Amoebae

Mahmud, Rohela; Lim, Yvonne Ai Lian; Amir, Amirah

doi:10.1007/978-3-319-68795-7_3

Rohela Mahmud⁴,
Yvonne Ai Lian Lim⁴ &
Amirah Amir⁴

1698 Accesses
1 Citations

Abstract

Entamoeba histolytica has a worldwide prevalence, especially where sanitation is poor and is more common in developing countries of the tropics. Majority of cases are asymptomatic. Among the numerous types of free living amoeba (FLA) found in water and soil, a few are potentially pathogenic and can cause human infections.

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Entamoeba histolytica

Distribution

Entamoeba histolytica has a worldwide prevalence, especially where sanitation is poor and is more common in developing countries of the tropics. Majority of cases are asymptomatic.

Habitat

Entamoeba histolytica is found in the human colon.

Morphology

Entamoeba histolytica occurs in 3 forms.

1.
Trophozoite
2.
Precyst
3.
Cyst

Trophozoite is the vegetative form of the parasite and the only form present in tissues. It is irregular in shape and varies in size from 12 to 60 μm; average being 20 μm (Fig. 3.1a). It has a cytoplasm which consists of ectoplasm and endoplasm. Ectoplasm is clear and transparent. Endoplasm is finely granular and contains nucleus, food vacuoles and phagocytosed erythrocytes. Pseudopodia are finger-like projections formed by movements of ectoplasm in one direction.

Its nucleus is spherical and contains central karyosome. The nuclear membrane is lined by a rim of evenly distributed chromatin. It reproduces by binary fission. It is killed by drying, heat and chemical sterilization.

The trophozoites undergo encystment in the intestinal lumen. Before encystment, the trophozoite extrudes its food vacuoles and rounds up to form a precystic stage, measuring 10–20 μm in size. It contains a large glycogen vacuole and chromatoid bars. It secretes a cyst wall to become cyst.

The cyst is spherical in shape. Immature cyst contains a single nucleus, a glycogen vacuole and chromatoid bars which are cigar shaped with rounded ends. The chromatoid bars are visible in saline. With iron haematoxylin stain, nuclear chromatin and chromatoid bodies appear deep blue or black. When stained with iodine, the glycogen mass appears golden brown while the nuclear chromatin and karyosome bright yellow. Mature cyst contains 4 nuclei. It measures 10–20 μm in size (Fig. 3.1b). The glycogen mass and chromatoid bars disappear in mature cyst. The cyst wall is highly resistant to gastric juice and unfavourable environmental conditions.

Life Cycle (Fig. 3.2)

(1) The cysts (usually found in formed stools) and trophozoites (in loose stools) are passed out in faeces of infected human. (2) Cysts are ingested via contaminated food or water. (3) In the intestine, the cysts undergo excystation and form trophozoites (4). (5) As the trophozoite passes down the intestine, it undergoes encystation and is excreted in the faeces.

Entamoeba histolytica completes its life cycle in human host. In the majority of cases, E. histolytica remains as a commensal in the large intestine. They are carriers or asymptomatic cyst passers and are responsible for maintenance and transmission of infection in the community.

Pathogenesis and Clinical Features

Entamoeba histolytica causes intestinal and extraintestinal amoebiasis.

The lumen-dwelling amoebae do not cause any illness. They cause disease only when trophozoites invade the intestinal tissues. The trophozoite penetrates the epithelial cells in the colon, aided by its movement and histolysin, a tissue lytic enzyme, which damages the mucosal epithelium. Amoebic lectin mediates adherence. Mucosal penetration produces discrete ulcers with pinhead centre and raised edges. Sometimes, the invasion remains superficial and heals spontaneously. The ulcers are multiple and are confined to the colon, being most numerous in the caecum and recto-sigmoidal region. The intervening mucous membrane between the ulcers remains healthy.

The amoebic ulcer is flask shaped in cross-section. Multiple ulcers may coalesce to form large necrotic lesions with ragged and undermined edges and are covered with brownish slough. The ulcers generally do not extend deeper than submucosal layer. Amoebae are seen at the periphery of the lesions and extending into the surrounding healthy tissues. Clinical manifestations are diarrhoea, vague abdominal symptoms and dysentery. This may resemble bacillary dysentery. The ulcers may involve the muscular and serous coats of the colon, causing perforation and peritonitis. Blood vessel erosion may cause haemorrhage. Deep ulcers form scars and may lead to strictures and partial obstruction. A granulomatous pseudotumoral growth may develop on the intestinal wall from a chronic ulcer. This amoebic granuloma or amoeboma may be mistaken for a malignant tumour. The incubation period for intestinal amoebiasis varies from 1 to 4 months.

Liver involvement is the most common extraintestinal complication of intestinal amoebiasis. About 5–10% of patients with intestinal amoebiasis will develop amoebic liver abscess (ALA). ALA arises from haematogenous spread of amoebic trophozoites from colonic mucosa or by direct extension. Often, ALA patients do not present with bowel symptoms. Liver damage may not be directly caused by the amoebae, but by lysosomal enzymes and cytokines from the inflammatory cells surrounding the trophozoites. The centre of the abscess contains thick brown pus (anchovy sauce), which is liquefied necrotic liver tissue free of amoeba. The trophozoite is in the wall of the abscess. Liver abscess may be multiple or more often solitary, usually located in the upper right lobe of the liver. Jaundice develops only when lesions are multiple or when they press on the biliary tract. Large, untreated abscess may rupture into the lungs and pericardium. The incidence of liver abscess is more common in adult males. Other complications of intestinal amoebiasis are as shown in Table 3.1.

Table 3.1 Other complications of intestinal amoebiasis

Full size table

Immunity

Infection with invasive strains will activate both humoral and cellular immune responses. Systemic antibodies can be demonstrated within a week of invasive infection. Infection confers some degree of protection against the recurrence of invasive colitis and liver abscess in endemic areas.

Diagnosis

1.
Diagnosis of intestinal amoebiasis
1. (a)
  Microscopic examination
  
  Demonstration of cysts or trophozoites in stool sample. Since excretion of cysts in the stool is often intermittent, at least 3 consecutive specimens should be examined. Trophozoite and cyst of E. histolytica have similar morphology to E. dispar and E. moshkovskii which are non-pathogens. Molecular technique can differentiate these 3 species. Fixed stool smear can be stained with trichrome to demonstrate cysts and trophozoites.
2. (b)
  Sigmoidoscopy for mucosal scrapings
  
  Direct wet mount and iron haematoxylin staining to demonstrate trophozoites.
3. (c)
  Stool culture
  
  Stool culture is a sensitive method in diagnosing chronic and asymptomatic intestinal amoebiasis. However, it is not a routine method of diagnosis.
4. (d)
  Serodiagnosis
  
  Serological test is positive only in invasive amoebiasis.
5. (e)
  Molecular diagnosis
  
  Polymerase chain reaction (PCR) to detect E. histolytica in stool and to differentiate between the other species that are non-pathogens (E. dispar and E. moshkovskii).
2.
Diagnosis of extraintestinal amoebiasis
1. (a)
  Microscopic examination
  
  Demonstration of trophozoites in pus aspirated from the wall of liver abscess. The pus obtained from the centre of the abscess may not contain amoebae as they are confined to the wall of the abscess. Cysts are not found in extraintestinal lesions. Stool examination rarely can detect E. histolytica cyst.
2. (b)
  Molecular diagnosis
  
  PCR of pus aspirated from ALA
3. (c)
  Serodiagnosis

Treatment

1.
Luminal amoebicides: Diloxanide furoate, iodoquinol, paromomycin and tetracycline act in the intestinal lumen but not in tissues.
2.
Tissue amoebicides: Emetine and chloroquine are effective in systemic infection, but less effective in the intestine.
3.
Both luminal and tissue amoebicides: Metronidazole (750–800 mg 3 times daily for 5–10 days), tinidazole and ornidazole act on both sites.

Carriers should also be treated because of the risk of transmitting the infection to others. Paromomycin or iodoquinol should be used in these cases. Although metronidazole and tinidazole are both luminal and tissue amoebicides, neither of them reach adequate levels in the gut lumen. Therefore, patients with ALA should also receive treatment with a luminal agent to ensure eradication of infection. Paromomycin (25–35 mg/kg/day, divided into 3 doses for 7 days) is the drug of choice.

Prevention and Control

1.
Boil drinking water
2.
Wash fruits and vegetables in clean water before eating
3.
Detection and treatment of carriers and prohibit them from food handling
4.
Health education

Note: It is important to distinguish between Entamoeba histolytica and Entamoeba coli cyst and trophozoite. Entamoeba coli is a commensal intestinal protozoa and it is non-pathogenic. The cyst of E. coli is large, 10–30 μm in size and mature cyst has 8 nuclei (Fig. 3.3a). Its chromatoid bodies are splinter like. The trophozoite of E. coli is large, measuring 20–50 μm, does not contain ingested red blood cells and does not invade tissues (Fig. 3.3b). Its life cycle is the same as that of E. histolytica.

Pathogenic Free-Living Amoebae (FLA)

Among the numerous types of FLA found in water and soil, a few are potentially pathogenic and can cause human infections.

1.
Naegleria fowleri causes primary amoebic meningoencephalitis (PAM)
2.
Acanthamoeba spp. cause granulomatous amoebic encephalitis (GAE) and amoebic keratitis (AK).

A few instances of GAE caused by Balamuthia spp. have also been reported. While PAM and AK occur in previously healthy individual, GAE has been associated with immunodeficient patients.