Amoebic dysentery is a severe intestinal infection caused by a single-celled parasite called Entamoeba histolytica. It produces bloody diarrhea, stomach pain, and fever, and is estimated to kill more than 55,000 people worldwide each year. The parasite spreads through contaminated water, food, or contact with infected fecal matter, and it’s most common in tropical and subtropical regions with limited sanitation.
Most people who swallow the parasite never get seriously ill. Many become asymptomatic carriers, passing infectious cysts in their stool without realizing it. But when the parasite invades the intestinal wall, the result is the painful, bloody form of the disease known as amoebic dysentery.
How the Parasite Causes Damage
Infection begins when you swallow mature cysts of the parasite, typically in contaminated water or unwashed food. These cysts have tough outer walls that protect them through the acidic environment of your stomach. Once they reach the small intestine, each cyst breaks open and releases active parasites called trophozoites, which travel to the large intestine and begin multiplying.
In many cases, the parasites stay in the intestinal cavity without causing harm. But when they turn invasive, they penetrate the mucous lining of the colon and kill epithelial cells using specialized proteins that punch holes in cell membranes. This creates deep, flask-shaped ulcers that spread beneath the surface of the intestinal wall and cause small hemorrhages. That tissue destruction is what produces the bloody stool characteristic of amoebic dysentery.
Symptoms to Recognize
Mild amebiasis (the broader infection) can cause loose stools, stomach cramping, and general discomfort. Amoebic dysentery is the more severe form, with three hallmark symptoms: abdominal pain, bloody or mucus-streaked stool, and fever. Episodes of diarrhea may alternate with periods of constipation, and the pain often concentrates in the lower abdomen.
Symptoms can take anywhere from a few days to several weeks to appear after exposure. This long, variable incubation period means people sometimes develop the illness well after returning from travel to a high-risk area. Without treatment, symptoms can persist for weeks or flare repeatedly.
How It Differs From Bacterial Dysentery
The word “dysentery” simply means diarrhea with blood or mucus, and it has two main causes. Amoebic dysentery is caused by a parasite. Bacillary dysentery is caused by bacteria, most commonly Shigella. The distinction matters because the treatments are different.
Bacillary dysentery tends to come on more suddenly, with high fever and frequent watery stools that quickly turn bloody. Amoebic dysentery often develops more gradually, with intermittent symptoms that can wax and wane over weeks. A stool test is usually needed to tell them apart definitively, since the symptoms overlap considerably.
Diagnosis
The most common first step is microscopic examination of a stool sample. A technician looks for the parasite’s cysts or active forms under a microscope. The challenge is that E. histolytica looks nearly identical to a harmless relative called E. dispar, which doesn’t need treatment. The only way to tell them apart under a microscope is if the parasites have visibly ingested red blood cells, a feature considered the classic sign of the disease-causing species.
Because microscopy alone often can’t distinguish between the two species, reference laboratories use DNA-based testing (PCR) as the preferred method for a definitive diagnosis. Antigen detection tests on stool samples offer another option. These molecular tools are especially important for avoiding unnecessary treatment in people carrying the harmless species, and for confirming the diagnosis in ambiguous cases. Stool culture followed by enzyme analysis has been considered the gold standard, though it’s not widely available and isn’t perfectly sensitive.
Treatment
Amoebic dysentery requires a two-phase treatment approach. The first phase targets the active parasites invading the intestinal wall. Doctors typically prescribe a course of antiparasitic medication lasting 3 to 5 days, depending on severity. The second phase uses a different type of medication to eliminate any cysts lingering in the intestinal cavity. Skipping this second step is a common reason for relapse, because surviving cysts can reactivate the infection.
Most people with uncomplicated intestinal disease recover fully with appropriate treatment. Symptoms generally begin improving within a few days of starting medication, though completing the full course of both phases is essential.
When the Infection Spreads Beyond the Gut
The most concerning complication is a liver abscess. Roughly 2% to 5% of people with intestinal amebiasis develop this, which happens when parasites penetrate blood vessels in the colon wall and travel to the liver. About 80% of patients with a liver abscess develop symptoms within 2 to 4 weeks of exposure, including fever, a dull ache below the right rib cage, and cough. Weight loss is common in cases that develop more slowly.
Only 10% to 35% of people with a liver abscess also have active gut symptoms like diarrhea, which can make the diagnosis tricky. Stool microscopy catches the parasite in only 10% to 40% of liver abscess cases. Instead, diagnosis relies on imaging (ultrasound, CT, or MRI) combined with blood tests for antibodies against the parasite. These antibody tests are over 95% sensitive once the illness has been present for more than a week, though they can miss early cases.
Liver abscesses respond well to the same antiparasitic medications used for intestinal disease, sometimes with a longer course. Draining the abscess is usually unnecessary. In rare cases, the parasite can also reach the lungs or brain, though this is uncommon.
How It Spreads and How to Avoid It
The parasite’s cysts are the infectious form. They’re shed in the stool of infected people (including asymptomatic carriers) and can survive days to weeks in the environment, particularly in water. The active trophozoite form dies quickly outside the body and can’t survive stomach acid, so it’s not a transmission risk.
Contaminated drinking water is the primary route of infection. The cysts are resistant to standard chlorination at the levels used in many municipal water systems, which is why the disease persists in areas with treated but inadequately purified water. Boiling water effectively kills cysts. Filtering through a fine enough filter also works. In endemic areas, uncooked vegetables washed in contaminated water and fruits that aren’t peeled are common sources.
For travelers to regions where the infection is common, practical steps include drinking only bottled or boiled water, avoiding ice in drinks, peeling fruits yourself, and skipping raw salads. Handwashing with soap is effective at removing cysts from skin, making it a simple but critical prevention measure, especially after using the bathroom and before preparing food.