A 56 year-old man was referred for colonoscopy after experiencing three months of watery diarrhea and crampy abdominal pain. He denied fevers or chills, hematochezia, recent travel, sick contacts, or recently taking antibiotics. His primary care physician referred him for colonoscopy after he was found to have a leukocytosis of 15,200 with 70% neutrophils and 5.4% eosinophils as well as negative bacterial stool culture and C. diff toxin.
The ulcerations encountered on colonoscopy are characteristic for invasive Entamoeba histolytica infection. There were numerous discrete 3-4mm clean based well demarcated ulcerations with slightly heaped up edges found scattered throughout the colon from cecum to rectum. Other non-specific findings include mucosal thickening and erythema. Friable ulcerations can also be found. Amebic colitis may increase the likelihood of perforation during colonoscopy.
Pathologic examination of biopsies taken from the ulcerations reveal acute inflammation. On low power, one can occasionally find “flask-shaped” ulcerations on histologic cross section. In these H&E slides, there is a large burden of Entamoba histolytica organisms at the luminal surface. Many of these trophozoites contain phagocytosed red blood cells, which is an important distinguishing feature from strains such as Entamoeba dispar, which is a noninvasive parasite, or Entamoeba moshkovskii, which can be found in humans but has not been associated with disease.
E. histolytica is a protozoan parasite that causes 40-50 million patients to have colitis or extra-intestinal disease annually and is responsible for roughly 40,000 deaths annually worldwide. This infection is most commonly found in tropical climates or areas with poor sanitation. Africa, India, Mexico and parts of Central and South America have the highest number of patients with amebic colitis. Risk factors for amebic colitis in the United States of America include travel to the previously mentioned countries, immigrants, refugees, men who have sex with men, and contact with institutions that have poor sanitary conditions.
Amebic cysts remain viable for weeks to months in the environment and are ingested during consumption of fecally contaminated food or water or during oral-anal sexual contact. Excystation involves the formation of eight trophozoites from each cyst and occurs in the small intestine. These trophozoites then migrate to the colon where they can adhere to galactose/N-acetylgalactosamine residues on colonic epithelial cells.
Approximately 90% of patients have a self-limited asymptomatic infection while roughly 10% of patients experience invasive disease (amebic colitis) caused by tissue destruction by the trophozoites once they have penetrated through the intestinal mucous layer. Less than 1% of patients experience extra-intestinal disease such as amebic liver abscess, brain abscess or rarely pericardial or pleural involvement.
Diagnosis of E. histolytica can be made by direct stool examination but requires a fresh specimen and an experienced technician or clinician. Multiple samples are often required to make the diagnosis. Stool antigen (multiple modalities) and serology (indirect hemagglutination) are more sensitive and specific for E. histolytica, however the latter may represent previous infection which can be particularly problematic in populations where there exists a higher prevalence of amebic colitis.
All E. histolytica infections should be treated. The goal of treatment of amebic colitis is to eliminate trophozoites and eradicate intestinal carriage of the organism. Various treatment regimens have been approved and should include either metronidazole, tinidazole, orindazole, or nitazoxanide for trophozoite elimination. The second part of treatment is with either paromomycin, iodoquinol, or diloxanide furoate for luminal cyst elimination. Our patient was treated with metronidazole for trophozoite elimination along with iodoquinol for eliminating luminal cysts. Of note, no regimen is completely effective and follow-up stool examinations should be performed.
Our patient was asymptomatic after treatment with this regimen. Soon after he began treatment, his wife began experiencing bloody diarrhea and eventually was diagnosed with amebic colitis. Upon further history, she recalled requiring frequent treatments for amebic colitis during her childhood in her hometown of El Paso, Texas, near the Mexican border. She endorsed recent travel to El Paso and also had engaged in oral-anal sex with her husband. She too was treated with metronidazole and iodoquinol and was also symptom free one month later.
Haque et al. “Amebiasis” New England Journal of Medicine. April 17, 2003, 348 (16): 1565.
Leder et al. Intestinal Entamoeba histolytica amebiasis UpToDate 2010 website accessed July 28, 2010
Suresh Pola, MD, University of California San Diego, Veterans Affairs San Diego Medical Center