Diseases

Amebiasis (Amoebiasis) : Causes, Symptoms, Diagnosis, Treatment & Prevention

Amebiasis (Amoebiasis) : Definition, Etiology, Epidemiology, Pathogenesis, Signs, Symptoms, Stool Color, Life Cycle, Management, & Prevention

Definition

  • Amebiasis is an intestinal infection with the protozoan Entamoeba histolytica which is spread between humans by its cysts.
  • E. histolytica has unique isoenzymes, surface antigens, DNA markers, and virulence properties that distinguish it from other genetically related and morphologically identical species, such as E. dispar and E. moshkovskii.

Transmission

E. histolytica is acquired by ingestion of viable cysts from fecally contaminated water, food, or hands.

  • Food-borne exposure: Most prevalent and particularly likely when food handlers are shedding cysts or food is being grown with feces-contaminated soil, fertilizer, or water.
  • Drinking of contaminated water,
  • Oral and anal sexual practices and—in rare instances—direct rectal inoculation through colonic irrigation devices.

Epidemiology

  • About 10% of the world’s population is infected with Entamoeba, the majority with noninvasive Entamoeba dispar.
  • 3rd most common cause of death from parasitic disease (after schistosomiasis and malaria). Invasive colitis and liver abscesses are sevenfold more common among men than among women; this difference has been attributed to a disparity in complement-mediated killing.
  • Areas of highest incidence of Entamoeba infection (due to inadequate sanitation and crowding) include most developing countries in the tropics, particularly Mexico, India, and nations of Central and South America, tropical Asia, and Africa.
  • Main groups at risk for amebiasis in developed countries are returned travelers, recent immigrants, MSM, military personnel, and inmates of institutions.

Pathogenesis

Ingestion of cysts of E. histolytica

Both trophozoites and cysts are found in the intestinal lumen, but only trophozoites of E. histolytica invade tissue

Parasite may invade the mucous membrane of the large bowel, producing lesions (flask-shaped ulcers, varying greatly in size and surrounded by healthy mucosa) that are maximal in the caecum but found as far down as the anal canal

Amoebic ulcers may cause severe haemorrhage but rarely perforate the bowel wall

Amoebic trophozoites can emerge from the vegetative cyst from the bowel and be carried to the liver in a portal venule

Clinical Features

About 90% of infections are asymptomatic, and the remaining 10% produce a spectrum of clinical syndromes ranging from dysentery to abscesses of the liver or other organs.

1. Intestinal Amebiasis (Amoebic dysentery)

The most common type of amebic infection is asymptomatic cyst passage. Even in highly endemic areas, most patients harbor E. dispar. Symptomatic amebic colitis develops 2–6 weeks after the ingestion of infectious E. histolytica cysts.

Gradual onset of lower abdominal pain & mild diarrhea

Malaise, weight loss, diffuse lower abdominal or back pain

If cecal involvement: May mimic acute appendicitis

More fulminant intestinal infection (Rare but occurs predominantly in children)

Toxic megacolon (severe bowel dilation with intramural air)

 

  • Patients with full-blown dysentery may pass 10–12 stools per day. The stools contain little fecal material and consist mainly of blood and mucus.
  • In contrast to those with bacterial diarrhea, fewer than 40% of patients with amebic dysentery are febrile.
  • Virtually all patients have heme-positive stools.
  • Patients receiving glucocorticoids are at risk for severe amebiasis.

The association between severe amebiasis complications and glucocorticoid therapy emphasizes the importance of excluding amebiasis when inflammatory bowel disease is suspected.

An occasional patient presents with only an asymptomatic or tender abdominal mass caused by an ameboma, which is easily confused with cancer on barium studies. A positive serologic test or biopsy can prevent unnecessary surgery in this setting.

The syndrome of post–amebic colitis—i.e., persistent diarrhea following documented cure of amebic colitis—is controversial; no evidence of recurrent amebic infection can be found, and re-treatment usually has no effect.

2. Extraintestinal (Invasive) Amoebiasis such as Hepatic amoebiasis, Cutaneous amoebiasis, Pulmonary amoebiasis

Amebic Liver Abscess : Extraintestinal infection by E. histolytica most often involves the liver. Of travelers who develop an amebic liver abscess after leaving an endemic area, 95% do so within 5 months.
Young patients with an amebic liver abscess are more likely than older patients to present in the acute phase with prominent symptoms of <10 days’ duration.

  • Most patients are febrile and have right-upper quadrant pain, which may be dull or pleuritic in nature and may radiate to the shoulder.
  • Point tenderness over the liver and right-sided pleural effusion are common.
  • Jaundice is rare. Although the initial site of infection is the colon, fewer than one-third of patients with an amebic abscess have active diarrhea.

Laboratory Diagnosis

Definitive diagnosis of amoebiasis depends on the demonstration of E.histolytica trophozoites or its cysts in stools, tissues or discharges from the lesions. Cultures are not employed for routine diagnosis. Immunological tests are not helpful for diagnosis of intestinal infection but may be of use in extraintestinal amoebiasis.

Intestinal Amoebiasis

Acute amoebic dysentery : The disease has to be differentiated from bacillary dysentery.

Process

No prior administration of antiamoebic drugs, bismuth, kaolin or mineral oil

Stool sample (To be collected directly into a wide mouthed container and examined without delay).

Examination of three separate samples is recommended. Inspection for

  • Macroscopic: Stool is copious, semi-liquid, brownish black in colour and contains foul smelling faecal material intermingled with blood and mucus. It is acid in reaction. It does not adhere to the container.
  • Microscopic features: Cellular exudate is scanty and consists of a few pus cells, epithelial cells and macrophages. The red cells are aggregated and yellowish or brownish red in colour. Charcot-Leyden crystals are often present.
  • Routine examination:  For other parasites.

Chronic Amoebiasis and Carriers : Sigmoidoscopy may show amoebic ulcers in the colon, from which biopsy tissue may be taken for direct microscopy and histopathology. Identification of asymptomatic carriers is important in epidemiological survey and in screening persons employed in food handling occupations.

Extraintestinal (Invasive) Amoebiasis

Hepatic amoebiasis : In diffuse hepatic amoebiasis (amoebic hepatitis) without localised abscess formation, laboratory diagnosis may be difficult. Often stool examination is negative for amoebae and a history of dysentery may be absent. In such cases serological tests can be helpful.

  • Complement fixation test
  • Indirect haemagglutination (IHA)
  • Latex agglutination (LA)
  • Gel diffusion precipitation (GDP)
  • Cellulose acetate membrane precipitation (CAP) test
  • Counter current immunoelectrophoresis (CIE)
  • Enzyme linked immunosorbent assay (ELISA).

Differential Diagnosis

Two non-pathogenic Entamoeba species (E. dispar and E. moshkovskii) are morphologically identical to E. histolytica, and are distinguishable only by molecular techniques, isoenzyme studies or monoclonal antibody typing. However, only E. histolytica causes amoebic dysentery or liver abscess.

The differential diagnosis of intestinal amebiasis includes bacterial diarrheas caused by Campylobacter, enteroinvasive Escherichia coli and species of Shigella, Salmonella, and Vibrio.

Treatment

Drug Therapy for Amebiasis is as following :

Indication Therapy
Asymptomatic carriage Luminal agent: Iodoquinol (650-mg tablets), 650 mg tid for 20 days; or Paromomycin (250-mg tablets), 500 mg tid for 10 days
Acute colitis Metronidazole (250- or 500-mg tablets), 750 mg PO or IV tid for 5–10 days; or
Tinidazole, 2 g/d PO for 3 days
plus
Luminal agent: Iodoquinol (650-mg tablets), 650 mg tid for 20 days; or Paromomycin (250-mg tablets), 500 mg tid for 10 days
Amebic liver abscess Metronidazole, 750 mg PO or IV for 5–10 days; or Tinidazole, 2 g PO once; or Ornidazole,a 2 g PO once
plus
Luminal agent: Iodoquinol (650-mg tablets), 650 mg tid for 20 days; or Paromomycin (250-mg tablets), 500 mg tid for 10 days

Prevention

  • Personal precautions against contracting amoebiasis consist of not eating fresh, uncooked vegetables or drinking unclean water.
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