Amebiasis | Entamebiasis

Amebiasis

(Entamebiasis)

Amebiasis is infection with Entamoeba histolytica. It is acquired by fecal-oral transmission. Infection is commonly asymptomatic, but symptoms ranging from mild diarrhea to severe dysentery may occur. Extraintestinal infections include liver abscesses. Diagnosis is by identifying E. histolytica in stool specimens and confirmed with immunoassays-detecting antigen in the stool, or by serologic tests if extraintestinal disease is suspected. Treatment for symptomatic disease is with metronidazole or tinidazole followed by paromomycin or another drug active against cysts in the lumen of the colon.Separated from other Protozoans by trophozoite forms having pseudopods

Appear as two forms in: The trophozoite and cyst forms

Trophozoites are delicate, fragile & motile

Human infections occur through cyst ingestion in contaminated food or drink

Only trophozoites replicate by binary fission

Encystation occur when environment is not conducive for trophozoite multiplication in ileocecal area of intestine.

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Four species of Entamoeba are morphologically indistinguishable, but molecular techniques show that they are different species:

 

• E. histolytica (pathogenic)
• E. dispar (harmless colonizer, more common)
• E. moshkovskii (less common, uncertain pathogenicity)
• E. bangladeshi (less common, uncertain pathogenicity)

Amebiasis is caused by E. histolytica and tends to occur in regions with poor socioeconomic conditions and poor sanitation. The parasite is present worldwide, but most infections occur in Central America, western South America, western and southern Africa, and the Indian subcontinent. In countries with sanitary food and water supplies (eg, US), most cases occur among recent immigrants and travelers returning from endemic regions.

 

 

Entamoeba species exist in 2 forms:

 

1. Trophozoite
2. Cyst

Trophozoite

The motile trophozoites feed on bacteria and tissue, reproduce, colonize the lumen and the mucosa of the large intestine, and sometimes invade tissues and organs. Trophozoites predominate in liquid stools but rapidly die outside the body and, if ingested, would be killed by gastric acids. Some trophozoites in the colonic lumen become cysts that are excreted with stool.

This is the invasive form of the parasite. They have a single nucleus with numerous food vacuoles.found in soft, watery faeces and in tissues.Able to produce proteolytic enzyme to lyse the tissues.15-30 um, irregular shape, amoeboid, pseudopodium.Sometimes they contain RBCs in the cytoplasm. 
Trophozoite die quickly outside the body.

 

E. histolytica trophozoites can adhere to and kill colonic epithelial cells and polymorphonuclear leukocytes (PMNs) and can cause dysentery with blood and mucus but with few PMNs in stool. Trophozoites also secrete proteases that degrade the extracellular matrix and permit invasion into the intestinal wall and beyond. Trophozoites can spread via the portal circulation and cause necrotic liver abscesses. Infection may spread by direct extension from the liver to the right pleural space, lung, or skin, or rarely through the bloodstream to the brain and other organs.

 

Cysts predominate in formed stools and resist destruction in the external environment. They may spread directly from person to person or indirectly via food or water. Amebiasis can also be sexually transmitted by oral-anal contact.At the stage between the trophozoite and the complete grown cyst is called pre-cyst in which it is larger than the pure grown cyst (10-20µm) in size.
The pure grown cyst (cyst) is the infective stage with the average of 12µm contains four nuclei (quadrinucleate cyst)
The cytoplasm is granular and contains rod like materials but there is no blood cells and food particles.
It contains the chitinous wall which is resistant to the gastric acid of the stomach.
It is found in the semi-formed or formed stool but it cant be found in the dysentery stool.

Transmission of amebiasis

 Food and water containing the cystic stage of the parasite are the main sources of infection.

Infected humans themselves especially cyst carriers are the main reservoir of the infection. Note that there is no animal reservoirs.

The infection can be transmitted by either 

    Faecal-oral route, Vectors (flies and cockroaches mechanically transmit cysts) and Sexual contact.

Symptoms and Signs of Amebiasis

• Most people with amebiasis are asymptomatic but chronically pass cysts in stools.
• Symptoms that occur with tissue invasion in the colon usually develop 1 to 3 weeks after ingestion of cysts and include
• Intermittent diarrhea and constipation
• Flatulence
• Cramping abdominal pain
• Tenderness over the liver or ascending colon and fever may occur, and stools may contain mucus and bery, common in the tropics, manifests with episodes of frequent semiliquid stools that often contain blood, mucus, and live trophozoites. Abdominal findings range from mild tenderness to frank abdominal pain, with high fevers and toxic systemic symptoms. Abdominal tenderness frequently accompanies amebic colitis. Sometimes, fulminant colitis complicated by toxic megacolon or peritonitis may develop.

 

Between relapses, symptoms diminish to recurrent cramps and loose or very soft stools, but emaciation and anemia may develop. Symptoms suggesting appendicitis may occur. Surgery in such cases may result in peritoneal spread of amebas.(see also how mucus look like after ovulation)

Chronic amebiasis can results into other infection of other organs like:

 

1. Chronic amebic infection of the colon

Chronic amebic infection of the colon can mimic inflammatory bowel disease and manifests as intermittent nondysenteric diarrhea with abdominal pain, mucus, flatulence, and weight loss. Chronic infection may also manifest as tender, palpable masses or annular lesions (amebomas) in the cecum and ascending colon. Ameboma may be mistaken for colon carcinoma or pyogenic abscess.

 

• Hepatic or other extraintestinal amebic disease

Extraintestinal amebic disease originates from infection in the colon and can involve any organ, but a liver abscess is the most common.

 

Liver abscess is usually single and in the right lobe. It can manifest in patients who have had no prior symptoms, is more common among men than among women (7:1 to 9:1), and may develop insidiously. Symptoms include pain or discomfort over the liver, which is occasionally referred to the right shoulder, as well as intermittent fever, sweats, chills, nausea, vomiting, weakness, and weight loss. Jaundice is unusual and low grade when present. The abscess may perforate into the subphrenic space, right pleural cavity, right lung, or other adjacent organs (eg, pericardium).

 

• Skin lesions are occasionally observed, especially around the perineum and buttocks in chronic infection, and may also occur in traumatic or operative wounds.

 

Diagnosis of Amebiasis

Intestinal infection: Microscopic examination, enzyme immunoassay of stool, molecular tests for parasite DNA in stool, and/or serologic testing

Extraintestinal infection: Imaging and serologic testing or a therapeutic trial with an amebicide

Nondysenteric amebiasis may be misdiagnosed as irritable bowel syndrome, regional enteritis, or diverticulitis. A right-sided colonic mass may also be mistaken for cancer, tuberculosis, actinomycosis, or lymphoma.

 

Amebic dysentery may be confused with shigellosis, salmonellosis, schistosomiasis, or ulcerative colitis. In amebic dysentery, stools are usually less frequent and less watery than those in bacillary dysentery. They characteristically contain tenacious mucus and flecks of blood. Unlike stools in shigellosis, salmonellosis, and ulcerative colitis, amebic stools do not contain large numbers of white blood cells because trophozoites lyse them.

 

Hepatic amebiasis and amebic abscess must be differentiated from other hepatic infections and tumors. Patients with amebic liver abscess often present with right upper quadrant pain and fever. Amebic liver abscess is more common in men and younger adults exposed to endemic areas, whereas pyogenic liver abscess is more common in older patients. Also, symptoms of echinococcosis are unusual until the cyst grows to 10 cm in diameter, and hepatocellular carcinoma usually has no symptoms other than those caused by chronic liver disease. However, imaging and laboratory tests and tissue biopsy are often needed. Testing typically includes complete blood count (CBC), liver tests, and abdominal CT. Patients with pyogenic liver abscess often have left shift on white blood cell count, elevated serum bilirubin concentration, history of gallstones, and diabetes mellitus. Amebic liver abscess generally does not cause a left shift on white blood cell counts or elevated serum bilirubin concentration.

 

Diagnosis of amebiasis is supported by finding amebic trophozoites, cysts, or both in stool or tissues; however, pathogenic E. histolytica are morphologically indistinguishable from nonpathogenic E. dispar, as well as E. moshkovskii and E. bangladeshi, which are of uncertain pathogenicity. Immunoassays that detect E. histolytica antigens in stool are sensitive and specific and are done to confirm the diagnosis. Specific DNA detection assays for E. histolytica using polymerase chain reaction are available at diagnostic reference laboratories and have very high sensitivity and specificity.

 

 

 

Pearls & Pitfalls

Microscopic examination of stool is usually negative in patients with extraintestinal amebiasis.

Treatment of Amebiasis

Initially, metronidazole, tinidazole, or sometimes nitazoxanide

Iodoquinol, paromomycin, or diloxanide furoate subsequently for cyst eradication

For gastrointestinal symptoms and extraintestinal amebiasis, one of the following is used:

 

Oral metronidazole 500 to 750 mg 3 times a day in adults (12 to 17 mg/kg 3 times a day in children) for 7 to 10 days

Tinidazole 2 g orally once/day in adults (50 mg/kg [maximum 2 g] orally once/day in children > 3 years) for 3 days for mild to moderate gastrointestinal symptoms, 5 days for severe gastrointestinal symptoms, and 3 to 5 days for amebic liver abscess

Metronidazole and tinidazole should not be given to pregnant women. Alcohol must be avoided because these drugs have a disulfiram-like effect. In terms of gastrointestinal adverse effects, tinidazole is generally better tolerated than metronidazole.

 

Nitazoxanide is an effective alternative for noninvasive intestinal amebiasis (500 mg orally 2 times a day for 3 days taken with food), but efficacy against invasive disease is not known; therefore, it should only be used if other treatments are contraindicated (1, 2).

 

Therapy for patients with significant gastrointestinal symptoms should include rehydration with fluid and electrolytes and other supportive measures.

 

Although metronidazole, tinidazole, and nitazoxanide have some activity against E. histolytica cysts, they are not sufficient to eradicate cysts. Consequently, a 2nd oral drug is used to eradicate residual cysts in the intestine.

 

Prevention of Amebiasis

• To prevent amebiasis, contamination of food and water with human feces must be avoided—a problem complicated by the high incidence of asymptomatic carriers. 
• Uncooked foods, including salads and vegetables, and potentially contaminated water and ice should be avoided in areas with poor sanitation. 
• Boiling water kills E. histolytica cysts. 
• The effectiveness of chemical disinfection with iodine- or chlorine-containing compounds depends on the temperature of the water and amount of organic debris in it. 
• Portable filters provide various degrees of protection.
• Work continues on the development of a vaccine, but none is available yet.
• Improvement of living standards and sanitary conditions
• Proper disposal of faeces
• Early detection and treatment of cases
• No vaccine available
• Boiling or treating water with iodine crystals
• Cysts are resistant to Chlorination

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Key Points

• E. histolytica is often asymptomatic, but can cause intestinal symptoms, dysentery, or liver abscesses.
• Diagnose amebic intestinal infection using stool antigen tests, molecular tests for DNA, or microscopy.
• Diagnose amebic liver abscess using ultrasonography, CT, or MRI, or serologic tests, which are most helpful when previous infection is considered unlikely (eg, in travelers to endemic areas), or a therapeutic trial of an amebicide.
• Treat with metronidazole or tinidazole to eliminate amebic trophozoites, followed by iodoquinol or paromomycin to kill cysts in the intestine.