Entamoeba histolytica (Amebiasis)

Description of the Pathogen

E. histolytica is a pseudopod-forming, nonflagellated protozoan parasite. Historically, it was considered the only invasive species of parasites in the Entamoeba genus infecting humans (including E. histolytica, E. dispar, E. moshkovskii, E. bangladeshi, E. coli, E. hartmanni, E. polecki, and E. gingivalis; see Chapter 264 ). However, E. moshkovskii and the newly discovered E. bangladeshi have been recently associated with diarrhea in children. Morphologically, E. histolytica is indistinguishable from E. dispar, E. moshkovskii, and E. bangladeshi . The genome was first sequenced in 2005. ^,

The life cycle of E. histolytica consists of an infectious cyst and an invasive trophozoite ( Fig. 263.1 ). The infectious cyst is 10–20 μm in diameter, contains four nuclei, and is surrounded by a refractile wall ( Fig. 263.2A ). Trophozoites are 10–60 μm in diameter, have one nucleus with a central karyosome, and possess progressive motility ( Fig. 263.2B ). The quadrinucleate cyst is highly resilient and can survive in the environment for weeks to months. E. histolytica infection occurs through fecal-oral transmission when cysts are ingested in contaminated food or water or through direct person-to-person contact. Excystation occurs in the small or large bowel, giving rise to eight daughter trophozoites. Trophozoites can colonize or invade the large bowel, but cysts are never found in tissue. About 10% of colonized individuals develop invasive disease characterized by flask-shaped colonic ulcers due to disruption of the intestinal mucosal barrier by the trophozoite. Trophozoites also can enter the bloodstream through the portal venous system and spread to other organs. Interaction of the amebic strain with bacterial flora as well as host factors (e.g., malnutrition, diet, sex, age, immune status) may influence whether infection results in colonization or invasive disease. Trophozoites reproduce by binary fission and encyst within the colon, excrete cysts into the environment in the stool, and thus perpetuate the life cycle.

Pathogenesis

Amebic invasion of the colonic mucosa proceeds through the contact-dependent process of adherence, cytolysis, and phagocytosis/trogocytosis, thus resulting in the tissue damage for which E. histolytica is named. ^,

First, trophozoites adhere to colonic mucins and epithelial cells through binding of an amebic lectin, galactose/ N -acetyl- d -galactosamine (Gal/GalNAc) to host glycoconjugates. Then, secreted proteolytic enzymes disrupt the intestinal barrier and facilitate tissue penetration by trophozoites. Trophozoites kill host epithelial and immune cells, resulting in tissue destruction.

Target cell killing is contact-dependent and mediated by the amebic Gal/GalNAc lectin. ^, The heavy subunit of lectin is encoded by a multigene family, is highly expressed in trophozoites, and is highly conserved in E. histolytica isolates worldwide, thus having important implications for vaccine development. ^, Lectin has additional functions in addition to adherence, including modulation of the immune response, such as by inhibiting assembly of the membrane attack complex of human complement on the surface of the ameba. ^,

The mechanism of cytolysis has been the subject of intense investigation. In lymphocytes killed in vitro by E. histolytica, compaction of nuclear chromatin, cytoplasmic condensation, and membrane blebbing occur, suggesting an apoptotic mechanism of cell killing. Furthermore, activation of host cell caspases occurs within minutes of contact by trophozoites, and caspase 3 activation specifically by E. histolytica leads to activation of apoptosis. Recently, it has been demonstrated that E. histolytica ingests distinct pieces of host cells, resulting in eventual cell death through a process called trogocytosis (from the Greek trogo, which means “to nibble”).

A number of other virulence factors, including production of proteases and secretion of a macrophage migration inhibitory factor protease, contribute to pathogenesis, tissue invasion, and immune evasion by E. histolytica . After invasion of the intestine, the organism can spread through the bloodstream to the liver.

Immunity

Epidemiology

The greatest morbidity and mortality due to E. histolytica occurs in resource-poor nations, where sanitary conditions are poor, particularly in the Indian subcontinent, Africa, and Central and South America, leading some to propose that amebiasis should be considered a neglected tropical disease. E. histolytica is considered a category B (second highest) priority emerging infectious disease/pathogen by the US National Institute of Allergy and Infectious Diseases.

E. histolytica and E. dispar infect about 500 million people worldwide. Fortunately, most people previously believed to be infected with E. histolytica carry E. dispar, which does not cause human disease. With the discovery and new understanding of E. bangladeshi and E. moshkovskii, there is an expanding spectrum of Entamoeba species that can be pathogenic in humans. The best estimate is that approximately 40–50 million symptomatic E. histolytica infections occur worldwide annually. Estimates are limited by the inability to distinguish between E. histolytica and its related species before the advent of more specific diagnostic tests.

Prevalence estimates vary widely depending on diagnostic methodology and population studied. In a representative sample of the general population of Mexico, overall seroprevalence was 8.4% and was higher in school-age children. In Bangladesh, seroprevalence in children is about 50% by age 5 years. In Fortaleza, Brazil, 10% of the sample population was positive for E. histolytica by stool antigen testing.

The Global Burden of Disease Study estimated that 1300 deaths of children aged <5 years, and 11,300 deaths of all ages, were attributable to amoebiasis in 2013, compared with 55,000 annual deaths of all ages in 2010. ^, Overall, the burden of diarrheal diseases appears to be decreasing over time but remains a significant cause of morbidity and mortality.

In the US, amebiasis is the third most common cause of parasitic infections, with the prevalence of E. histolytica estimated at 4%. Approximately 2900 cases of amebiasis annually were reported to the Centers for Disease Control and Prevention (CDC) in 1993 and 1994, mostly in adults. ^, In 1994 19% of cases were in individuals aged <19 years. Since 1994, amebiasis is no longer a US nationally notifiable disease. From 1990–2007, 134 deaths in the US were attributed to amebiasis, 9 of which were in children aged <14 years, with a declining trend over time.

GeoSentinel data point to amebiasis as a significant cause of GI illness in returning travelers. It is the third most frequently isolated pathogen (after Giardia and Campylobacter ) in returning travelers diagnosed with infectious GI disease, with an estimated rate of 14 per 1000 returned travelers.

Most individuals with amebiasis are infected by ingestion of food or water contaminated with feces containing cysts, but other modes of transmission, including sexual and nosocomial (through direct inoculation associated with contaminated devices), have been reported. ^, Groups at increased risk in resource-rich nations include immigrants from endemic areas, long-term visitors to endemic areas, people in group homes and institutions for the mentally disabled, men who have sex with men, and people with HIV infection. Over the past few decades, there has been an increase in sexually transmitted E. histolytica in Australia and East Asian resource-rich nations such as Japan, Taiwan, and Korea. In Japan, nearly 80% of amebic cases diagnosed are in men who have sex with men, and positive serology for E. histolytica was seen in 21% of roughly 1300 patients with HIV infection.

Disease is more severe in elderly, young, or malnourished individuals, individuals on immunosuppression, and pregnant women. The relationship between malnutrition and morbidity associated with diarrhea, as well as effects on development, is well documented.

Asymptomatic amebiasis appear to occur equally in males and females. Overall, invasive disease appears to occur more frequently in male adults but not children. Intestinal amebiasis occurs roughly equally in males and females. Amebic liver abscess occurs more frequently in male adults, but not children, with some studies estimating a 10-fold higher risk in men. Recently, a molecular mechanism to explain this discrepancy was found: in a mouse model, higher testosterone levels resulted in lower IFNγ produced from natural killer T cells and thus larger liver abscesses.

Clinical Manifestations

Infection with E. histolytica ranges from asymptomatic carriage to invasive disease involving the GI tract or dissemination causing amebic liver abscess and other complications. The majority of E. histolytica infections do not result in clinical disease; in fact, only an estimated 1 in 4 infections progress to the development of symptoms. This discrepancy is due to both pathogen factors like genotype and host factors.