Amebiasis Pathogenesis

Pathogenesis of Amebiasis

• Trophozoites divide and cause extensive local necrosis in the large intestine
• Invasion into the deeper mucosa can lead to secondary involvement of other organs, primarily the liver, lungs, brain, and heart
• Extraintestinal amebiasis is associated with trophozoites

Epidemiology of Amebiasis

• E. histolytica has a worldwide distribution, with highest incidence in tropical and subtropical regions with poor sanitation and contaminated water
• 90% of infections are asymptomatic, while the remaining produce a spectrum of clinical syndromes
• Patients infected with E. histolytica pass non-infectious trophozoites and infectious cysts in their stools, making them the main source of water and food contamination
• Symptomatic amebiasis is usually sporadic, while epidemic form is a result of direct person-to-person faecal-oral spread under conditions of poor personal hygiene

Non-Pathogenic Protozoa

• Entamoeba gingivalis:
  • Lives primarily on exudate from the margins of gums
  • Thrives best on unhealthy gums
  • Can be transferred by kissing, using contaminated utensils, or droplet spray of the mouth
  • Can be treated or prevented by proper care of the teeth and gums
• Entamoeba coli:
  • Transmitted through fecal contamination
  • Depends on personal hygiene
• Endolimax nana:
  • Lumen dweller in the large intestine
  • Results from ingestion of viable cyst in polluted water or contaminated food
• Iodamoeba buetschili:
  • Natural habitat is the lumen of the large intestine, primarily in the caecum
  • Natural parasite of man and lower primates

Symptoms of Amoebiasis

• Diarrhea/Dysentery
• Nausea
• Abdominal Pain
• Ulceration of the intestinal mucosa

Life Cycle and Laboratory Diagnosis

• Microscopic examination of faeces for trophozoite and cysts
• Trophozoite is very large, varying in length from 50 to 200μm and in width from 40 to 70μm
• Surface is covered with cilia

Treatment

• Drug of choice is tetracycline
• Alternative agents are iodoquinol and metronidazole

Medical Helminthology

• Concerned with the study of helminthes or parasitic worms
• Helminthes are trophoblastic metazoa (multi-cellular organisms)
• Causes of high morbidity and mortality worldwide
• Causes anemia and malnutrition
• Reduces academic performance in children
• Causes economic loss in domestic animals

Introduction to Helminthes

• Age-dependent distribution of infections from geo-helminthes and schistosomes
• Children disproportionately carry the burden of schistosomes and geo-helminthes
• Sources of parasites include:
  • Contaminated soil (Geo-helminthes)
  • Water (cercariae of blood flukes)
  • Food (Taenia in raw meat)
  • Blood sucking insects or arthropods (as in filarial worms)
  • Domestic or wild animals harboring the parasite (as in echinococcus in dogs)
  • Person to person (as in Enterobius vermicularis, Hymenolopis nana)
  • Oneself (auto-infection) as in Enterobius vermicularis

Dientamoeba fragilis

• Initially classified as an amoeba, but internal structures of the trophozoite are typical of a flagellate
• No cyst stage has been described
• Worldwide distribution
• Transmission occurs through faecal-oral routes
• Most infections are asymptomatic, with colonization of the cecum and upper colon
• Symptoms include abdominal discomfort, flatulence, intermittent diarrhea, anorexia, and weight loss
• Therapeutic agent of choice is iodoquinol, with tetracycline and paromomycine as acceptable alternatives

Other Flagellates

• Trichomonas hominis:
  • Lives in the caecal area of the large intestine
  • Feeds on bacteria
  • Considered to be non-pathogenic
  • Transmission occurs in the trophic form
  • No treatment is indicated
• Trichomonas tenax:
  • Recovered from the mouth, specifically in tartar from the teeth
  • No known cyst stage
  • Trophozoite has a pyriform shape and is smaller and more slender than that of T. hominis
  • Diagnosis is based on the recovery of the organism from the teeth, gums, or tonsillar crypts
  • No therapy is indicated
• Chilomastix mesnli:
  • Has both a trophozoite and cyst stage
  • Lives in the cecal region of the large intestine, where the organism feeds on bacteria and debris
  • Considered to be non-pathogenic
  • No treatment is recommended