Amebiasis

Etiology

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. These organisms are spread via the oral-fecal route. The infected cysts are often found in contaminated food and water. Rare cases of sexual spread have also been reported.

Epidemiology

Amebiasis occurs worldwide but is predominantly 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 100000 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.[4]

Pathophysiology

E. histolytica is a pseudopod-forming, protozoal parasite that causes proteolysis and tissue lysis. Humans are the natural hosts. Amoebic infection occurs by the 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. Ingestion of only a small number of organisms can cause disease.

Histopathology

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. In some patients, flask-shaped ulcers are seen.

History and Physical

Although most cases of amebiasis are asymptomatic, many patients with E. histolytica present with a spectrum of illness. The incubation period from amebiasis is between 2 to 4 weeks.

Symptoms range from mild abdominal cramps and watery diarrhea to severe colitis producing bloody diarrhea with mucus. Young people tend to have a more severe disease compared to older individuals. Fulminant colitis can present with bloody diarrhea in some patients. Risk factors include the use of corticosteroids, poor nutrition, young age, and pregnancy. Toxic megacolon can be a complication and is associated with very high mortality.

A few patients may develop invasive extraintestinal disease. The most common extraintestinal manifestations are an amoebic liver abscess. A liver abscess develops in less than 4% of patients and may occur within 2 to 4 weeks after the initial infection. Liver abscess usually presents with right upper quadrant pain, fever, and tenderness to palpation.

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.

Evaluation

Amebiasis can be diagnosed by a demonstration of the organism using direct microscopy of stools or rectal swabs. However, the organisms are seen in only 30% of patients.

Antigen detection using an enzyme-linked immunosorbent assay and polymerase chain reaction techniques is often done. However, the most promising detection method 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.[5][1]

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.

Blood tests may reveal the following:

• Elevated WBC
• Eosinophilia
• Elevated bilirubin and transaminase enzymes
• Mild anemia
• Elevated ESR

Imaging studies may be required depending on the presentation. Ultrasound can identify a liver abscess.

Treatment / Management

The primary therapy for symptomatic amebiasis requires hydration and the use of metronidazole and/or tinidazole. These two agents are dosed as follows:

• Metronidazole dosing for adults is 500 mg orally every 6 to 8 hours for 7 to 14 days. 
• Tinidazole adult dosing is 2 g orally each day for 3 days.

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.[6][7][8]

Differential Diagnosis

• Colitis caused by E. coli, Yersinia, or Campylobacter
• Pericarditis
• Perforated bowel
• Diverticulitis
• Hepatitis A
• Cholecystitis
• Shigellosis/salmonellosis

Prognosis

If left untreated, amoebic infections have very high morbidity and mortality. In fact, mortality is second only to malaria. Amoebic infections tend to be most severe in the following populations:

• Pregnant women
• Postpartum women
• Neonates
• Malnourished individuals
• Individuals who are on corticosteroids
• Individuals with malignancies

When the condition is treated, the prognosis is good, but recurrent infections are common in some parts of the world. 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, potentially increasing the mortality rate. Amoebic pericarditis and pulmonary amebiasis have a high mortality rate exceeding 20%.

Today with effective treatment, mortality rates are less than 1% in patients with uncomplicated disease. However, rupture of an infected amebic liver abscess carries a high mortality.

Complications

• Toxic megacolon
• Fulminant necrotizing colitis
• Rectovaginal fistula
• Ameboma
• Intraperitoneal rupture of liver abscess
• Secondary bacterial infection
• Extension of infection from the liver into the pericardium or pleura
• Dissemination in the brain
• Bowel perforation
• Stricture of the colon
• Gastrointestinal bleeding
• Empyema