Entamoeba hystolytica

• Entamoeba histolytica


  Only pathogenetic Entamoeba

  
• Entamoeba histolytica


  • Morphologically indistinguishable with the non-pathogenic Entamoeba dispar and Entamoeba moshkovskii
  • Differentiated through isoenzyme analysis, polymerase chain reaction (PCR), Restriction Fragment Length Polymorphism (RFLP), and typing with monoclonal antibodies
• Davao Doctor Hospital was the first laboratory to offer the differentiation of E. dispar and E. histolytica through serologic testing
• Finding E. dispar and E. moshkovskii still warrants treatment even without pathogenicity to humans
• Host


  • Humans
  • Non-human primates
• Infective stage


  Quadrinucleated cyst
• Diagnostic stage


  • Trophozoite
  • Cyst
• Transmission


  • Ingestion
  • Sexual
  • Direct colonic inoculation through contaminated enema equipment
• The last mode of transmission (direct colonic inoculation through contaminated enema equipment) is now rare because most enema equipment are disposable - no longer reused
• Site of Excystation


  Small intestine
• Site of Encystation


  Large intestine
• Habitat


  Large intestine
• E. histolytica is known to exhibit erratic parasitism

  
• World Prevalence


  • Before PCR-RFLP: 10%
  • After PCR-RFLP: 1 - 5%
• 50 million cases/year
• 40,000 - 100,000 deaths/year
• Entamoeba histolytica is the third most important parasitic disease (after malaria and schistosomiasis)

  
• Entamoeba histolytica is the second top cause of mortality among parasitic protozoans (after malaria)

  
• Factors affecting the prevalence


  • Level of sanitation
  • Crowding
  • Socio-economic status
  • Cultural habits
  • Age
• Cyst


  • Measuring 10 - 20 um
  • Highly refractile cyst wall
  • 1 - 4 nuclei
  • Infective stage: Quadrinucleated cyst
  • Resistant to gastric acidity and desiccation
  • Can survive in moist environment for several weeks
  • Rod-shaped (or cigar-shaped) chromatoidal bars with rounded ends
• Trophozoite


  • Measuring 12 - 60 um, oval
  • Highly motile: Progressive and directional movement
  • Pseudopod as locomotory apparatus
  • Invasive form causing colitis and liver abscess
  • With ingested red blood cells seen in the cytoplasm of the amoeba as pale greenish, refractile bodies in unstained amoeba (nucleus not visible)
  • Multiply by binary fission

  
• Life Cycle of Amoeba

  1. Infective stage is the quadrinucleated cyst. Cyst can survive the gastric acid, travels to the small intestine where it excysts. It then travels to the large intestine and establishes itself their to become trophozoite, or multiply (some encyst), and both stages exit the host in the fecal material. Passed cyst may be uninucleated to trinucleated. Trophozoites die in the external environment while the cyst resist desiccation and harsh environment and can stay in the environment for several weeks before it is ingested by a definitive host

  2. Aberrant infection of the lungs and brain is possible

  
• Steps involved in Amoebic Killing of Host Cells

  1. Receptor-mediated adherence of amoebae to target cells

  2. Amoebic cytolysis of target cells

  3. Amoebic phagocytosis of killed target cells
• Receptor-mediated adherence of amoebae to target cells


  Mediated by amoebal galactose-inhibitable adherence lectin
• Amoebic cytolysis of target cells

  • Pore-forming protein amoebapore, lipases, cysteine proteases
  • Proteases and lipases are responsible for the breakdown of intestinal mucosa leading to amebic ulcer
  • Proteases and lipases are not enough to destroy the muscle layer of the intestine leading to intestinal perforation. Thus, instead of burrowing through the wall, it invades surrounding mucosa, causing the characteristic flask-shaped ulcer
  • However, heavy infection is capable of perforating the intestine. Commensal bacteria are released into the peritoneum causing peritonitis
• Amoebic cytolysis of target cells


  Activation of caspase-3 resulting to apoptosis
• The epithelium is avascular, but in the submucosa, it is well-vascularized. Invasion of the protozoa may reach the blood vessel and enter the circulation, causing abberant amebic infections

  
• The right lobe of the liver is common to amebic liver abscess due to the liver venous drainage
• Infection on the left lobe of the liver may cause infection of the heart due to their proximity
• Infection on the upper part of the liver may infect the diaphragm → pleural cavity → lungs
• Amoebic colitis


  • Gradual onset of abdominal pain and diarrhea with or without blood and mucus in the stool
  • Fever in 1/3 of patients
  • Some has diarrhea alternating with constipation
  • Dehydration rarely manifest especially when patients are well hydrated and nourished
  • Fulminant case: Severe bloody diarrhea, fever, and abdominal pain
  • Biopsy of the erythematous area with ulcerations reveal trophozoites
  • 60% of fulminant cases may result to serious complications like intestinal perforation and secondary bacterial peritonitis

  
• Bacillary dysentery


  • May be epidemic
  • Acute onset
  • Prodromal fever and malaise common
  • Vomiting common
  • Patient prostrate
  • Watery, bloody diarrhea
  • Odorless stool
  • Stool microscopy: Numerous bacilli, pus cells, macrophages, red cells, no Charcot-Leyden crystals
  • Abdominal cramps common and severe
  • Tenesmus common
  • Spontaneous recovery in a few days, weeks or more; no relapse

  
• Amebic dysentery


  • Seldom epidemic
  • Gradual onset
  • No prodromal features
  • No vomiting
  • Patient usually ambulant
  • Bloody diarrhea
  • Fishy odor stool
  • Stool microscopy: Few bacilli, red cells, trophozoites with ingested red blood cells, Charcot-Leyden crystals
  • Mild abdominal cramps
  • Tenesmus uncommon
  • Lasts for weeks; dysentery returns after remission; infection persists for years

  
• Amoeboma


  • Chronic granulomatous lesion, presenting as mass-like lesion with abdominal pain and history of dysentery
  • Mistaken as cancer
  • Appears as colon cancer in colonoscopy
  • Biopsy reveal trophozoites in granulomatous lesion
  • Treat with metronidazole

  
• Amoebic Liver Abscess


  • Most common extra-intestinal amoebiasis
  • Acute (<2 weeks): Fever, right upper quadrant pain
  • Chronic (>2 weeks): Significant weight loss
  • Anchovy sauce-like content of abscess
  • Right lobe more affected than left lobe
  • Serious complication is rupture into the pericardium for left lobe amoebic liver abscess, rupture into the pleural cavity, intraperitoneal rupture, and bacterial superinfection
• Inferior and superior mesenteric vein goes towards the right side of the liver, hence the right lobe is more affected than the left lobe
• Acute stage of amoebic liver abscess


  Right upper quadrant paint - Glisson's capsule have nociceptors (pain receptor) and when irritated, elicits pain
• Rupturing of the left lobe can cause cardiac arrythmia leading to death
• Other infections


  • Amoebic meningoencephalitis
  • Extension to the kidney
• Standard method of diagnosis


  • Microscopic detection of trophozoites and cysts
  • Fresh stool specimen should be examined within 30 minutes after defecation