Amebiasis (Amoebiasis) is a term for disease caused by the protozoan parasite Entamoeba histolytica that primarily infects the human bowel, but can also affect other organs (most commonly the liver). Entamoeba histolytica can exist in two forms – as a short-lived motile trophozoite with the ability to invade different organ systems, and as a long-surviving cyst that is able to colonize a patient.
Amebiasis can be found all around the world, although the highest burden is observed in developing countries. The World Health Organization estimates that each year approximately 50 million people worldwide suffer from an invasive form of amebiasis, leading to almost 100 thousand deaths. Therefore, Entamoeba histolytica is second only to the Plasmodium parasite (responsible for malaria) as a protozoan cause of death.
When the parasitic cyst is swallowed, it can cause an infection by excysting to the trophozoite stage in the human gastrointestinal tract. Different clinical classifications of amebiasis are based on the invasiveness and the site of infection, as well as response to different treatments.
Intestinal amebiasis represents an umbrella term for the whole spectrum of clinical intestinal diseases – including amebic colitis. Patients with the latter condition typically present with cramping abdominal pain present for several weeks, weight loss, as well as watery or (sometimes) bloody diarrhea.
The insidious onset, variable symptoms, and a plethora of differential diagnostic possibilities hinder swift diagnosis of this disease. Potential unusual manifestations of amebic colitis include ameboma, toxic megacolon, acute necrotizing colitis, cutaneous amebiasis, and perianal ulcerations (with fistula formation).
Amebic Liver Abscess
Amebic liver abscess is ten times more commonly observed in men than in women, while at the same time it is very rare in children. It must be emphasized that in a majority of patients with an amebic liver abscess there is no coexistent intestinal amebiasis.
Roughly 80% of patients with this condition present with acute symptoms of cough, fever, and dull abdominal pain in the right upper quadrant or epigastrium. If diaphragmatic surface of the liver is involved, right pleural pain or shoulder pain can be observed. Enlargement of the liver with point tenderness is a typical finding.
Amebic liver abscess can result in several complications if the abscess ruptures with extension into the peritoneum, pericardium, or pleural cavity. Furthermore, extrahepatic amebic abscesses (most notably in the brain, lung, and skin) have been described in the medical literature.
Diagnosis and Treatment of Amebiasis
Due to non-specific and often unreliable signs and symptoms, the diagnosis of amebiasis is confirmed in the microbiology or parasitology laboratory. It is pivotal to use the tests that can distinguish Entamoeba histolytica from morphologically identical (but non-pathogenic) Entamoeba dispar.
Enzyme-linked immunosorbent assay-based antigen detection tests that are specific for Entamoeba histolytica and polymerase chain reaction are used in modern laboratories. The detection of serum antibodies to amebae can aid in the diagnosis, albeit the patients can remain positive for years after infection, making it very difficult to distinguish new from past infections.
Non-invasive forms of amebiasis can be treated with lumen active agents such as paromomycin to eradicate trophozoites and cysts, whereas nitroimidazoles (particularly metronidazole) remain the dominant treatment approach in invasive amebiasis. Second-line agents include luminal amebicide diloxanide furoate, and antibiotics should be used in rare cases of fulminant amebic colitis.
Although improved sanitation could potentially lead to the eradication of amebiasis, it is not very likely that the whole world will be disease free in the foreseeable future; hence alternative measures should be pursued. One such approach is vaccine development that has shown significant progress, but also posed unanswered questions on the effectiveness of immune response, testing the vaccine in humans, and tackling major economic barriers.
Reviewed by Susha Cheriyedath, MSc