The liver fluke Opisthorchis viverrini is a small parasite that people catch by eating raw or undercooked freshwater fish, and in northeast Thailand it is the main cause of bile-duct cancer. The risk lies in a few specific raw-fish dishes, such as koi pla and uncooked pla ra, eaten again and again over years; fish that is cooked through does not carry it.
For a visitor, the practical message is simple. Enjoy Isaan food, but skip the dishes made with raw freshwater fish, and ask whether the fermented fish in a dish has been cooked. One meal will not give you cancer. It can, however, give you an infection that lasts for years if it is not found and treated, and that is worth avoiding.
This page explains the parasite, the dishes and the numbers from Thailand's Department of Disease Control (DDC), the World Health Organization (WHO) and the US Centers for Disease Control and Prevention (CDC). It is general information, not medical advice. For pla ra as a flavor, and for what else to eat in the region, see our guide to Thai dishes by region.
The Southeast Asian liver fluke
- Parasite
- Opisthorchis viverrini, a small flat worm living in the bile ducts
- Where it occurs
- Mainly northeast Thailand, Laos, Cambodia and central and southern Vietnam (CDC)
- How people catch it
- Raw or undercooked freshwater fish, including lightly salted, pickled or smoked fish
- Symptoms
- Usually none; sometimes indigestion, abdominal pain or jaundice (CDC)
- Long-term risk
- Chronic infection over many years is linked to bile-duct cancer (cholangiocarcinoma)
- Sources checked
- DDC, WHO and CDC pages, 19 October 2026
How the parasite gets from a river into a person
The CDC describes the cycle. Fluke eggs reach rivers, ponds and rice-field water, where freshwater snails eat them. The snails release tiny larvae, and the larvae burrow into freshwater fish, especially small fish of the carp family. When a person eats that fish raw or undercooked, the larvae survive, travel to the bile ducts of the liver and grow into adult flukes over about four months. Adult flukes can live for several years.
You cannot catch it from another person or from drinking water, according to the CDC. It comes from what is on the plate. Thorough cooking kills the larvae; the CDC gives a core temperature of at least 63 °C for fish.
The dishes that carry the risk
The DDC report on liver-fluke larvae in freshwater fish names the dishes that spread the infection. All of them are made with freshwater fish, or with pla ra (fermented fish), that has not been cooked through:
- Koi pla, a salad of raw, chopped freshwater fish with herbs, lime and chili.
- Raw fish larb, minced raw fish dressed like the meat version.
- Som tam with raw pla ra, papaya salad made with uncooked fermented fish.
- Jaew bong and other chili dips made with raw pla ra.
- Pla som and pla jom, lightly fermented or pickled fish eaten without cooking.
Fermenting and salting do not make raw fish safe. The CDC states plainly that lightly salted, smoked or pickled fish can still contain infectious parasites. This is the claim most often got wrong online, so it is worth remembering.
"Fermented" is not the same as "cooked". Pla ra can go into a dish raw or cooked, depending on the dish and the cook, and you cannot tell by looking. We could not find an official source confirming whether bottled, factory-made pla ra is pasteurized, so if you want to be sure, ask "pla ra suk mai?" (is the pla ra cooked?) or order the dish without it.
What the numbers say about Isaan
The DDC calls the liver fluke the main cause of cholangiocarcinoma in Thailand. Its figures, which are older and should always be read with their year, show the scale:
| Measure (DDC) | Figure |
|---|---|
| National infection rate, surveys | 11.8 % (1996), 8.7 % (2009), 5.1 % (2014) |
| People in the Northeast who had or had had the infection | An estimated 6 million+ (2009) |
| Deaths from liver and bile-duct cancer | About 15,900 a year (MOPH data 2009-2018) |
| Liver and bile-duct cancer mortality | 24.8 per 100,000 (2019); the leading cancer death in Thai men |
| Age of patients | About 55 % aged 40 to 60 |
The 5.1 percent from 2014 is the latest national figure in an official DDC document; we found no newer one. The trend is downward. Thailand ran a national "decade to eliminate liver fluke and cholangiocarcinoma" from 2016 to 2025, aiming to cut fish infection below 1 percent and to halve bile-duct cancer deaths by 2035. In its own 2019 survey of 4,711 carp-family fish from high-risk areas, the DDC found no larvae of this liver fluke, although it did find a smaller intestinal fluke in 12.3 percent of fish. One survey does not make raw fish safe, and the DDC does not say it does.
The problem is not unique to Thailand. The WHO notes that some villages in neighboring Laos have had infection rates above 80 percent, which matters if your trip continues over the Mekong from Nong Khai.
Why a single taste is a low risk, and a habit is not
The cancer risk comes from long-standing inflammation of the bile ducts, caused by infection that persists or keeps coming back over many years. The DDC links cholangiocarcinoma to more than 20 years of repeated infection, and both the CDC and the WHO describe chronic infection as the danger. The WHO classifies this fluke as a cause of cancer in humans.
A visitor who tries a spoonful of koi pla at a village celebration is in a very different position from someone who has eaten raw fish dishes every week since childhood. It is still better not to eat it, because even one meal can cause an infection. But if you already have, there is no reason to panic: the WHO says safe and effective medicines exist, and a doctor can test you.
Safer ways to eat fish in Isaan
You do not have to give up fish. The risk comes from raw freshwater fish, so the choices are easy:
- Grilled whole fish, cooked through until the flesh is opaque and flakes from the bone.
- Fish in hot soups and curries that have been boiled, not added at the last moment.
- Som tam thai, the sweeter papaya salad without pla ra, or a version where the vendor confirms the pla ra is cooked.
- Larb and nam tok made with cooked pork, chicken or beef rather than raw fish.
For everyday hygiene at stalls, such as turnover, ice and food that has sat out, see our article on avoiding stomach trouble from street food. If you have trouble explaining what you want, our page on ordering Thai food has useful phrases for asking about ingredients.
Symptoms, testing and when to see a doctor
Most infected people have no symptoms at all, the CDC says. When symptoms appear, they can include indigestion, nausea, abdominal pain, diarrhea or constipation, fever, an enlarged liver and jaundice. These are common symptoms with many causes, so they do not tell you on their own that you have a fluke. Sudden vomiting or diarrhea soon after a meal usually has other causes; our page on food poisoning and diarrhea in Thailand covers that.
Tell the doctor about raw fish in Isaan or Laos. See a doctor promptly if your skin or the whites of your eyes turn yellow, if you have pain under the right ribs that does not go away, or fever together with yellowing. Mention that you ate raw or fermented freshwater fish, and where. The CDC says the infection is usually found by looking for eggs in a stool sample, sometimes more than one, and that this test may not help if the exposure was many years ago.
In Thailand, a private or government hospital can arrange the tests; our guide to hospitals in Thailand explains how visitors pay. Back home, tell your own doctor where you traveled. In the Northeast, Khon Kaen University's cholangiocarcinoma institute and Srinagarind Hospital run screening for local people with a urine test and, from age 40, an ultrasound, according to a Thai news report; that program is aimed at residents of the region.
If your trip takes in the regional cities, our guides to Khon Kaen and Udon Thani cover the practical side, and the Isaan guide covers the whole region.
Frequently asked questions
Is koi pla safe to eat?
Not from a parasite point of view. Koi pla is made from raw freshwater fish, and Thailand's Department of Disease Control names it as one of the dishes that spread the liver fluke. Lime juice and chili do not kill the larvae; only proper cooking does.
Does fermenting fish kill liver flukes?
No, not reliably. The CDC says lightly salted, smoked or pickled fish can still contain infectious parasites. Pla ra that has been boiled or cooked is a different matter, so ask whether it is cooked, or order the dish without it.
Will eating raw fish once in Thailand give me cancer?
Very unlikely. The cancer link comes from chronic infection over many years, often decades of repeated exposure. A single meal can still cause an infection, so if you develop jaundice or lasting pain under the right ribs, see a doctor and mention the raw fish.
Is sea fish sushi in Bangkok a liver fluke risk?
This liver fluke comes from freshwater fish, according to the CDC, so it is not the concern with sea fish. Raw seafood carries other food-safety risks, which our street-food hygiene article covers in general terms.
Sources
- Department of Disease Control, Ministry of Public Health – Report on liver-fluke larvae in freshwater fish (Thai) (checked October 2026)
- World Health Organization – Foodborne trematode infections (fact sheet) (checked October 2026)
- US Centers for Disease Control and Prevention – About Opisthorchis (checked October 2026)
- Thansettakij – Liver-fluke and cholangiocarcinoma screening program in the Northeast (Thai) (checked October 2026)