Etiology:

Amebiais is the result of infection with the protozoa Entamoeba histolytica .

Entamoeba histolytica is an enteric protozoan  Excreted as cysts or trophozoites in stools of infected persons.

It has been classified into 2 species that morphologically are identical but genetically distinct 

• Entamoeba histolytica causes invasive disease

• E dispar  is a noninvasive parasite that does not cause disease. 

Life cycle and pathogenesis

• Humans are the principal host and reservoir of E. histolytica. 

  • Infection with E histolytica is transmitted via amebic cysts by the fecal-oral route. 

  • Human to human transmission occurs by ingestion of cysts. 

  • Transmission occasionally has been associated with contaminated food, water, and enema equipment. Up to 10% of the world's population is infected. Food and water-borne spread occur. 

  • Infected patients excrete cysts intermittently, sometimes for years if untreated. 

  • Venereal transmission is common among homosexuals    

• Ingested cysts, which are unaffected by gastric acid, undergo excystation in the alkaline small intestine and produce trophozoites that
  infect the colon. 

• Cysts that subsequently develop are the source of transmission, especially from asymptomatic cyst excreters. 

• The incubation period is variable, ranging from a few days to months or years, but commonly is 1 to 4 weeks.

• In most people E. histolytica is a harmless commensal. Virulence is strain related, and associated with certain isoenzyme patterns and surface antigens.

• Appproximately 10% of infected patients develop invasive amebiasis

• All of the infected patients have anti-amebic serum antinody response

Pathogenesis:

• Trophozoites adhere to colonic mucin, disrupting mucosal barrier with proteolytic enzymes, and contact dependent lysis of host cells, including inflammatory cells.

• Lectin mediated adherence is important, along with the capacity to lyse host cells on contact. 

• Virulence is increased with passage through humans.

• Malnutrition, corticosteroids, pregnancy and childhood render the host more susceptible to invasion. 

• Lysis of colonic epithelial cells produces small mucosal ulcerations. 

• There is edema and hyperemia, but minimal inflammatory response. 

• The inflammatory cells are also lysed, thus explaining the paucity of leukocytes

• Is associted with the occurrence of antigen-specific suppression of cell mediated rsponse

• Trophozoites are present in the base of the ulcer. The lesion spreads laterally in the submucosa, producing a flask-shaped ulcer. 

• Granulation tissue and fibrotic thickening will occasionally produce an ameboma.

• Spread to the portal circulation, and rarely to lung, spleen or brain results in metastatic abscesses.  

Epidemiology:

Entamoeba histolytica can be found worldwide but is more prevalent in persons of lower socioeconomic status who live in developing countries where the prevalence of amebic infection may be as high as 50%. 

Groups at increased risk of amebiasis in developed nations include 

• immigrants from endemic areas

• long-term visitors to endemic areas

• institutionalized persons

• men who have sex with men

Clinical Manifestations:

• Noninvasive intestinal infection, which may be asymptomatic (and most likely due to Entamoeba dispar)

  •  May have ill-defined intestinal tract complaints but generally tolerate the infection. In most people E. histolytica is a harmless commensal. 

• Intestinal amebiasis (amebic colitis) 

  • generally have 1 to 3 weeks of increasing diarrhea progressing to grossly bloody dysenteric stools with lower abdominal pain and tenesmus. 

  • Weight loss is common, and fever occurs in one third of patients. 

  • Symptoms may be chronic and may mimic symptoms of inflammatory bowel disease.

  • Progressive involvement of the colon may produce toxic megacolon, fulminant colitis, ulceration of the colon and perianal area, and, rarely, perforation. 

  • Paucity of WBC's in stools

• Ameboma 

  • An ameboma is an annular lesion of the cecum or ascending colon that may be mistaken for colonic carcinoma or as a tender extrahepatic mass mimicking a pyogenic abscess. 

  • Amebomas usually resolve with antiamebic therapy and do not require surgery.

• Extraintestinal disease:

  • In a small percentage of patients, extraintestinal disease may occur with involvement of the lungs, pericardium, brain, skin, and genitourinary tract, but the liver is the most common site. 

  • Presentation of liver abscess may be acute with fever and abdominal pain, tachypnea, and liver tenderness and hepatomegaly, or chronic with weight loss, vague abdominal symptoms, and irritability. Rupture of abscesses into the abdomen or chest may lead to death.

  •  Pericardial abscesses with tamponade may occur. Evidence of recent intestinal infection frequently is absent.

Diagnostic Tests:

• The diagnosis of intestinal amebiasis is established by the identification of the organism in stool or sigmoidoscopic aspirates. 

  • Trophozoites can be seen on wet mounts, while cysts can be seen on concentrated specimens. 

  • Three or more specimens may be required for diagnosis.

  • Specimens of stool, endoscopy scrapings (not swabs), and biopsies, should be examined by wet mount within 30 minutes of collection and fixed in formalin and polyvinyl alcohol (available in kits) for concentration and permanent staining.

  • Entamoeba histolytica is relatively indistinguishable from the noninvasive more prevalent E dispar; trophozoites containing ingested red blood cells are more likely to be E histolytica. Polymerase chain reaction, isoenzyme analysis, and antigen detection assays can differentiate E histolytica and E dispar.

• Endoscopic biopsy of ulcers and demonstration of organisms

• Enzyme immunoassays detecting E. histolytica antigens in stool have been developed.

• Indirect hemagglutination and enzyme immunoassay are the most sensitive for detecting, antibody.  

  • Serology is useful in diagnosing extra-intestinal amebiasis.

  • Serum antibody tests may be helpful, primarily for the diagnosis of amebic dysentery (85% positive) and extraintestinal amebiasis with liver involvement (99% positive). 

  • Patients who are asymptomatic cyst excreters generally have negative serologic assays for E histolytica. 

  • Results of the standard serologic tests are negative in persons infected with E dispar.

• Ultrasonography and computed tomography can effectively identify liver abscesses and other extraintestinal sites of infection.

• CT guided fine needle aspiration: Aspirates from a liver abscess usually show neither trophozoites nor leukocytes. The trophozoites are found in tissue at the periphery of the liver abscess.

Treatment 

• Fluid and blood replacement, relief of symptoms and eradication of the organism are important. 

• Treatment involves elimination of the tissue-invading trophozoites as well as organisms in the intestinal lumen. 

  • The drug of choice  to eliminate the tissue-invading trophozoites is metronidazole, which is effective against all forms of amebiasis. 

  • It should be combined with diloxanide to cure organisms in the intestinal lumen. 

• Alternatives are tetracycline, paromomycin, and dehydroemetine.

• Entamoeba dispar infection does not require treatment. 

• Corticosteroids and antimotility drugs administered to persons with amebiasis can worsen symptoms and the disease process. 

• To prevent spontaneous rupture of an abscess, patients with large liver abscesses may benefit from percutaneous or surgical aspiration.

Immunity

• Cure of invasive amebiasis is associted with resistance to recurrent disease and immunity for a year or more.

  • Serum antibody and an amebicidal cell-mediated immune response with lymphokine-activated macrocytes and a CD8 subset of cytotoxic lymphocytes serving as effector cells.

Control Measures:

• Isolation of the Hospitalized Patient: Standard precautions are recommended for symptomatic and asymptomatic patients.

• Hand washing after defecation

• Sanitary disposal of fecal material

• Treatment of drinking water . Boiling is the only reliable way to kill the cysts.

• Avoid uncooked food such as vegetables and salads in endemic areas

• Sexual transmission may be controlled by the use of condoms