Entamoeba histolytica is a microscopic, single-celled parasitic protozoan that causes amoebiasis. This organism colonizes the human gastrointestinal tract, leading to conditions ranging from no symptoms to life-threatening complications. While found worldwide, the parasite is most frequent in tropical and subtropical regions lacking clean water and adequate sanitation. Globally, an estimated 50 million people develop amoebic colitis or extraintestinal disease each year.
How Infection Occurs
Infection with E. histolytica begins with ingesting the parasite’s infective stage, the cyst, primarily through the fecal-oral route. The cyst is a hardy form protected by a wall, allowing it to survive for days or weeks in the external environment, including soil, water, and contaminated food. This resistance, even to common water treatments like chlorination, makes the cyst the sole agent of transmission. Ingesting contaminated water or food, particularly raw produce fertilized with human waste, is the most common means of transmission.
Once inside the host, the cyst passes through the stomach and excysts in the small intestine, releasing the active, disease-causing form called the trophozoite. Trophozoites migrate to the large intestine, where they multiply and can either stay within the intestinal lumen or begin to invade the tissue. Some trophozoites transform back into the cyst stage, which is then shed in the feces to continue the transmission cycle. Direct person-to-person spread is also possible through poor hygiene practices or oral-anal contact.
Recognizing the Signs of Amoebiasis
The outcome of an E. histolytica infection varies widely. Approximately 90% of infected individuals remain asymptomatic carriers, passing cysts without experiencing illness. This noninvasive or luminal amoebiasis means the parasite remains confined to the intestine and does not breach the tissue barrier. These carriers continuously shed infectious cysts, posing a public health risk. When symptoms appear, they typically develop within two to four weeks following exposure.
The symptomatic form begins with intestinal amoebiasis, often called amoebic colitis. Trophozoites invade the lining of the large intestine, using specialized enzymes to degrade the tissue. This invasion results in symptoms including abdominal pain, severe diarrhea, and tenesmus (the feeling of incomplete evacuation). Extensive invasion progresses to amoebic dysentery, marked by frequent, watery stools containing visible blood and mucus, sometimes accompanied by fever.
In more severe cases, trophozoites penetrate the intestinal wall and travel through the bloodstream, leading to extraintestinal amoebiasis. The most frequent complication is the development of an Amoebic Liver Abscess (ALA), occurring when parasites lodge in the liver via the portal vein. Symptoms of an ALA include fever, right upper abdominal pain, and sometimes referred pain to the right shoulder. Liver abscesses can develop without preceding symptoms of intestinal dysentery. Less commonly, the parasite spreads to distant sites, including the lungs, brain, or skin.
Testing and Eradication
Confirming an E. histolytica infection requires specific laboratory testing, as symptoms mimic other intestinal illnesses. Diagnosis for intestinal amoebiasis often begins with a stool Ova and Parasite (O&P) examination under a microscope to look for cysts or trophozoites. This method has limitations because E. histolytica is morphologically identical to the non-pathogenic species, E. dispar, which does not require treatment.
Specific and sensitive methods are preferred to distinguish the pathogenic species. Advanced diagnostic tools include antigen detection tests, which identify specific proteins of E. histolytica in the stool. Polymerase Chain Reaction (PCR) testing is the most accurate method, detecting the parasite’s unique genetic material for clear differentiation between species. For suspected extraintestinal disease, such as an Amoebic Liver Abscess, diagnosis relies on imaging, like ultrasound or CT scans, to visualize the abscess.
A blood test, known as serology, detects antibodies against E. histolytica and is useful for confirming extraintestinal infection. Treatment involves a dual-drug regimen designed to eliminate parasites in both the tissue and the intestinal lumen. Invasive disease, including dysentery or liver abscess, is treated first with a tissue-active drug (e.g., metronidazole or tinidazole) to kill trophozoites in the bloodstream and tissues. This is followed by a luminal agent (e.g., paromomycin or iodoquinol), which acts directly on parasites remaining in the intestine to clear residual cysts and prevent recurrence.