Amoebiasis

 

Amoebiasis:

Diseases caused by Entamoeba histolytica and other species.

P.K. Ghatak, MD

Amoeba: it is a unicellular organism. The cytoplasm of the amoeba is jelly like and contains mitochondria, other organelle, a large nucleus and various sized vacuoles. The cytoplasm is encased within a double layered membrane. The outer layer of the membrane is tough and elastic. Amoeba crawls in a peculiar fashion and known as amoeboid movement. There are microvilli attached to the outer membrane which prevent the amoeba in getting tucked to any solid surface and the outpouring foot processes – pseudopods, help them float in water. Amoeba surrounds food particles by encircling it with pseudopods and incorporate it in its cytoplasm and forms a vacuole and after digesting it with various enzymes, the waste is eliminated. Amoeba propagates by binary division. In rare occasions, it produces multiple identical daughters. The cellular form of the amoeba is called trophozoites, and it also has a cyst form. The cysts are infective form.

Three species of amoeba are pathogenic for humans.

Entamoeba histolytica. It is the primary amoeba causing intestinal diseases and its various other sequels.

Acanthamoeba. It causes keratitis of the eyes. People wearing contact lenses are susceptible to infection with this amoeba and if keratitis is not properly treated it can produce corneal ulcer and loss of vision.

Naegleria fowleri is responsible for a highly fetal disease, amoebic meningoencephalitis.

Entamoeba histolytica.

Humans are the primary host and in endemic areas the local people develop a partial immunity and 90 % of infections produce no symptoms because perhaps the people develop a kind of parasitic tolerance with the amoeba colonies in the colon mucosa. People who are immune depressed or on immune suppressed therapy, specially steroids, are venerable to severe illness. Visitors are susceptible to acute amoebic dysentery and often develop symptoms after returning home. The amoebic cysts are infectious and the mode of infection is oral-fecal route. The incubation period is 2 to 4 weeks.

The cysts pass unchanged from the stomach to the terminal ilium, and in here the cysts break open and release young trophozoites. Trophozoites release protein digestive enzymes and phagocytize the surface enterocytes. The amoeba is antigenic to humans. The body responds by both innate and acquired immune systems. This results in acute inflammation and minute abscess formation in the colon. As abscess ruptures forming ulcers. Ulcers causes in mucus dysentery, abdominal cramps and frequency of bowel movements. Low grade fever and other systemic symptoms also develop.

Direct extension of colon infection to the right lobe of the liver, through the right hemidiaphragm, produces liver abscess and then to right lower lobe consolidation of the lung and lung abscess and pleural effusion.

Complications and sequels.

The following complication may develop -

Colon perforation, toxic megacolon, fulminant colitis - specially in the immune compromised victims. Chronic non-dysentery colitis – specially common in local population. A localized mass formation in the colon called Ameboma. Anal and perianal fistula. In some cases the amoeba borrows through the colon wall and enters blood circulation and distant organs are infected. This may result in peritonitis, ascites, pericarditis and pleurisy. Fallopian tube and uterine amoebic ulcers, left lobe of the liver abscess, abscess of the spleen and the brain.

Naegleria fowleri.

N.fowleri lives in warm water of lakes, ponds and untreated swimming pools. It enters the body through the nose. The amoeba penetrates the cribriform plate of the roof of the nose and enters the base of the brain, and a meningo-encephalitis follows. Because of the presence of a blood brain barrier, no amoeba killing agent can be used to control the infection. This ends in a catastrophe, the fatality rate is over 90 %.

Diagnosis:

Examination of stool for amoeba cysts used to be the only reliable method. In the past over diagnosis of Entamoeba histolytica had happened due to another amoeba, Entamoeba dispar, which is far more common intestinal parasite and it does not cause any human diseases, but the cysts are identical with the pathogenic amoeba species. The PCR can be used to detect amoebic systemic infections. At present, distant infections and the brain infections depend on this test.

Treatment:

In the past Metronidazole was used indiscriminately and now in endemic area the amoeba is resistant to it. Other imidazole compounds also fell to the same fate. Emetine injections are discontinued because of cardiotoxicity. Chloroquine and other synthetic quinine compounds are in use in tissue invasive types of lesions like abscess in the liver etc. Broad spectrum antibiotics are sometimes combined with Chloroquine.

A chat below, taken from CDC showing recommendation for Entamoeba histolytica infection:

Table. Recommended Treatment for Entamoeba histolytica Infection in Adults

Infection Type	Agent	Dosage
Asymptomatic	Paromomycin	25 to 30 mg/kg/d in 3 divided doses for 7 days
Invasive disease	Metronidazole	750 mg 3 times daily for 10 days or 2.4 g once daily for 2–3 days
Invasive disease	Tinidazole	2 g once daily for 3 days
Amebic liver abscesses	Metronidazole	750 mg 3 times daily for 10 days
Amebic liver abscesses	Tinidazole	2 g once daily for 3–5 days
Amebic liver abscesses	Chloroquine*	600 mg once daily for 2 days, then 300 mg once daily for 14 to 21 days

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