Embryonated eggs are discharged in the biliary ducts and in the stool (1). Eggs are ingested by a suitable snail intermediate host (2); there are more than 100 species of snails that can serve as intermediate hosts. Each egg releases a miracidium (2a), which goes through several developmental stages (sporocyst [2b], redia [2c], and cercaria [2d]. The cercaria is released from the snail and after a short period of free-swimming time in water, it penetrates the flesh of a freshwater fish, such as Cyclocheilichthys armatus or Puntius leiacanthus, where it encysts as a metacercaria (3). Humans are infected through ingestion of undercooked, salted, pickled, or smoked freshwater fishes (4). After ingestion, the metacercaria excysts in the duodenum (5) and ascends the biliary tract through the ampulla of Vater. Maturation to adulthood (right, O. viverrini; left, C. sinensis) takes approximately one month (6). The adult flukes (measuring 10–25 mm by 3–5 mm) reside in the small and medium-sized biliary ducts. In addition to humans, carnivorous animals can serve as reservoir hosts. (Adapted from http://www.dpd.cdc.gov/DPDx/HTML/Opisthorchiasis.htm.)

Morphologically this cancer falls into three subtypes: mass forming, periductal infiltrating, and intraductal. This classification is independent of the location of the affected bile duct, which may be intrahepatic (within the liver) or extrahepatic (outside the liver, excluding the gallbladder). The line between these subtypes tends to blur as the malignancy matures. Tumour dimensions, mode of spreading, and clinical manifestations are noted. Adapted from [36,37] by Giovanni Maki.

(A) Fluke-infected fish are plentiful in the local rivers such as the Chi River in Khon Kaen province, Thailand. (B) Local people catch the fish in nets and prepare the fish-based meals with local herbs, spices, and condiments. (C) The finished dish of koi-pla accompanied by rice and vegetables. This dish is a dietary staple of many northeastern Thai villagers and is a common source of infection with O. viverrini.

Increasing intensity of red represents increasing prevalence of O. viverrini, while increasing number of dots represents increasing cancer rates. In general, higher O. viverrini prevalence correlates with a higher CCA burden, although sporadic anthelmintic therapy has influenced this relationship. It should be noted that even one spot represents significant cancer rates anywhere else in the world. *Truncated age-standardised incidence from 35–64 years. **Age-standardised incidence of CCA throughout registered regions. Adapted from [20].

The three proposed pathways linking the parasite to CCA initiation are mechanical damage (in yellow), molecular products (in blue), and immunopathology (in green). Combined, these mechanisms result in several common elements (purple) that all lead to DNA damage. The inhibition of a normal DNA damage response is the final oncogenic factor proposed to dramatically increase the likelihood of a malignant transformation. This invariably leads to progression of CCA. (Adapted from [10].) The photographs underneath the schematic show two livers with advanced CCA.