What Is Dysentery? Symptoms, Causes, and How It Differs From Diarrhea

Clinically reviewed by Dr. Ponlawat Pitsuwan, Physician, Doctor Samui. Last reviewed: August 2026

You felt fine at breakfast. By afternoon, after a plate of seafood at a beach bar or a fruit shake from a Chaweng market stall, you are running to the bathroom every hour. That alone could be traveller’s diarrhea. But when there is blood or mucus in the toilet, that changes things.

Dysentery is a tropical gut infection we see regularly at our Chaweng clinic, especially after busy dive weeks, waterfall trips, and street food tours. It is treatable, but the wrong move early on, like reaching for Imodium, can make it significantly worse. Here is what you need to know.

What Is Dysentery?

Dysentery is an infection of the intestines that causes bloody or mucus-filled diarrhea, stomach cramping, and fever. The key word is blood. Watery diarrhea without blood is not dysentery. When blood shows up, the gut lining is being damaged, and that means you need a diagnosis, not just fluids and rest.

One symptom that often goes unmentioned: tenesmus. That is the painful urge to pass stool where you strain and strain but pass almost nothing, or just a small amount of bloody mucus. If that is happening to you right now, it is a strong sign this is dysentery, not ordinary food poisoning.

Amoebic vs Bacillary Dysentery — Which Is More Likely in Koh Samui?

There are two types, and the treatment for each is different. That is exactly why guessing at a cure without a test is not a good idea.

Bacillary dysentery is caused by Shigella bacteria. It hits fast, usually within one to three days of exposure. You get sudden fever, cramps, and bloody stool. This is the more common type in short-stay tourists who have been eating street food or swimming in freshwater.

Amoebic dysentery is caused by a parasite called Entamoeba histolytica. It can take weeks to show up. Some people feel fine on Koh Samui but get sick after they fly home. Symptoms build gradually: cramping that worsens over days, loose stool that eventually turns bloody. Both types need different drugs, which is why a stool test matters before you take anything.

Dysentery Symptoms — What to Look For

The signature symptom is diarrhea with visible blood or mucus. Beyond that: fever, stomach cramps, nausea, and that tenesmus sensation of painful straining with very little to show for it. You may need the bathroom ten or more times a day.

With bacillary dysentery, symptoms come on sharply within one to three days of exposure. With amoebic, the onset is slower, often starting as general stomach discomfort before escalating to bloody stool days or weeks later.

Dehydration builds fast in Koh Samui’s heat. Watch for a dry mouth, dark urine, dizziness when you stand, and no urination for more than six hours. In children, these signs appear faster than most parents expect.

Dysentery vs Diarrhea — How to Tell the Difference

The simple rule: if there is blood in your stool, treat it as dysentery until a doctor tells you otherwise.

Why Dysentery Is a Real Risk in Koh Samui

Koh Samui’s heat and humidity are near-ideal conditions for bacterial growth in food and water. The island has no shortage of exposure points: street food stalls, beach buffets, resort seafood nights, fruit shakes blended with tap water, and ice at open-air bars that may not come from a filtered source.

Freshwater is a risk too. Swimming at Na Muang waterfall or other inland pools can put you in contact with water that carries both Shigella and Entamoeba. Tourists who mix adventurous eating with waterfall swimming are among the most exposed visitors on the island.

Long-stay expats are not immune either. Residents who have been here for months sometimes assume they have built up tolerance. They have not. Amoebic dysentery is endemic in Thailand and does not discriminate between first-timers and long-term islanders.

How Dysentery Is Diagnosed and Treated

A stool culture is the right test. It identifies whether the cause is bacterial or parasitic, which determines which drug you need. At Doctor Samui, we can collect a sample at the clinic or during a hotel visit and send it to the lab.

For bacillary dysentery, ciprofloxacin or azithromycin is typically prescribed. For amoebic dysentery, metronidazole or tinidazole is the standard course. These are completely different drug classes, and taking the wrong one will not help.

One critical point: do not take loperamide, sold as Imodium. It slows down your gut, which sounds useful, but in dysentery it can trap the infection inside and make things significantly worse. If you have blood in your stool or a fever, put the Imodium down and contact a doctor instead.

What You Can Do Right Now While You Wait for a Doctor

Start oral rehydration salts immediately. You can get ORS sachets from any 7-Eleven or pharmacy on Koh Samui. Mix one sachet in 200 ml of bottled water and sip steadily. Do not gulp large amounts if you are nauseous.

Avoid dairy, spicy food, raw fruit, and anything fried. Plain rice and boiled chicken are fine. Drink bottled water only. Skip the ice, and when you are already sick on Koh Samui, assume bar ice is not from a filtered source.

Rest, stay close to a bathroom, and contact a doctor the same day. Not tomorrow morning.

Red-Flag Signs — See a Doctor Immediately

Most cases of dysentery are unpleasant but manageable with prompt treatment. Koh Samui’s heat makes dehydration escalate faster than it would at home.

Go to the Chaweng clinic or call for a hotel visit right away if you have a fever above 39°C, blood in every stool, no urination for six or more hours, or you feel faint when standing. If a child under five is affected, do not wait at all. Young children dehydrate seriously fast and often cannot tell you how bad they feel.

Amoebic dysentery left untreated can rarely spread to the liver and cause an abscess requiring more intensive treatment. That is not common, but it is the reason you do not leave bloody diarrhea untested and hope it passes.

Preventing Dysentery on Koh Samui

Eat food that is freshly cooked and served hot. Avoid buffet seafood that has been sitting out, particularly at busy resort nights where shellfish can be left unrefrigerated. Order fruit shakes from places that use bottled or filtered water, and ask. Most vendors will tell you.

Skip ice at beach bars if you are already feeling off. Wash your hands before every meal, not only after the bathroom. If you are swimming at Na Muang or any freshwater pool, keep your mouth closed.

For a full breakdown of what to expect if your child is the one who is sick, our diarrhea treatment page for Koh Samui covers the clinic visit step by step, including what causes dysentery symptoms and when IV fluids are needed.