable barriers to stimulating an effective acquired immune response. Aiding vaccine design is the demonstration that several recombinant antigens, including the Gal/GalNAc-specific lectin, provide protection in animal models of amebiasis and that human immunity is linked to intestinal IgA against the lectin. The clonal-population structure of E. histolytica and, specifically, the high degree of sequence conservation of the Gal/GalNAc-specific lectin suggest that a vaccine could be broadly protective.50 Finally, the absence of epidemiologically significant animal reservoirs suggests that herd immunity could interrupt fecaloral transmission in humans. The challenges will be to design vaccines capable of eliciting durable mucosal immunity, to understand the correlates of acquired immunity, and most important, to enlist the continued support of industrialized nations to combat diarrheal diseases of children in developing countries.
Supported in part by grants from the Howard Hughes Medical Institute (to Drs. Haque and Houston) and from the Burroughs Wellcome Fund, the Lucille P. Markey Trust, and the National Institutes of Health (to Dr. Petri).
Dr. Petri has reported receiving royalties from a patent license agreement with TechLab for a diagnostic test for amebiasis; these royalties accrue to the American Society of Tropical Medicine and Hygiene without benefit to Dr. Petri.
From the International Centre for Diarrhoeal Disease Research, Dhaka, Bangladesh (R.H.); the University of Vermont, Burlington (C.D.H.); and the University of Virginia, Charlottesville (M.H., E.H., W.A.P.).
Address reprint requests to Dr. Petri at the Division of Infectious Diseases and International Health, University of Virginia Health System, Rm. 2115, MR4 Bldg., P.O. Box 801340, Charlottesville, VA 22908-1340, or at email@example.com.