Amebiasis: Entamoeba Histolytica
Entamoeba histolytica is a protozoan infection of humans that is found worldwide and is globally responsible for up to 100,000 deaths annually. It is endemic in most temperate and tropical areas of the world, particularly in areas with poor socioeconomic development and limited sanitation. Serologic evidence of prior or current infection with E. histolytica is present in 5% to 50% of individuals in impoverished populations. E. histolytica is infectious in the cyst form. Transmission usually occurs as a result of contamination of food or water, but may also occur by means of oral–anal contact. Infection is common in developing countries. In Europe and the United States, infection is most commonly seen in individuals who have lived in endemic areas of the world. Institutionalized individuals are also at increased risk of infection. Sexually active men who have sex with men are also at increased risk of infection. There does not appear to be an increased risk of invasive disease in persons with HIV infection.
A morphologically identical, but nonpathogenic protozoan, Entamoeba dispar, also infects the human gastrointestinal tract. It can be distinguished from E. histolytica by antigenic, genetic, and immunologic methods. Infection is 10 times more prevalent with E. dispar than with E. histolytica; however, the former does not cause invasive disease. Many older epidemiologic studies relied on microscopic diagnosis alone, and inadvertently incorporated both pathogenic and nonpathogenic species in estimates of prevalence of infection.
E. histolytica has a simple life cycle involving an infectious cyst and an ameboid trophozoite phase. Cysts may survive in the external environment for several weeks to months, especially in damp conditions and temperatures between −5°C and 40°C. After ingestion by humans, cysts excyst in the small intestine, each forming eight daughter trophozoites. These motile trophozoites can adhere to the intestinal wall; it is in this form that they may invade the mucosa, causing symptomatic invasive disease. In the colon, trophozoites encyst to complete the life cycle and are excreted. Trophozoites do not survive outside the human host.
The incubation period for intestinal amebiasis is usually 1 to 4 weeks, but ranges from a few days to months. Infection with E. histolytica is asymptomatic in up to 90% of cases, but may cause a range of gastrointestinal symptoms from mild diarrhea to severe colitis with bloody diarrhea. Symptoms result from penetration of the trophozoites through the mucosal barrier with invasion of the colon wall. In the absence of frank blood, stool is usually positive for occult blood. Fever is present in less than half of cases. Fecal leukocytosis is usually less marked than in bacterial colitis. Rarely, progression of invasive intestinal disease may lead to severe colitis and bowel perforation. On occasion, granulation tissue formation surrounding a localized area of invasive disease may form an inflammatory mass (an “ameboma”) and manifest as a tender abdominal mass.