Amoebiasis

Amoebiasis is a disease caused by Entamoeba histolytica, a protozoa which is found worldwide. Humans are the natural reservoir of E. histolytica, and infection occurs via faecal-oral transmission (e.g. contaminated hands, water, or food, and oral-anal sex). There is a higher incidence of amoebiasis in developing countries where barriers between human faeces and food and water supplies are inadequate. Risk factors for amoebiasis in developed countries include travellers to endemic regions, men who have sex with men, and immunosuppressed or institutionalised people.

The life cycle of E. histolytica includes the formation of cysts and trophozoites, both of which are passed in faeces. Cysts can survive days to weeks in the external environment and are mainly responsible for transmission of disease.

Annually an estimated 50 million people are infected by invasive E. histolytica, leading to 100, 000 deaths.

What are the symptoms of amoebiasis?

Many cases of amoebiasis are asymptomatic with the cysts and trophozoites remaining confined to the intestinal lumen (inside the tube of the intestine). However in some patients the trophozoites invade the intestinal mucosal wall leading to bloody diarrhoea and colitis. The trophozoites can also invade the bloodstream and spread the infection to other organs including the liver (most common), lung, heart, brain, and skin.

Cutaneous amoebiasis is very rare, but easily diagnosed and treated. E. histolytica can spread to the skin and mucous membranes either by:

• contiguous spread of internal disease to neighbouring regions – e.g. rectal amoebiasis may spread to the perianal skin, vulva, or the penis of sexual contacts during anal intercourse; liver abscesses may extend to the skin of the abdominal wall
• external inoculation with contaminated hands – e.g. to the face

Diagnosis

The most common method of diagnosis is microscopic identification of E. histolytica cysts and trophozoites in faeces, liver abscess aspirates, or biopsy samples. Note: E. histolytica cannot be distinguished microscopically from the harmless E. dispar organism. Confirmation of E. histolytica infection requires serology, antigen detection, or identification of E. histolytica genetic material:

• Serology assays (blood tests) can detect E. histolytica immunoglobulins (antibodies) that are produced in response to the infection. Detectable E. histolytica-specific antibodies may persist for years after successful treatment, so the presence of antibodies does not necessarily indicate current infection
• E. histolytica antigens can be detected from stool samples
• Polymerase chain reaction can also confirm the diagnosis by identifying E. histolytica genetic material from faeces, biopsy specimens, and liver abscess aspirates

Treatment

• Asymptomatic E. histolytica infection should be treated with a luminal agent, such as iodoquinol, paromomycin, or diloxanide furoate. (These agents are not registered in New Zealand but may be obtained through their manufacturers). The aim of treatment in these patients is to eradicate infection and prevent further shedding of cysts into the environment
• Treatment of invasive disease usually consists of oral or intravenous metronidazole, immediately followed by iodoquinol, paromomycin, or diloxanide furoate to eradicate colonisation of the intestine
• Alternative treatments include tinidazole, emetine hydrochloride, and pentamidine

Following treatment, invasive amoebiasis carries a good prognosis. Fulminant colitis and liver abscess rupture are associated with higher mortality rates.

Prevention

• Prevent faecal contamination of food and water through improved sanitation, hygiene, and water treatment
• In endemic areas, water should be boiled for more than one minute, and uncooked vegetables should be washed with a detergent soap and soaked in acetic acid or vinegar for 10-15 minutes before consumption
• Avoid sexual practices that involve faecal-oral contact
• Screen family members or close contacts of an index case