Entamoeba histolytica is a single-celled parasite that infects the human intestine and causes a disease called amebiasis. Around 50 million people develop symptomatic infections each year, and roughly 100,000 die from it, making it the third leading cause of death among parasitic infections worldwide. Most people who carry the parasite never feel sick, but in a minority of cases it can invade the intestinal wall and spread to other organs, most commonly the liver.
How the Parasite Works
E. histolytica exists in two forms during its life cycle. The first is a hardy, dormant shell called a cyst, which measures about 12 to 15 micrometers across (far too small to see without a microscope). Cysts are the infectious form. You swallow them through contaminated water or food, and they pass through stomach acid intact. Once they reach the lower part of the small intestine, each cyst opens and releases the second form: active, feeding cells called trophozoites.
Trophozoites are slightly larger, typically 15 to 20 micrometers but sometimes stretching up to 60. They colonize the large intestine, where they multiply by simple division. In many people, they live quietly in the gut without causing damage. But when they do turn invasive, they burrow into the intestinal lining, creating flask-shaped ulcers. The trophozoites can also enter the bloodstream and travel to the liver or, rarely, to the lungs or brain. Meanwhile, some trophozoites in the gut re-form into cysts, which pass out in stool and can infect the next person.
How It Spreads
Transmission follows a fecal-oral route. That means the cysts from an infected person’s stool end up being swallowed by someone else, usually through drinking water contaminated with sewage or eating food washed or prepared with that water. The parasite is most common in tropical and subtropical regions with limited sanitation infrastructure, and travelers to these areas are at elevated risk.
One reason E. histolytica is difficult to control is that its cysts are remarkably tough. They can survive for several months in water at freezing temperatures and are extremely resistant to standard chlorine levels used in water treatment. Heat is more effective: cysts die within 30 minutes at 45°C (113°F) and within 5 minutes at 50°C (122°F). Boiling water reliably kills them.
Symptoms of Intestinal Infection
About 90% of people infected with E. histolytica have no symptoms at all. They carry the parasite and shed cysts in their stool without knowing it, which is a major reason the infection keeps spreading.
For the roughly 10% who do get sick, symptoms usually appear within 2 to 4 weeks of swallowing the cysts, though they can show up later. Mild cases involve loose stools and cramping abdominal pain. The more severe form, called amebic dysentery, produces bloody stool, significant stomach pain, and fever. Because the trophozoites physically destroy tissue in the colon wall, the bleeding can be substantial. Symptoms sometimes come and go over weeks, which can make the infection easy to dismiss early on.
Liver Abscess and Other Complications
The most serious complication is an amebic liver abscess, which occurs when trophozoites travel from the intestine to the liver through the portal vein. This can happen weeks or even months after the initial gut infection, and sometimes the person never had noticeable intestinal symptoms at all.
A liver abscess typically causes pain in the upper right side of the abdomen, fever, and general fatigue. If a doctor drains the abscess (which is only done in select cases), the fluid has a distinctive thick, odorless, chocolate-brown appearance often described as “anchovy paste.” Blood tests for antibodies against the parasite are over 95% sensitive in established infections, though they can miss early cases during the first week of illness. Imaging with ultrasound or CT scan usually reveals the abscess clearly.
Left untreated, a liver abscess can rupture into the chest cavity or abdominal space, which is life-threatening. With appropriate treatment, though, most people recover fully.
How It’s Diagnosed
Diagnosing E. histolytica is trickier than it sounds, because under a standard microscope it looks identical to a harmless relative called Entamoeba dispar. The two species produce cysts and trophozoites of the same size and shape. The only visual clue that distinguishes them is the presence of ingested red blood cells inside the trophozoites, a feature specific to E. histolytica, but this isn’t always visible.
When a lab can’t tell the two apart visually, the report will read “E. histolytica/E. dispar,” meaning the sample could be either species. To confirm a true E. histolytica infection, doctors rely on antigen detection kits or molecular tests that identify genetic material specific to the pathogenic species. Standard stool microscopy alone has limited sensitivity, catching only 10% to 40% of liver abscess cases, for example, because trophozoites may not be present in stool by that stage.
Treatment
Amebiasis is treated in two steps. The first targets trophozoites that have invaded tissue, using an antibiotic that penetrates the intestinal wall and liver. The second step uses a different medication that works only inside the gut to eliminate any remaining cysts and prevent the person from continuing to spread the parasite. Both steps are necessary because no single drug handles both forms effectively.
For people who carry cysts without symptoms, only the gut-targeted medication is needed. Treatment is generally well tolerated, and most people recover without lasting effects. Liver abscesses usually respond to medication alone; drainage is reserved for large abscesses or cases that don’t improve.
Who Is Most at Risk
People living in areas with poor sanitation bear the greatest burden. The parasite circulates widely in parts of Central and South America, sub-Saharan Africa, and South Asia. Travelers to these regions, especially those eating street food or drinking untreated water, face meaningful risk. People living in institutional settings with shared facilities and those who are immunocompromised also have higher rates of infection and more severe outcomes.
Because asymptomatic carriers silently pass cysts, the parasite persists even in communities where only a fraction of infected people become visibly ill. Boiling drinking water, thorough handwashing, and careful food preparation remain the most effective ways to break the cycle of transmission.