Amebiasis (Amoebiasis) : Epidemiology, Pathogenesis, Symptoms, Diagnosis, Treatment & Prevention
Definition
- Amebiasis is an infection with the intestinal protozoan Entamoeba histolytica which is spread between humans by its cysts.
- 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.
- 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.
- 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 is most prevalent and is particularly likely when food handlers are shedding cysts or food is being grown with feces-contaminated soil, fertilizer, or water.
- Besides the drinking of contaminated water, less common means of transmission include 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.
- Amebiasis results from infection with E. histolytica and is the third 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.
- The potential of E. moshkovskii to cause diarrhea, weight loss, and colitis was recently demonstrated in a mouse model of cecal infection. However, the pathogenic potential of this species is not clear.
- 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.
- The main groups at risk for amebiasis in developed countries are returned travelers, recent immigrants, MSM, military personnel, and inmates of institutions.
Pathogenesis
- Cysts of E. histolytica are ingested in water or uncooked foods contaminated by human faeces. Infection may also be acquired through anal/oral sexual practices. In the colon, trophozoite forms emerge from the cysts.
- Both trophozoites and cysts are found in the intestinal lumen, but only trophozoites of E. histolytica invade tissue.
- The parasite may invade the mucous membrane of the large bowel, producing lesions that are maximal in the caecum but found as far down as the anal canal. These are flask-shaped ulcers, varying greatly in size and surrounded by healthy mucosa.
- A localised granuloma (amoeboma), presenting as a palpable mass in the rectum or a filling defect in the colon on radiography, is a rare complication which should be differentiated from colonic carcinoma.
- 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. They can multiply rapidly and destroy the liver parenchyma, causing an abscess. The liquid contents at first have a characteristic pinkish colour, which may later change to chocolate-brown (like
anchovy sauce). - Cutaneous amoebiasis, though rare, causes progressive genital, perianal or peri-abdominal surgical wound ulceration.
Clinical syndromes & features
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.
- A gradual onset of lower abdominal pain and mild diarrhea is followed by malaise, weight loss, and diffuse lower abdominal or back pain.
- Cecal involvement may mimic acute appendicitis.
- 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.
- More fulminant intestinal infection, with severe abdominal pain, high fever, and profuse diarrhea, is rare and occurs predominantly in children.
- Patients may develop toxic megacolon, in which there is severe bowel dilation with intramural air.
- Patients receiving glucocorticoids are at risk for severe amebiasis.
- Uncommonly, patients develop a chronic form of amebic colitis, which can be confused with inflammatory bowel disease.
- 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-upperquadrant 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.
Older patients from endemic areas are more likely to have a subacute course lasting 6 months, with weight loss and hepatomegaly. About one-third of patients with chronic presentations are febrile. Thus, the clinical diagnosis of an amebic liver abscess may be difficult to establish because the symptoms and signs are often nonspecific. Since 10–15% of patients present only with fever, amebic liver abscess must be considered in the differential diagnosis of fever of unknown origin.
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. The stool sample has to be collected directly into a wide mouthed container and examined without delay. Prior administration of antiamoebic drugs, bismuth, kaolin or mineral oil may interfere with demonstration of the trophozoite. It should be inspected for macroscopic and microscopic features, as well as routine examination for other parasites also. Examination of three separate samples is recommended.
- Macroscopic appearance: The stool is copious, semiliquid, 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 appearance: The 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. But this finding is only suggestive, because they may also occur in some other bowel
disorders such as ulcerative colitis and malignancy. In freshly passed motion unmixed with urine or antiseptics, actively motile trophozoites of E.histolytica can be demonstrated in unstained saline mounts.The presence of ingested erythrocytes clinches the identity of E.histolytica, as they are not found in any other intestinal amoeba. Stained films may not be necessary as a routine for diagnosis in acute cases, but trichrome or iron-haematoxylin stained films provide the most dependable identification and differentiation.
Culture and serology are not routinely employed. Serology is usually negative in early cases and in the absence of deep invasion.
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).
While IHA and LA are highly sensitive, they often give false-positive results. They remain positive for several years even after successful treatment.
Gel precipitation tests are less sensitive, but more specific.
ELISAs are both sensitive and specific and like GDP and CIE become negative within six months of successful treatment. Highly sensitive radioimmunoassay (RIA) and DNA probes have been introduced for detection of amoeba antigens in blood pus and faeces but these are too complex for routine use.
In case of liver abscess when diagnostic aspiration is done the pus obtained from the centre of the abscess may not contain amoebae as they are confined to the periphery. The fluid draining after a day or two is more likely to contain the trophozoite. Aspirates from the margins of the abscess also would show the trophozoites. Cysts are never seen in extraintestinal lesions.
Differential Diagnosis
The differential diagnosis of intestinal amebiasis includes bacterial diarrheas caused by Campylobacter, enteroinvasive Escherichia coli and species of Shigella, Salmonella, and Vibrio.
- Although the typical patient with amebic colitis has less prominent fever than in these other conditions as well as heme-positive stools with few neutrophils, correct diagnosis requires bacterial cultures, microscopic examination of stools, and amebic serologic testing.
- Because of the variety of presenting signs and symptoms, amebic liver abscess can easily be confused with pulmonary or gallbladder disease or with any febrile illness with few localizing signs, such as malaria or typhoid fever. Once radiographic studies have identified an abscess in the liver, the most important differential diagnosis is between amebic and pyogenic abscess. Patients with pyogenic abscess typically are older and have a history of underlying bowel disease or recent surgery. Amebic serology is helpful, but aspiration of the abscess, with Gram’s staining and culture of the material, may be required for differentiation of the two diseases.
Treatment
Drug Therapy for Amebiasis is as following :
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Prevention
- Personal precautions against contracting amoebiasis consist of not eating fresh, uncooked vegetables or drinking unclean water.
