Entamoeba histolytica

Introduction

Entamoeba histolytica is the only amoebic species capable of invading tissues & causing disease.

Epidemiology

• endemic is 80% of the world
  • often seen in travelers from Central America, South America, Africa & Asia
• 2nd leading cause of death from parasitic infections
• AIDS patients not more susceptible to invasive infections
• adult male/female ratio 7-10/1, but equal gender distribution for children & elderly
• infection results from ingestion of cystic protozoa in fecally contaminated soil, food or water
• maturation of cysts into trophozoites occurs in the terminaal ileum, cecum, or colon
• trophozoites penetrate the colonic mucosa & may produce tissue secretory bloody diarrhea & colitis

Pathology

• causes dysentery
• hemagenous spread may lead to abscess from deposition of trophozoites in liver & rarely pulmonary, cardiac or brain tissue
• liver abscesses most common extraintestinal manifestation
  • chocolate-colored fluid or paste
  • acellular debris, necrotic hepatocytes
  • trophozoites seen on microspopy < 20% of cases
    • generally in periphery of abscess
  • eosinophils, with surrounding neutrophils
• galactose binding receptor on amoeba binds to mucin-containing cells
• trophozoites
  • measure 10-60 um
  • commensal forms are generally 15-20 um
  • invasive forms are generally > 20 um
  • progressively motile on wet mounts with hyaline finger-like pseudopods
  • unstained nucleus is not visible
  • with invasive disease, some trophozoites may contain ingested erythrocytes (pathognomonic)
  • both pathogenic & commensal forms may contain ingested bacteria in the cytoplasm
  • stained specimens, peripheral nuclear chromatin is evenly distributed along nuclear membrane
• cysts
  • spherical in shape
  • measure 10-20 um in diameter (generally 12-15 um)
  • precyst has single nucleus without refractile cyst wall
  • mature cyst has 4 nuclei, each 1/6 the diameter of the cyst
  • glycogen is generally diffuse
  • chromatoid bodies: elongated bars

Clinical manifestations

• incubation period 7-21 days
• colitis
• manifestations of liver abscess
  • fever, right upper quadrant pain
  • < 1/3 with bloody diarrhea or history of diarrhea
  • hepatomgaly, anemia & weight loss with chronic infection
• 90% of infections are asymptomatic, including those with liver absecess
• in different parts of the world, amoeba have predilection for different organs, South Africa liver, Egypt intestine

Laboratory

• Entamoeba histolytica serology
  • IgA, IgM, IgG response; no IgE response
• liver function tests
  • elevated serum ALP in 80%
  • serum ALT may also be elevated
• complete blood count (CBC) may show leukocytosis
  • no eosinophilia
• urinalysis may show proteinuria
• stool for ova & parasites
  • Entamoeba histolytica identified in stool
  • direct microscopy identifies trichrome-stained cysts < 18% of cases
• fine needle aspiration of abscess
  • failure of response to therapy (3-5 days)
  • risks include amoebic peritonitis, inadvertant puncture of echinococcal cyst
  • PCR of aspirate may be useful
• Entamoeba histolytica antigen
• Entamoeba histolytica DNA
  • Entamoeba histolytica+E dispar+E ecuadoriensis+E nuttalli DNA
• Entamoeba histolytica 18S rRNA

* image of cyst

Diagnostic procedures

• flexible sigmoidoscopy
  • diffuse colitis
  • intestinal biopsy
    • flask-shaped intestinal ulcers

Radiology

• chest X-ray may be abnormal
• imaging of liver is primary diagnostic tool
  • computed tomography
  • ultrasound
  • magnetic resonance imaging
  • single abscess in right lobe of liver (70-80%)
• radiologic resolution of abscess may take 2 years
• gallium scan may distinguish amoebic (cold)* vs pyogenicb(hot) abscess

* rim of amoebic abscess may be bright (surrounding neutrophils)

Differential diagnosis

• Cyclospora cayetanensis
  • nausea/vomiting, explosive watery diarrhea
  • recovery of oocysts in feces
• Giardia lamblia
  • explosive watery diarrhea, steatorrhea, foul smelling stool
  • associated with contaminated fresh water, campinng ...
• Cryptosporidium
  • self-limited watery diarrhea, except HIV1 & other immunocompromised hosts
  • public water supply, recreational water facilities
• Isospora
  • watery diarrhea
  • self-limited except HIV1 & other immunocompromised hosts
• Microsporidia
  • immunocompromised hosts

Management

• asymptomatic carriers (lumenal agents)
  • iodoquinol 650 mg PO TID for 20 days
  • diloxanide furoate 500 mg PO TID for 10 days
  • paromomycin 500 mg PO TID
• acute colitis &/or liver abscess
  • metronidazole 750 mg PO or IV TID for 5-10 days, plus ceftriaxone
  • metronidazole plus lumenal agent
  • lumenal agents (see above)
• other agents
  • tinidazole 2 g PO
  • nitazoxanide (Alinia) 500 PO BID with food for 3 days
  • ornidazole 2 g PO ( not available in USA)
• aspiration & percutaneous drainage of liver abscesses may be indicated
• corticosteroids are contraindicated
  • may result in toxic megacolon & death

More general terms

• Entamoeba

Additional terms

• amebiasis
• trophozoite