Amebiasis or amoebic dysentery is a common parasitic enteral infection. It is caused by any of the amoebas of the Entamoeba group. Amoebiasis may present with no symptoms or mild to severe symptoms including abdominal pain, diarrhea, or bloody diarrhea. Severe complications may include inflammation and perforation resulting in peritonitis. People affected may develop anemia.
If the parasite reaches the bloodstream, it can spread through the body and end up in the liver causing amoebic liver abscesses. Liver abscesses can occur without previous diarrhea. Diagnosis is typically by stool examination using a microscope. An increased WBC count may be present. The most accurate test is specific antibodies in the blood.
Prevention of amoebiasis is by improved sanitation. Two treatment options are possible, depending on the location. Amoebiasis in tissue is treated with metronidazole, tinidazole, nitazoxanide, dehydroemetine or chloroquine. A luminal infection is treated with diloxanide furoate or iodoquinoline. Effective treatment may require a combination of medications. Infections without symptoms require treatment, but infected individuals can spread the parasite to others.
Amoebiasis is present all over the world. Each year, about 40,000 to 110,000 people die from amoebiasis infection.
The protozoan Entamoeba histolytica causes amebiasis. There are three species of intestinal amoebas. Entamoeba histolytica causes most symptomatic diseases. Entamoeba dispar is nonpathogenic, and Entamoeba moshkovskii is reported increasingly, but its pathogenicity is unclear.
Amebiasis occurs worldwide but predominantly is seen in developing countries due to decreased sanitation and increased fecal contamination of water supplies. Globally, approximately 50 million people contract the infection, with over 100,000 deaths due to amebiasis reported annually. The principal source of infection is ingestion of water or food contaminated by feces containing E. histolytica cysts. Hence, travelers to developing countries can acquire amebiasis when visiting the endemic region. Those who are institutionalized or immunocompromised are also at risk. The organism E. histolytica is viable for prolonged periods in the cystic form in the environment. It can also be acquired after direct inoculation of the rectum, from anal or oral sex, or from equipment used for colonic irrigation. Despite the global public health burden, there are no vaccines or prophylactic medications to prevent amebiasis.
E. histolytica is a pseudopod-forming, protozoal parasite that causes proteolysis and tissue lysis. Humans are the natural hosts. Amoebic infection occurs by ingestion of mature cysts in fecally-contaminated food or water or from the hands. Excystation of the mature cysts occurs in the small intestine and trophozoites are released; the trophozoites then move to the large intestine. The trophozoites increase by binary fission and produce cysts. Both stages pass in the feces. The cysts can survive days to weeks in the external environment because of the protection provided by the cyst wall. The cyst is responsible for further transmission of the parasite.
Histology of the intestinal infection is nonspecific. It usually reveals discrete ulcers, mucosal thickening, and edematous mucosa. Sometimes flask-shaped ulcers may be seen in the submucosal layers.
Although most cases of amebiasis are asymptomatic, many patients with E. histolytica present with a spectrum of illness. Symptoms range from mild abdominal cramps and watery diarrhea to severe colitis producing bloody diarrhea with mucus. A few patients may develop an invasive extraintestinal disease. The most common extraintestinal manifestations are an amoebic liver abscess. An amoebic liver abscess may rupture into the pleural cavity or pericardium, presenting as pleural or pericardial effusion; however, this is a rare occurrence. Rarely, amebiasis may affect the heart, brain, kidneys, spleen, and skin. One can also develop proctocolitis, toxic megacolon, peritonitis, brain abscess, and pericarditis. Hence, amebiasis is a leading parasitic cause of death in humans.
Amebiasis can be diagnosed by a demonstration of the organism using direct microscopy of stools or rectal swabs. Antigen detection using an enzyme-linked immunosorbent assay and polymerase chain reaction techniques is often done. However, the most promising method of detection is the loop-mediated isothermal amplification assay because of its rapidity, operational simplicity, high specificity, and sensitivity. An ultrasound or CT scan evaluates for extraintestinal amebiasis.
Cultures can be done from fecal or rectal biopsy specimens or liver aspirates. Cultures are not always positive, with a success rate of about 60%.
Liver aspiration using CT-guided imaging is often performed when there is a collection in the liver. The liver aspiration usually reveals a chocolate-like or thick, dark viscous fluid. Liver aspiration is indicated when the abscess is large or there is a threat of imminent rupture.
A colonoscopy is done to obtain scrapings of the mucosal surface. It is appropriate when the stool studies are negative for amebiasis.
The primary therapy for symptomatic amebiasis requires hydration and use of metronidazole and/or tinidazole. Luminal agents such as paromomycin and diloxanide furoate are also used. An amoebic liver abscess can be managed by aspiration using CT guidance in combination with metronidazole. Surgery is sometimes required to treat massive gastrointestinal bleeding, toxic megacolon, perforated colon, or liver abscesses not amenable to percutaneous drainage.
If left untreated, amoebic infections have very high morbidity and mortality. In fact, the mortality is second only to malaria. Amoebic infections tend to be most severe in the following populations:
When the condition is treated, the prognosis is good, but in some parts of the world, recurrent infections are common. The mortality rates after treatment are less than 1%. However, amoebic liver abscesses may be complicated by an intraperitoneal rupture in 5% to 10% of cases, which can increase the mortality rate. Amoebic pericarditis and pulmonary amebiasis have a high mortality rate exceeding 20%.
Once the diagnosis of amebiasis is made, consult with the general surgeon, gastroenterologist, and infectious disease specialist.
Prevent amebiasis by maintaining good personal hygiene, sanitation, and avoiding high-risk sexual practice. The E. histolytica cysts are relatively resistant to disinfection of water with chlorine. Drinking boiled or bottled water is advised.