Title: Cholangiocarcinoma Thailand: The Liver Fluke Behind the Rates
Author: Arthur Harmash
Published: 2026-10-08
Source: https://lymyvoyage.com/articles/cholangiocarcinoma-thailand-the-liver-fluke-behind-the-rates
Citations:
- https://wjgnet.com/1007-9327/full/v23/i39/7160.htm
- https://pmc.ncbi.nlm.nih.gov/articles/PMC12541340/
- https://pmc.ncbi.nlm.nih.gov/articles/PMC4130346
- https://pmc.ncbi.nlm.nih.gov/articles/PMC4459438
- https://inchem.org/documents/iarc/vol61/m61-2.html
- https://www.cancer.org/cancer/types/bile-duct-cancer/about/key-statistics.html
- https://www.cancer.org/cancer/types/bile-duct-cancer/detection-diagnosis-staging/survival-by-stage.html
- https://bmjopen.bmj.com/content/9/3/e023217
- https://doi.org/10.1016/j.hpb.2019.10.010
- https://europepmc.org/article/PMC/PMC11458967
- https://europepmc.org/article/PMC/PMC11526638
- https://www.cdc.gov/liver-flukes/hcp/clinical-overview-opisthorchis/index.html
---
Cholangiocarcinoma, cancer of the bile ducts, is rare in most Western countries. A 2017 nationwide study from Thailand cited rates of 1.2 cases per 100,000 people in the UK and 1.6 in the US.[^1] It put Thailand's rate at 22.9.[^1]
In Thailand's north-east, rates reported from the Khon Kaen province registry are higher still. Researchers link most of that gap to one cause, a liver fluke caught from raw freshwater fish.[^2] This article looks at how strong that link is, and what it means for travellers and people with roots in the region.
Bile duct cancer hits about 1.6 in 100,000 Americans a year. A north-east Thai registry reported 94.8 per 100,000 men. Is a raw-fish parasite really why?
The parasite is the liver fluke Opisthorchis viverrini. The World Health Organization's cancer agency has classed it as a cause of cancer in humans since 1994.
A 2025 model of north-east Thai data attributed 92% of bile duct cancers diagnosed from 1997 to 2009 to the fluke. Infection usually started in childhood.
The counterargument: the published rates disagree. Khon Kaen figures range from about 40 to 135 per 100,000, depending on the years, the sex and the method.
What the data cannot show: a risk for a short-term traveller. The studies measured people infected for decades, starting in early childhood.
The fluke drives the Thai numbers, and ultrasound screening there finds more tumours early. If you eat raw freshwater fish in the region, tell your doctor. Save this.
## How far apart the rates are
The 2017 study used hospital admissions across Thailand from 2008 to 2013. It counted 39,421 patients with cholangiocarcinoma.[^1] Its rates are crude, meaning not adjusted for age, and they vary sharply by region. The north-east recorded about 29 cases per 100,000, the south about 3.[^1]
The north-east's provincial registry reports much higher rates. A 2008 review gave age-standardised rates in Khon Kaen of 94.8 per 100,000 men and 39.4 per 100,000 women, for 1988 to 1991.[^3] A 2015 profile of the province's screening programme cited 135.4 for men and 43.0 for women.[^4]
These figures do not contradict each other so much as measure different things. The registry counts one high-risk province and adjusts for age. The nationwide study averages whole regions and does not.
## The parasite behind the gap
In 1994 the International Agency for Research on Cancer (IARC), the World Health Organization's cancer agency, classed Opisthorchis viverrini as carcinogenic to humans.[^5] People catch it by eating raw or undercooked freshwater fish that carry the fluke's larvae.[^5]
The agency estimated in 1994 that about nine million people were infected, and at least a third of north-east Thailand's population.[^5] A 2008 review put Thai infections at about six million, with prevalence reaching 70% in some areas.[^3] The two estimates are 14 years apart, after years of treatment and health education.
The strongest recent evidence comes from a 2025 study in the Proceedings of the National Academy of Sciences. It modelled infection and cancer data from north-east Thailand.[^2] It attributed 92% of cholangiocarcinoma cases diagnosed there from 1997 to 2009 to the fluke.[^2] Infected people had a 4.9% lifetime chance of the cancer, about 14 times the risk without infection.[^2]
The same study points to falling exposure. Among people born before 1990, 88% had been exposed. Among those born after, 32% had.[^2]
## What the headline numbers leave out
**The cancer takes decades.** The 2025 model estimated about 28 years from infection to the genetic damage behind the cancer, then about 32 more to diagnosis.[^2] First infection typically came at about two years old in the older cohorts.[^2]
**Survival is poor everywhere.** In the US, about 8,000 people a year are diagnosed with bile duct cancer.[^6] The American Cancer Society puts five-year survival at 10% for cancers inside the liver and 13% for those outside it.[^7] In Khon Kaen, most patients are diagnosed late and five-year survival is below 10%.[^4]
**Screening finds earlier tumours, but the survival gain is less certain.** The Cholangiocarcinoma Screening and Care Program (CASCAP) offers ultrasound to residents of the north-east aged 40 and over.[^4][^8] By 2019 it had enrolled 394,026 people.[^8] Of 762 cases studied, 84.5% found by screening were early-stage, against 21.6% among patients who came to hospital with symptoms.[^9]
A 2024 study of 711 cases found five-year survival of 53.9% after screening and 21.9% without it.[^10] A second 2024 study, of patients who had surgery, found a median survival of 51 months after screening against 38 months otherwise. That difference was not statistically significant.[^11] Both were retrospective. Neither can fully separate living longer from simply being diagnosed earlier.
## What this means for travellers and diaspora
The US Centers for Disease Control and Prevention (CDC) says the fluke mainly occurs in north-east Thailand, Laos, Cambodia and central and southern Vietnam.[^12] It lists two at-risk groups: travellers who eat raw or undercooked freshwater fish there, and immigrants from those areas.[^12]
The risk in the Thai data comes from decades of repeated infection from early childhood.[^2] No study reviewed here measured cancer risk from a short trip.
Infection is preventable and treatable. The CDC says cooking fish to 145°F (63°C), or freezing it properly, kills the parasite.[^12] Praziquantel, a two-day course of tablets, is the standard treatment.[^12]
## How to use this
If you eat raw or undercooked freshwater fish in north-east Thailand, Laos, Cambodia or Vietnam, mention it to your doctor. Ask whether testing for the fluke makes sense, since the CDC notes infections can last for years.[^12]
If you grew up in the region and ate those dishes as a child, say so at check-ups. In Thailand, people aged 40 and over in the north-east can join the CASCAP ultrasound screening.[^4] Outside Thailand, ask your doctor whether an ultrasound is worth doing.
[^1]: https://wjgnet.com/1007-9327/full/v23/i39/7160.htm — Chaiteerakij R. et al., World Journal of Gastroenterology, "Characteristics and outcomes of cholangiocarcinoma by region in Thailand: A nationwide study" (published Oct 2017). Accessed Oct 5, 2026.
[^2]: https://pmc.ncbi.nlm.nih.gov/articles/PMC12541340/ — Crellen T. et al., Proceedings of the National Academy of Sciences, "Natural history of liver fluke infection underpins epidemiological patterns of biliary cancer" (published Oct 2025). Accessed Oct 5, 2026.
[^3]: https://pmc.ncbi.nlm.nih.gov/articles/PMC4130346 — Sripa B. and Pairojkul C., Current Opinion in Gastroenterology, "Cholangiocarcinoma: lessons from Thailand" (published May 2008). Accessed Oct 5, 2026.
[^4]: https://pmc.ncbi.nlm.nih.gov/articles/PMC4459438 — Khuntikeo N. et al., BMC Cancer, "Cohort profile: cholangiocarcinoma screening and care program (CASCAP)" (published 2015). Accessed Oct 5, 2026.
[^5]: https://inchem.org/documents/iarc/vol61/m61-2.html — International Agency for Research on Cancer, IARC Monographs Volume 61, "Infection with liver flukes (Opisthorchis viverrini, Opisthorchis felineus and Clonorchis sinensis)" (published 1994). Accessed Oct 5, 2026.
[^6]: https://www.cancer.org/cancer/types/bile-duct-cancer/about/key-statistics.html — American Cancer Society, "Key Statistics for Bile Duct Cancer" (revised Oct 11, 2024). Accessed Oct 5, 2026.
[^7]: https://www.cancer.org/cancer/types/bile-duct-cancer/detection-diagnosis-staging/survival-by-stage.html — American Cancer Society, "Survival Rates for Bile Duct Cancer", citing SEER data for 2015-2021 (revised Jul 2, 2025). Accessed Oct 5, 2026.
[^8]: https://bmjopen.bmj.com/content/9/3/e023217 — Chamadol N. et al., BMJ Open, "Association between periductal fibrosis and bile duct dilatation among a population at high risk of cholangiocarcinoma" (published Mar 2019). Accessed Oct 5, 2026.
[^9]: https://doi.org/10.1016/j.hpb.2019.10.010 — Khuntikeo N. et al., HPB, "A comparison of the proportion of early stage cholangiocarcinoma found in an ultrasound-screening program compared to walk-in patients" (published Oct 2019). Accessed Oct 5, 2026.
[^10]: https://europepmc.org/article/PMC/PMC11458967 — Chamadol N. et al., Heliyon, "Improvement of survival outcomes of cholangiocarcinoma by ultrasonography surveillance: Multicenter retrospective cohorts" (published Sep 2024). Accessed Oct 5, 2026.
[^11]: https://europepmc.org/article/PMC/PMC11526638 — Thanasukarn V. et al., World Journal of Surgical Oncology, "Improving postoperative survival in cholangiocarcinoma: development of surgical strategies with a screening program in the epidemic region" (published Oct 2024). Accessed Oct 5, 2026.
[^12]: https://www.cdc.gov/liver-flukes/hcp/clinical-overview-opisthorchis/index.html — US Centers for Disease Control and Prevention, "Clinical Overview of Opisthorchis" (published Jun 24, 2024). Accessed Oct 5, 2026.