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Infectious Disease and Microbiology – Amebiasis
Amebiasis is a protozoal infection caused by Entamoeba histolytica. Infection may result in asymptomatic colonization, diarrhea, colitis, or invasive extra-intestinal disease, most commonly liver abscess. Although approximately 10% of the global population is estimated to be infected, symptomatic disease develops in fewer than 10% of infected individuals, and only a small proportion of those with intestinal infection progress to invasive disease.
Prevalence varies geographically, ranging from less than 5% in developed countries to 20–30% in tropical regions. In the United States, prevalence is estimated at under 4%. Infection occurs at all ages and affects both sexes equally. In endemic areas, risk factors include low socioeconomic status, poor sanitation, and overcrowding. In low-prevalence regions, infection is more common among immigrants and travelers from endemic areas, institutionalized individuals, and men who have sex with men. Severe disease is more likely in neonates, pregnant women, malnourished individuals, and those receiving corticosteroid therapy.
Humans are the only reservoir. Transmission occurs via the fecal–oral route, typically through ingestion of contaminated water or food. Chlorination does not reliably destroy cysts; boiling water is required for effective decontamination. Proper washing of vegetables with potable water or treatment with detergent and vinegar reduces risk. The organism exists in two forms: the trophozoite, which may contain ingested erythrocytes, and the quadrinucleate cyst, which is the infective form. After ingestion, excystation occurs in the small intestine, releasing trophozoites that colonize the colon. Most infected individuals remain asymptomatic cyst passers. In invasive disease, trophozoites penetrate the colonic mucosa, producing characteristic flask-shaped ulcers. Cysts can survive for months in moist environments.
Several Entamoeba species infect humans, but E. histolytica is the primary pathogenic species. Morphologically similar nonpathogenic species include Entamoeba dispar, Entamoeba moshkovskii, Entamoeba hartmanni, and Entamoeba coli.
Clinical manifestations depend on the extent of invasion. Symptoms of intestinal disease usually develop within four weeks of cyst ingestion. Asymptomatic infection is common. Noninvasive disease presents with mild diarrhea. Amebic colitis or dysentery causes crampy abdominal pain, bloody and mucoid diarrhea, rectal bleeding (especially in children), fever in about one-third of patients, weight loss, and anorexia. Extra-intestinal disease most frequently involves the liver, with abscess formation developing approximately three months after infection. Patients present with fever and right upper quadrant pain, and half may not recall prior colitis. Abscess rupture can cause peritonitis, empyema, or pleural involvement. Cerebral amebiasis is rare but presents with headache, nausea, vomiting, and altered mental status.
On examination, colitis may produce diffuse abdominal tenderness, distention, and signs of peritonitis in severe cases. Liver abscess is associated with right upper quadrant tenderness, hepatomegaly, and rarely jaundice.
Diagnosis relies on stool microscopy, antigen detection, serology, imaging, and sometimes biopsy. Stool ova and parasite examination may reveal trophozoites with ingested red blood cells. Multiple stool samples increase sensitivity. Fecal antigen detection by ELISA is more sensitive than microscopy but less sensitive than PCR. Serology is useful in invasive disease and liver abscess, though antibodies may persist for years and are less helpful in endemic regions. Leukocytosis without eosinophilia is common in invasive disease, and liver abscess may show elevated alkaline phosphatase and mild transaminase elevations. Imaging with ultrasound, CT, or MRI typically demonstrates a right-lobe liver abscess. Colonoscopy may reveal friable mucosa and flask-shaped ulcers. Aspiration of a liver abscess yields characteristic brown, odorless “anchovy paste” material, although organisms are often not recovered from the fluid itself.
Differential diagnosis includes ulcerative colitis, Crohn’s disease, colorectal carcinoma, diverticulitis, abdominal abscess, pyogenic liver abscess, hepatoma, echinococcal cyst, and irritable bowel syndrome.
Treatment depends on clinical presentation. Asymptomatic luminal infection should be treated to prevent invasive disease, typically with paromomycin for seven days, or alternatives such as diloxanide furoate or iodoquinol. Amebic colitis requires metronidazole or tinidazole followed by a luminal agent such as paromomycin or iodoquinol to eradicate cysts. Amebic liver abscess is treated with metronidazole followed by a luminal agent. Large abscesses greater than 3 cm may require needle aspiration or drainage, whereas smaller abscesses usually respond to medical therapy alone.
Follow-up imaging is recommended after treatment of liver abscess to confirm resolution, which may take several months. Amebiasis carries significant morbidity and mortality in developing countries. Complications include fulminant colitis with toxic megacolon, perforation, peritonitis, formation of amebomas that mimic colon cancer, bowel obstruction, and rupture of liver abscess into pleural or pericardial spaces.
Amebiasis is a protozoal infection caused by Entamoeba histolytica. Infection may result in asymptomatic colonization, diarrhea, colitis, or invasive extra-intestinal disease, most commonly liver abscess. Although approximately 10% of the global population is estimated to be infected, symptomatic disease develops in fewer than 10% of infected individuals, and only a small proportion of those with intestinal infection progress to invasive disease.
Prevalence varies geographically, ranging from less than 5% in developed countries to 20–30% in tropical regions. In the United States, prevalence is estimated at under 4%. Infection occurs at all ages and affects both sexes equally. In endemic areas, risk factors include low socioeconomic status, poor sanitation, and overcrowding. In low-prevalence regions, infection is more common among immigrants and travelers from endemic areas, institutionalized individuals, and men who have sex with men. Severe disease is more likely in neonates, pregnant women, malnourished individuals, and those receiving corticosteroid therapy.
Humans are the only reservoir. Transmission occurs via the fecal–oral route, typically through ingestion of contaminated water or food. Chlorination does not reliably destroy cysts; boiling water is required for effective decontamination. Proper washing of vegetables with potable water or treatment with detergent and vinegar reduces risk. The organism exists in two forms: the trophozoite, which may contain ingested erythrocytes, and the quadrinucleate cyst, which is the infective form. After ingestion, excystation occurs in the small intestine, releasing trophozoites that colonize the colon. Most infected individuals remain asymptomatic cyst passers. In invasive disease, trophozoites penetrate the colonic mucosa, producing characteristic flask-shaped ulcers. Cysts can survive for months in moist environments.
Several Entamoeba species infect humans, but E. histolytica is the primary pathogenic species. Morphologically similar nonpathogenic species include Entamoeba dispar, Entamoeba moshkovskii, Entamoeba hartmanni, and Entamoeba coli.
Clinical manifestations depend on the extent of invasion. Symptoms of intestinal disease usually develop within four weeks of cyst ingestion. Asymptomatic infection is common. Noninvasive disease presents with mild diarrhea. Amebic colitis or dysentery causes crampy abdominal pain, bloody and mucoid diarrhea, rectal bleeding (especially in children), fever in about one-third of patients, weight loss, and anorexia. Extra-intestinal disease most frequently involves the liver, with abscess formation developing approximately three months after infection. Patients present with fever and right upper quadrant pain, and half may not recall prior colitis. Abscess rupture can cause peritonitis, empyema, or pleural involvement. Cerebral amebiasis is rare but presents with headache, nausea, vomiting, and altered mental status.
On examination, colitis may produce diffuse abdominal tenderness, distention, and signs of peritonitis in severe cases. Liver abscess is associated with right upper quadrant tenderness, hepatomegaly, and rarely jaundice.
Diagnosis relies on stool microscopy, antigen detection, serology, imaging, and sometimes biopsy. Stool ova and parasite examination may reveal trophozoites with ingested red blood cells. Multiple stool samples increase sensitivity. Fecal antigen detection by ELISA is more sensitive than microscopy but less sensitive than PCR. Serology is useful in invasive disease and liver abscess, though antibodies may persist for years and are less helpful in endemic regions. Leukocytosis without eosinophilia is common in invasive disease, and liver abscess may show elevated alkaline phosphatase and mild transaminase elevations. Imaging with ultrasound, CT, or MRI typically demonstrates a right-lobe liver abscess. Colonoscopy may reveal friable mucosa and flask-shaped ulcers. Aspiration of a liver abscess yields characteristic brown, odorless “anchovy paste” material, although organisms are often not recovered from the fluid itself.
Differential diagnosis includes ulcerative colitis, Crohn’s disease, colorectal carcinoma, diverticulitis, abdominal abscess, pyogenic liver abscess, hepatoma, echinococcal cyst, and irritable bowel syndrome.
Treatment depends on clinical presentation. Asymptomatic luminal infection should be treated to prevent invasive disease, typically with paromomycin for seven days, or alternatives such as diloxanide furoate or iodoquinol. Amebic colitis requires metronidazole or tinidazole followed by a luminal agent such as paromomycin or iodoquinol to eradicate cysts. Amebic liver abscess is treated with metronidazole followed by a luminal agent. Large abscesses greater than 3 cm may require needle aspiration or drainage, whereas smaller abscesses usually respond to medical therapy alone.
Follow-up imaging is recommended after treatment of liver abscess to confirm resolution, which may take several months. Amebiasis carries significant morbidity and mortality in developing countries. Complications include fulminant colitis with toxic megacolon, perforation, peritonitis, formation of amebomas that mimic colon cancer, bowel obstruction, and rupture of liver abscess into pleural or pericardial spaces.
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