Entamoeba histolytica

Amoeba infection (Entamoeba histolytica)

• Amoebiasis caused by Entamoeba histolytica has world wide distribution with more prevalence in tropical countries

• Etiopathogenesis

  • Entamoeba histolytica has two forms

    • Trophozoite (motile form measuring 12 to 60μm)

    • Cyst (immotile form measuring 10 to 20μm)

  • Cysts are infectious forms which survive in the environment for weeks to months

  • Mode of transmission – direct fecal-oral route

  • Ingested cysts resists gastric digestion and pass to the ileocecal region

  • In the ileocecal region, they excyst and divide, giving rise to trophozoites that than colonize the colon

  • Trophozoites produce enzymes (like glycosidase, galactosidase, mannosidase, fucosidase,xylosidase, glucosidase, amylase and hyaluronidase) which would degrade the mucosa and help the organisms to  attach to the epithelial cells by adherence lectin

  • This also protects the organism from the attack of complement complex and is also involved in signaling cell lysis

  • Proteases causes the lysis of epithelial cells and degrade the extracellular matrix, helping the organism to invade the tissue

  • Once the organism enters the tissue, it interacts with neutrophils and macrophages

  • It inhibits the migration of macrophages by producing macrophage locomotion – inhibitory factor

• Factors which affect severity of disease are

  • poor nutrition

  • Tropical climate

  • Decreased host immunocompetence

  • Integrity of mucosal barrier

  • Altered colonic bacterial flora

  • Trauma

  • Different zymodemes (enzyme patterns)

  • genetic differences in a cysteine proteases that serves as virulence factor

• Clinical features

  • Age – all ages including infants are affected

  • Many patients may remain asymptomatic

  • Mild cases of amebiasis present with mild diarrhea, cramps or abdominal discomfort often with mucus and blood in stools

  • Intestinal amebiasis presents in 4 clinically recognized forms:

    • Typical dysenteric infection with intermittent bloody diarrhea and lower abdominal cramps

    • As fulminant amebic colitis

    • Amebic appendicitis

    • Localized mass lesion in the colon, the ameboma

• Dysenteric infection, initially starts with loose stools, malodorous flatus and recurrent bouts of diarrhea. Intermittent constipation can also be seen

• With in 3 to 4 days, number of stools may increase to 25 per day

• Patient may develop severe dehydration, abdominal pain and hypotension

• Generalized peritonitis, toxic megacolon or rectal prolapse may develop

• Amebomas –

  • Present as palpable mass

  • Causes numerous symptoms including alternating diarrhea and constipation, weight loss and low-grade fever

• Gross findings

  • Ulcerations are more often seen in cecum followed by the sigmoid colon, descending colon and rectum

  • Developing ulcers which have irregular, hyperemic mucosal outlines with markedly undermined, overhanging edges, producing a characteristic flask like shape

  • Intervening mucosa is normal

  • Ulcers may vary in size from few mm to several centimeters

  • Amebomas present as localized, often circumferential areas of bowel wall thickening, strictures, mucosal excrescences or large tumor like masses

  • They result from persistent ulceration and granulation tissue formation with fibroblastic proliferation and inflammation and may be up to 15cms in diameter

• Microscopic findings

  • Specific diagnostic finding is identification of the organism in either the trophozoites or cyst formation

  • Trophozoites are large round to ovoid structures, varying from 6μm to 40μm

  • They contain voluminous pinkish purple cytoplasm with distinctive foamy, vacuolated or granular appearance

  • Cytoplasm is PAS positive

  • Amoebae contain ingested RBC’s which can be demonstrated by Heidenhain iron hematoxylin stain

  • In tissue sections amoebae may be surrounded by clear space, a fixation induced shrinkage artefact

  • Amoebic ulcers are flask shaped with necrotic debris and fibrin covering base of the ulcers.

  • Amoebae lie in the lamina propria surrounded by inflammatory response. Some times organisms are found in tissue space and in small vessels. In severe cases they are found in abdominal cavity and serosal fat

  • Patient with long standing infections and exuberant tissue reactions develop tumorous exophytic cicatrical, inflammatory masses known as amoebomas. Histologically amoebomas consist of granulation tissue with round cell infiltration and giant cells

• Investigation

  • Stool examination

  • Special stains used to identify trophozoites are 

    • PAS

    • H&E

    • Trichrome 

    • Phosphotungstic acid hematoxylin

  • Other tests

    • Serological tests

    • DNA hybridization

• Treatment and prognosis

  • Trophozoites may migrate to liver producing amebic liver abscess

  • They may also invade adjacent structures, including lung, peritoneum and pericardium

  • Metronidazole along with diiodohydroxyquin or paromomycin